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20-01-2011, 04:30 PM


The objective of this project and implimentation is to design an embedded system which can control appliances remotely. This project and implimentation is designed to study the basic implementation of DTMF (Dual Tone Multi Frequency).
The DTMF signal is composed of high frequencies & low frequencies. Each key is assigned a low frequency and a high frequency. When a particular key is pressed, the frequencies those are specific to that particular key are transmitted. Therefore, no two keys will have a same set of low & high frequencies.
We use DTMF encoder & decoder to develop this switch. The DTMF encoder transmits the set of two frequencies related to a particular key and the decoder on receiving the set, converts it to related digital signals. These digital signals are fed to Microcontroller, and can be used to control appliances or nodes remotely. The embedded software for the controller is implemented using ‘C’ language.
The system is designed and developed using 8051 µController.


Telephone Controlled Switch or DTMF based remote switch implies control of devices at a remote location via circuit interfaced to the remote telephone line/device by dialing specific DTMF (dual tune multi frequency) digits from a local telephone. Using this system the user can access any load from a remote location. Each load will be given a code number. All he has to do is dial the corresponding code through a local phone.
This system can selectively turn on or off multiple loads one at a time or switch off all the loads simultaneously. It also provides you feedback when the circuit is in energized state and also sends an acknowledgement indicating action with respect to the switching on of each load and switching off of all loads (together). This is a micro controller aided system where a ring detector, auto lifter and DTMF decoder work together to place a call and a relay driver connects the required load. This makes the task of the various operators easy as they can sit anywhere and control the status of the various loads.
In the existing system if an operator in and industry has to control a certain load there is no other method than being physically present at the site of the load. If he has to switch off a system after a particular period he will have to wait till the completion of the operation. But our project and implimentation titled "Telephone controlled switch" rectifies this disadvantage of conventional systems. This system as its name suggests help the user to control the status of various loads from any remote location through a telephone connection.
As mentioned this system implies control of devices at a remote location via circuit interfaced to the remote telephone line/device by dialing specific DTMF (dual tune multi frequency) digits from a local telephone. For this each of the loads that the operator intends to control is given a code number and is connected to the telephone line through relay drivers and micro controller. So when the user dials the respective codes the corresponding loads can be accessed. Then its status can be controlled using some other code. All this is made possible by programming the micro controller.
This project and implimentation telephone controlled load activator has the following features.
1. It can control multiple load (on/off/status of each load)



An embedded system is a special-purpose computer system designed to perform one or a few dedicated functions, sometimes with real-time computing constraints. It is usually embedded as part of a complete device including hardware and mechanical parts. In contrast, a general-purpose computer, such as a personal computer, can do many different tasks depending on programming. Embedded systems have become very important today as they control many of the common devices we use.
Since the embedded system is dedicated to specific tasks, design engineers can optimize it, reducing the size and cost of the product, or increasing the reliability and performance. Some embedded systems are mass-produced, benefiting from economies of scale.

Physically, embedded systems range from portable devices such as digital watches and MP3 players, to large stationary installations like traffic lights, factory controllers, or the systems controlling nuclear power plants. Complexity varies from low, with a single microcontroller chip, to very high with multiple units, peripherals and networks mounted inside a large chassis or enclosure.

In general, "embedded system" is not an exactly defined term, as many systems have some element of programmability. For example, Handheld computers share some elements with embedded systems — such as the operating systems and microprocessors which power them — but are not truly embedded systems, because they allow different applications to be loaded and peripherals to be connected.

An embedded system is some combination of computer hardware and software, either fixed in capability or programmable, that is specifically designed for a particular kind of application device. Industrial machines, automobiles, medical equipment, cameras, household appliances, airplanes, vending machines, and toys (as well as the more obvious cellular phone and PDA) are among the myriad possible hosts of an embedded system. Embedded systems that are programmable are provided with a programming interface, and embedded systems programming is a specialized occupation.

Certain operating systems or language platforms are tailored for the embedded market, such as Embedded Java and Windows XP Embedded. However, some low-end consumer products use very inexpensive microprocessors and limited storage, with the application and operating system both part of a single program. The program is written permanently into the system's memory in this case, rather than being loaded into RAM (random access memory), as programs on a personal computer are.

We are living in the Embedded World. You are surrounded with many embedded products and your daily life largely depends on the proper functioning of these gadgets. Television, Radio, CD player of your living room, Washing Machine or Microwave Oven in your kitchen, Card readers, Access Controllers, Palm devices of your work space enable you to do many of your tasks very effectively. Apart from all these, many controllers embedded in your car take care of car operations between the bumpers and most of the times you tend to ignore all these controllers.

In recent days, you are showered with variety of information about these embedded controllers in many places. All kinds of magazines and journals regularly dish out details about latest technologies, new devices; fast applications which make you believe that your basic survival is controlled by these embedded products. Now you can agree to the fact that these embedded products have successfully invaded into our world. You must be wondering about these embedded controllers or systems. What is this Embedded System?

The computer you use to compose your mails, or create a document or analyze the database is known as the standard desktop computer. These desktop computers are manufactured to serve many purposes and applications.
You need to install the relevant software to get the required processing facility. So, these desktop computers can do many things. In contrast, embedded controllers carryout a specific work for which they are designed. Most of the time, engineers design these embedded controllers with a specific goal in mind. So these controllers cannot be used in any other place.
Theoretically, an embedded controller is a combination of a piece of microprocessor based hardware and the suitable software to undertake a specific task.
These days designers have many choices in microprocessors/microcontrollers. Especially, in 8 bit and 32 bit, the available variety really may overwhelm even an experienced designer. Selecting a right microprocessor may turn out as a most difficult first step and it is getting complicated as new devices continue to pop-up very often.

In the 8 bit segment, the most popular and used architecture is Intel's 8031. Market acceptance of this particular family has driven many semiconductor manufacturers to develop something new based on this particular architecture. Even after 25 years of existence, semiconductor manufacturers still come out with some kind of device using this 8031 core.
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For makers of low-cost consumer electronic devices who wish to integrate the LynxOS real-time operating system into their products, we offer special MSRP-based pricing to reduce royalty fees to a negligible portion of the device's MSRP.
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What is the difference between a Microprocessor and Microcontroller? By microprocessor is meant the general purpose Microprocessors such as Intel's X86 family (8086, 80286, 80386, 80486, and the Pentium) or Motorola's 680X0 family (68000, 68010, 68020, 68030, 68040, etc). These microprocessors contain no RAM, no ROM, and no I/O ports on the chip itself. For this reason, they are commonly referred to as general-purpose Microprocessors.

A system designer using a general-purpose microprocessor such as the Pentium or the 68040 must add RAM, ROM, I/O ports, and timers externally to make them functional. Although the addition of external RAM, ROM, and I/O ports makes these systems bulkier and much more expensive, they have the advantage of versatility such that the designer can decide on the amount of RAM, ROM and I/O ports needed to fit the task at hand. This is not the case with Microcontrollers.

A Microcontroller has a CPU (a microprocessor) in addition to a fixed amount of RAM, ROM, I/O ports, and a timer all on a single chip. In other words, the processor, the RAM, ROM, I/O ports and the timer are all embedded together on one chip; therefore, the designer cannot add any external memory, I/O ports, or timer to it. The fixed amount of on-chip ROM, RAM, and number of I/O ports in Microcontrollers makes them ideal for many applications in which cost and space are critical.
In many applications, for example a TV remote control, there is no need for the computing power of a 486 or even an 8086 microprocessor. These applications most often require some I/O operations to read signals and turn on and off certain bits.


In the Literature discussing microprocessors, we often see the term Embedded System. Microprocessors and Microcontrollers are widely used in embedded system products. An embedded system product uses a microprocessor (or Microcontroller) to do one task only. A printer is an example of embedded system since the processor inside it performs one task only; namely getting the data and printing it. Contrast this with a Pentium based PC. A PC can be used for any number of applications such as word processor, print-server, bank teller terminal, Video game, network server, or Internet terminal. Software for a variety of applications can be loaded and run. Of course the reason a pc can perform myriad tasks is that it has RAM memory and an operating system that loads the application software into RAM memory and lets the CPU run it.

In an Embedded system, there is only one application software that is typically burned into ROM. An x86 PC contains or is connected to various embedded products such as keyboard, printer, modem, disk controller, sound card, CD-ROM drives, mouse, and so on. Each one of these peripherals has a Microcontroller inside it that performs only one task. For example, inside every mouse there is a Microcontroller to perform the task of finding the mouse position and sending it to the PC. Table 1-1 lists some embedded products.


The generic 8051 architecture supports a Harvard architecture, which contains two separate buses for both program and data. So, it has two distinctive memory spaces of 64K X 8 size for both programmed and data. It is based on an 8 bit central processing unit with an 8 bit Accumulator and another 8 bit B register as main processing blocks. Other portions of the architecture include few 8 bit and 16 bit registers and 8 bit memory locations.

Each 8051 device has some amount of data RAM built in the device for internal processing. This area is used for stack operations and temporary storage of data.
This bus architecture is supported with on-chip peripheral functions like I/O ports, timers/counters, versatile serial communication port. So it is clear that this 8051 architecture was designed to cater many real time embedded needs.


 Optimized 8 bit CPU for control applications and extensive Boolean processing capabilities.
 64K Program Memory address space.
 64K Data Memory address space.
 128 bytes of on chip Data Memory.
 32 Bi-directional and individually addressable I/O lines.
 Two 16 bit timer/counters.
 Full Duplex UART.
 6-source / 5-vector interrupt structure with priority levels.
 On chip clock oscillator.
Now we may be wondering about the non-mentioning of memory space meant for the program storage, the most important part of any embedded controller. Originally this 8051 architecture was introduced with on-chip, ‘one time programmable’ version of Program Memory of size 4K X 8. Intel delivered all these microcontrollers (8051) with user’s program fused inside the device. The memory portion was mapped at the lower end of the Program Memory area. But, after getting devices, customers couldn’t change any thing in their program code, which was already made available inside during device fabrication.

Figure 4.1 - Block Diagram of the 8051 Core

So, very soon Intel introduced the 8051 devices with re-programmable type of Program Memory using built-in EPROM of size 4K X 8. Like a regular EPROM, this memory can be re-programmed many times. Later on Intel started manufacturing these 8031 devices without any on chip Program Memory.


ALE/PROG: Address Latch Enable output pulse for latching the low byte of the address during accesses to external memory. ALE is emitted at a constant rate of 1/6 of the oscillator frequency, for external timing or clocking purposes, even when there are no accesses to external memory. (However, one ALE pulse is skipped during each access to external Data Memory.) This pin is also the program pulse input (PROG) during EPROM programming.
PSEN : Program Store Enable is the read strobe to external Program Memory. When the device is executing out of external Program Memory, PSEN is activated twice each machine cycle (except that two PSEN activations are skipped during accesses to external Data Memory). PSEN is not activated when the device is executing out of internal Program Memory.
EA/VPP: When EA is held high the CPU executes out of internal Program Memory (unless the Program Counter exceeds 0FFFH in the 80C51). Holding EA low forces the CPU to execute out of external memory regardless of the Program Counter value. In the 80C31, EA must be externally wired low. In the EPROM devices, this pin also receives the programming supply voltage (VPP) during EPROM programming.
XTAL1: Input to the inverting oscillator amplifier.
XTAL2: Output from the inverting oscillator amplifier.
The 8051’s I/O port structure is extremely versatile and flexible. The device has 32 I/O pins configured as four eight bit parallel ports (P0, P1, P2 and P3). Each pin can be used as an input or as an output under the software control. These I/O pins can be accessed directly by memory instructions during program execution to get required flexibility.
These port lines can be operated in different modes and all the pins can be made to do many different tasks apart from their regular I/O function executions. Instructions, which access external memory, use port P0 as a multiplexed address/data bus. At the beginning of an external memory cycle, low order 8 bits of the address bus are output on P0. The same pins transfer data byte at the later stage of the instruction execution.
Also, any instruction that accesses external Program Memory will output the higher order byte on P2 during read cycle. Remaining ports, P1 and P3 are available for standard I/O functions. But all the 8 lines of P3 support special functions: Two external interrupt lines, two counter inputs, serial port’s two data lines and two timing control strobe lines are designed to use P3 port lines. When you don’t use these special functions, you can use corresponding port lines as a standard I/O. Even within a single port, I/O operations may be combined in many ways. Different pins can be configured as input or outputs independent of each other or the same pin can be used as an input or as output at different times. You can comfortably combine I/O operations and special operations for Port 3 lines.

All the Port 3 pins are multifunctional. They are not only port pins, but also serve the functions of various special features as listed below:
Port Pin Alternate Function
P3.0 RxD (serial input port)
P3.1 TxD (serial output port)
The alternate functions can only be activated if the corresponding bit latch in the port SFR contains a 1. Otherwise the port pin remains at 0.All 80C51 devices have separate address spaces for program and data memory, as shown in Figures 1 and 2. The logical separation of program and data memory allows the data memory to be accessed by 8-bit addresses, which can be quickly stored and manipulated by an 8-bit CPU. Nevertheless, 16-bit data memory addresses can also be generated through the DPTR register.

Program memory (ROM, EPROM) can only be read, not written to. There can be up to 64k bytes of program memory. In the 80C51, the lowest 4k bytes of program are on-chip. In the ROM less versions, all program memory is external. The read strobe for external program memory is the PSEN (program store enable). Data Memory (RAM) occupies a separate address space from Program Memory. In the 80C51, the lowest 128 bytes of data memory are on-chip. Up to 64k bytes of external RAM can be addressed in the external Data Memory space. In the ROM less version, the lowest 128 bytes are on-chip. The CPU generates read and write signals, RD and WR, as needed during external Data Memory accesses.

External Program Memory and external Data Memory may be combined if desired by applying the RD and PSEN signals to the inputs of an AND gate and using the output of the gate as the read strobe to the external Program/Data memory.

A number of 8052 registers can be considered "basic." Very little can be done without them and a detailed explanation of each one is warranted to make sure the reader understands these registers before getting into more complicated areas of development.

The Accumulator: If you've worked with any other assembly language you will be familiar with the concept of an accumulator register.

The Accumulator, as its name suggests, is used as a general register to accumulate the results of a large number of instructions. It can hold an 8-bit (1-byte) value and is the most versatile register the 8052 has due to the sheer number of instructions that make use of the accumulator. More than half of the 8052's 255 instructions manipulate or use the Accumulator in some way. For example, if you want to add the number 10 and 20, the resulting 30 will be stored in the Accumulator. Once you have a value in the Accumulator you may continue processing the value or you may store it in another register or in memory.
The "R" Registers: The "R" registers are sets of eight registers that are named R0, R1, through R7. These registers are used as auxiliary registers in many operations. To continue with the above example, perhaps you are adding 10 and 20. The original number 10 may be stored in the Accumulator whereas the value 20 may be stored in, say, register R4. To process the addition you would execute the command:
After executing this instruction the Accumulator will contain the value 30. You may think of the "R" registers as very important auxiliary, or "helper", registers. The Accumulator alone would not be very useful if it were not for these "R" registers.
The "R" registers are also used to store values temporarily. For example, let’s say you want to add the values in R1 and R2 together and then subtract the values of R3 and R4. One way to do this would be:
MOV A, R3 ; Move the value of R3 to accumulator
ADD A, R4 ; add the value of R4
MOV R5, A ; Store the result in R5
MOV A, R1 ; Move the value of R1 to Acc
ADD A, R2 ; add the value of R2 with A
SUBB A, R5 ; Subtract the R5 (which has R3+R4)
As you can see, we used R5 to temporarily hold the sum of R3 and R4. Of course, this isn't the most efficient way to calculate (R1+R2) - (R3 +R4) but it does illustrate the use of the "R" registers as a way to store values temporarily.
As mentioned earlier, there are four sets of "R" registers-register bank 0, 1, 2, and 3. When the 8052 is first powered up, register bank 0 (addresses 00h through 07h) is used by default. In this case, for example, R4 is the same as Internal RAM address 04h. However, your program may instruct the 8052 to use one of the alternate register banks; i.e., register banks 1, 2, or 3. In this case, R4 will no longer be the same as Internal RAM address 04h. For example, if your program instructs the 8052 to use register bank 1, register R4 will now be synonymous with Internal RAM address 0Ch. If you select register bank 2, R4 is synonymous with 14h, and if you select register bank 3 it is synonymous with address 1Ch.
The concept of register banks adds a great level of flexibility to the 8052, especially when dealing with interrupts (we'll talk about interrupts later). However, always remember that the register banks really reside in the first 32 bytes of Internal RAM.

The B Register The "B" register is very similar to the Accumulator in the sense that it may hold an 8-bit (1-byte) value. The "B" register is only used implicitly by two 8052 instructions: MUL AB and DIV AB. Thus, if you want to quickly and easily multiply or divide A by another number, you may store the other number in "B" and make use of these two instructions.
Aside from the MUL and DIV instructions, the "B" register are often used as yet another temporary storage register much like a ninth "R" register.

The Program Counter The Program Counter (PC) is a 2-byte address that tells the 8052 where the next instruction to execute is found in memory. When the 8052 is initialized PC always starts at 0000h and is incremented each time an instruction is executed. It is important to note that PC isn't always incremented by one. Since some instructions are 2 or 3 bytes in length the PC will be incremented by 2 or 3 in these cases.

The Program Counter is special in that there is no way to directly modify its value. That is to say, you can't do something like PC=2430h. On the other hand, if you execute LJMP 2430h you've effectively accomplished the same thing.
It is also interesting to note that while you may change the value of PC (by executing a jump instruction, etc.) there is no way to read the value of PC. That is to say, there is no way to ask the 8052 "What address are you about to execute?" As it turns out, this is not completely true: There is one trick that may be used to determine the current value of PC. This trick will be covered in a later chapter.

The Data Pointer: The Data Pointer (DPTR) is the 8052ís only user-accessible 16-bit (2-byte) register. The Accumulator, "R" registers, and "B" register are all 1-byte values. The PC just described is a 16-bit value but isn't directly user-accessible as a working register.

DPTR, as the name suggests, is used to point to data. It is used by a number of commands that allow the 8052 to access external memory. When the 8052 accesses external memory it accesses the memory at the address indicated by DPTR.

While DPTR is most often used to point to data in external memory or code memory, many developers take advantage of the fact that it's the only true 16-bit register available. It is often used to store 2-byte values that have nothing to do with memory locations.

The Stack Pointer: The Stack Pointer, like all registers except DPTR and PC, may hold an 8-bit (1-byte) value. The Stack Pointer is used to indicate where the next value to be removed from the stack should be taken from.
When you push a value onto the stack, the 8052 first increments the value of SP and then stores the value at the resulting memory location. When you pop a value off the stack, the 8052 returns the value from the memory location indicated by SP and then decrements the value of SP.

This order of operation is important. When the 8052 is initialized SP will be initialized to 07h. If you immediately push a value onto the stack, the value will be stored in Internal RAM address 08h. This makes sense taking into account what was mentioned two paragraphs above: First the 8051 will increment the value of SP (from 07h to 08h) and then will store the pushed value at that memory address (08h).

The addressing modes in the 80C51 instruction set are as follows:

Direct Addressing: In direct addressing the operand is specified by an 8-bit address field in the instruction. Only internal Data RAM and SFRs can be directly addressed.

Indirect Addressing: In indirect addressing the instruction specifies a register which contains the address of the operand. Both internal and external RAM can be indirectly addressed. The address register for 8-bit addresses can be R0 or R1 of the selected bank, or the Stack Pointer. The address register for 16-bit addresses can only be the 16-bit “data pointer” register, DPTR.

Register Instructions The register banks, containing registers R0 through R7, can be accessed by certain instructions which carry a 3-bit register specification within the opcode of the instruction. Instructions that access the registers this way are code efficient, since this mode eliminates an address byte. When the instruction is executed, one of the eight registers in the selected bank is accessed. One of four banks is selected at execution time by the two bank select bits in the PSW.

Register-Specific Instructions Some instructions are specific to a certain register. For example, some instructions always operate on the Accumulator, or Data Pointer, etc., so no address byte is needed to point to it. The opcode itself does that. Instructions that refer to the Accumulator as A assemble as accumulator specific opcodes.

Immediate Constants
The value of a constant can follow the opcode in Program Memory. For example,
MOV A, #100
loads the Accumulator with the decimal number 100. The same number could be specified in hex digits as 64H.

Indexed Addressing
Only program Memory can be accessed with indexed addressing, and it can only be read. This addressing mode is intended for reading look-up tables in Program Memory A 16-bit base register (either DPTR or the Program Counter) points to the base of the table, and the Accumulator is set up with the table entry number. The address of the table entry in Program Memory is formed by adding the Accumulator data to the base pointer. Another type of indexed addressing is used in the “case jump” instruction. In this case the destination address of a jump instruction is computed as the sum of the base pointer and the Accumulator data.
The CPU is the brain of the microcontrollers reading user’s programs and executing the expected task as per instructions stored there in. Its primary elements are an 8 bit Arithmetic Logic Unit (ALU ) , Accumulator (Acc ) , few more 8 bit registers , B register, Stack Pointer (SP ) , Program Status Word (PSW) and 16 bit registers, Program Counter (PC) and Data Pointer Register (DPTR).

The ALU (Acc) performs arithmetic and logic functions on 8 bit input variables. Arithmetic operations include basic addition, subtraction, and multiplication and division. Logical operations are AND, OR, Exclusive OR as well as rotate, clear, complement and etc. Apart from all the above, ALU is responsible in conditional branching decisions, and provides a temporary place in data transfer operations within the device.
B-register is mainly used in multiply and divides operations. During execution, B register either keeps one of the two inputs or then retains a portion of the result. For other instructions, it can be used as another general purpose register.
Program Status Word (PSW) keeps the current status of the ALU in different bits. Stack Pointer (SP) is an 8 bit register. This pointer keeps track of memory space where the important register information is stored when the program flow gets into executing a subroutine. The stack portion may be placed in any where in the on-chip RAM. But normally SP is initialized to 07H after a device reset and grows up from the location 08H. The Stack Pointer is automatically incremented or decremented for all PUSH or POP instructions and for all subroutine calls and returns.
Program Counter (PC) is the 16 bit register giving address of next instruction to be executed during program execution and it always points to the Program Memory space. Data Pointer (DPTR) is another 16 bit addressing register that can be used to fetch any 8 bit data from the data memory space. When it is not being used for this purpose, it can be used as two eight bit registers.


8051 has two 16 bit Timers/Counters capable of working in different modes. Each consists of a ‘High’ byte and a ‘Low’ byte which can be accessed under software. There is a mode control register and a control register to configure these timers/counters in number of ways.

These timers can be used to measure time intervals, determine pulse widths or initiate events with one microsecond resolution up to a maximum of 65 millisecond (corresponding to 65, 536 counts). Use software to get longer delays. Working as counter, they can accumulate occurrences of external events (from DC to 500 KHz) with 16 bit precision.


Each 8051 microcomputer contains a high speed full duplex (means you can simultaneously use the same port for both transmitting and receiving purposes) serial port which is software configurable in 4 basic modes: 8 bit UART; 9 bit UART; inter processor Communications link or as shift register I/O expander.

For the standard serial communication facility, 8051 can be programmed for UART operations and can be connected with regular personal computers, teletype writers, modem at data rates between 122 bauds and 31 kilo bauds. Getting this facility is made very simple using simple routines with option to elect even or odd parity. You can also establish a kind of Inter processor communication facility among many microcomputers in a distributed environment with automatic recognition of address/data. Apart from all above, you can also get super fast I/O lines using low cost simple TTL or CMOS shift registers.

A microprocessor as a term has come to be known is a general-purpose digital computer central processing unit. Although popularly known as a computer on a chip.

The microprocessor contains arithmetic and logic unit, program counter, Stack pointer, some working registers, clock timing circuit and interrupt circuits.

To make a complete computer one must add memory usually RAM & ROM, memory decoders, an oscillator and number of I/O devices such as parallel and serial data ports in addition special purpose devices such as interrupt handlers and counters.

The key term in describing the design of the microprocessor is “general purpose”. The hardware design of a microprocessor CPU is arranged so that a small or very large system can be configured around the CPU as the application demands.

The prime use of microprocessor is to read data, perform extensive calculations on that data and store those calculations in a mass storage device. The programs used by the microprocessor are stored in the mass storage device and loaded in the RAM as the user directs. A few microprocessor programs are stored in the ROM. The ROM based programs are primarily are small fixed programs that operate on peripherals and other fixed device that are connected to the system


Micro controller is a true computer on a chip the design incorporates all of the features found in a microprocessor CPU: arithmetic and logic unit, stack pointer, program counter and registers. It has also had added additional features like RAM, ROM, serial I/O, counters and clock circuit.
Like the microprocessor, a microcontroller is a general purpose device, but one that is meant to read data, perform limited calculations on that data and control it’s environment based on those calculations. The prime use of a microcontroller is to control the operation of a machine using a fixed program that is stored in ROM and that does not change over the lifetime of the system.
The design approach of a microcontroller uses a more limited set of single byte and double byte instructions that are used to move code and data from internal memory to ALU. Many instructions are coupled with pins on the IC package; the pins are capable of having several different functions depending on the wishes of the programmer.

The microcontroller is concerned with getting the data from and on to its own pins; the architecture and instruction set are optimized to handle data in bit and byte size.


1. The first and foremost criterion for choosing a microcontroller is that it must meet task at hands efficiently and cost effectively. In analyzing the needs of a microcontroller based project and implimentation we must first see whether it is an 8-bit, 16-bit or 32-bit microcontroller and how best it can handle the computing needs of the task most effectively. The other considerations in this category are:
(a) Speed: The highest speed that the microcontroller supports
(b) Packaging: Is it 40-pin DIP or QPF or some other packaging format?
This is important in terms of space, assembling and prototyping the
End product.
© Power Consumption: This is especially critical for battery-powered
(d) The amount of RAM and ROM on chip
(e) The number of I/O pins and timers on the chip.
(f) Cost per unit: This is important in terms of final product in which a microcontroller is used.
2. The second criteria in choosing a microcontroller are how easy it is to develop products around it. Key considerations include the availability of an assembler, debugger, a code efficient ‘C’ language compiler, emulator, technical support and both in house and outside expertise. In many cases third party vendor support for chip is required.

3. The third criteria in choosing a microcontroller is it readily available in needed quantities both now and in future. For some designers this is even more important than first two criteria’s. Currently, of leading 8–bit microcontrollers, the 89C51 family has the largest number of diversified (multiple source) suppliers. By suppliers meant a producer besides the originator of microcontroller in the case of the 89C51, which was originated by Intel, several companies are also currently producing the 89C51. Viz: INTEL, ATMEL, These companies include PHILIPS, SIEMENS, and DALLAS-SEMICONDUCTOR. It should be noted that Motorola, Zilog and Microchip Technologies have all dedicated massive resource as to ensure wide and timely availability of their product since their product is stable, mature and single sourced. In recent years they also have begun to sell the ASIC library cell of the microcontroller.

The 89C51/89C52/89C54/89C58 contains a non-volatile FLASH program memory that is parallel programmable. All three families are Single-Chip 8-bit Microcontrollers manufactured in advanced CMOS process and are derivatives of the 80C51 microcontroller family. All the devices have the same instruction set as the 80C51.

• 80C51 Central Processing Unit.
• On-chip FLASH Program Memory
• Speed up to 33 MHz
• Fully static operation
• RAM expandable externally up to 64 Kbytes
• 4 interrupt priority levels
• 6 interrupt sources
• Four 8-bit I/O ports
• Full-duplex enhanced UART
 Framing error detection
 Automatic address recognition
• Three 16-bit timers/counters T0, T1 (standard 80C51) and additional T2 (capture and compare)
• Power control modes
 Clock can be stopped and resumed
 Idle mode
 Power down mode
• Programmable clock out
• Second DPTR register
• Asynchronous port reset
• Low EMI (inhibit ALE)
• Wake up from power down by an external interrupt

General Description

The 89C51/89C52/89C54/89C58 FLASH reliably stores memory contents even after 10,000 erase and program cycles. The cell is designed to optimize the erase and programming mechanisms. In addition, the combination of advanced tunnel oxide processing and low internal electric fields for erase and programming operations produces reliable cycling.

• FLASH EPROM internal program memory with Chip Erase.
• Up to 64 k byte external program memory if the internal program memory is disabled (EA = 0).
• Programmable security bits.
• 10,000 minimum erase/program cycles for each byte.
• 10 year minimum data retention.
• Programming support available from many popular vendors.

XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier. The pins can be configured for use as an on-chip oscillator. To drive the device from an external clock source, XTAL1 should be driven while XTAL2 is left unconnected. There are no requirements on the duty cycle of the external clock signal, because the input to the internal clock circuitry is through a divide-by-two flip-flop. However, minimum and maximum high and low times specified in the data sheet must be observed.

A reset is accomplished by holding the RST pin high for at least two machine cycles (24 oscillator periods), while the oscillator is running. To insure a good power-on reset, the RST pin must be high long enough to allow the oscillator time to start up (normally a few milliseconds) plus two machine cycles. At power-on, the voltage on VCC and RST must come up at the same time for a proper start-up. Ports 1, 2, and 3 will asynchronously be driven to their reset condition when a voltage above VIH1 (min.) is applied to RST. The value on the EA pin is latched when RST is deasserted and has no further effect.

Stop Clock Mode

The static design enables the clock speed to be reduced down to 0 MHz (stopped). When the oscillator is stopped, the RAM and Special Function Registers retain their values. This mode allows step-by-step utilization and permits reduced system power consumption by lowering the clock frequency down to any value. For lowest power consumption the Power Down mode is suggested.

Idle Mode

In the idle mode (see Table 2), the CPU puts itself to sleep while all of the on-chip peripherals stay active. The instruction to invoke the idle mode is the last instruction executed in the normal operating mode before the idle mode is activated. The CPU contents, the on-chip RAM, and all of the special function registers remain intact during this mode. The idle mode can be terminated either by any enabled interrupt (at which time the process is picked up at the interrupt service routine and continued), or by a hardware reset which starts the processor in the same manner as a power-on reset.

Power-Down Mode

To save even more power, a Power Down mode (see Table 2) can be invoked by software. In this mode, the oscillator is stopped and the instruction that invoked Power Down is the last instruction executed. The on-chip RAM and Special Function Registers retain their values down to 2.0 V and care must be taken to return VCC to the minimum specified operating voltages before the Power Down Mode is terminated. Either a hardware reset or external interrupt can be used to exit from Power Down. Reset redefines all the SFRs but does not change the on-chip RAM. An external interrupt allows both the SFRs and the on-chip RAM to retain their values. To properly terminate Power Down the reset or external interrupt should not be executed before VCC is restored to its normal operating level and must be held active long enough for the oscillator to restart and stabilize (normally less than 10ms). With an external interrupt, INT0 and INT1 must be enabled and configured as level-sensitive. Holding the pin low restarts the oscillator but bringing the pin back high completes the exit. Once the interrupt is serviced, the next instruction to be executed after RETI will be the one following the instruction that put the device into Power Down.

Design Consideration

When the idle mode is terminated by a hardware reset, the device normally resumes program execution, from where it left off, up to two machine cycles before the internal reset algorithm takes control. On-chip hardware inhibits access to internal RAM in this event, but access to the port pins is not inhibited. To eliminate the possibility of an unexpected write when Idle is terminated by reset, the instruction following the one that invokes Idle should not be one that writes to a port pin or to external memory.


The ONCE (“On-Circuit Emulation”) Mode facilitates testing and debugging of systems without the device having to be removed from the circuit. The ONCE Mode is invoked by:
1. Pull ALE low while the device is in reset and PSEN is high;
2. Hold ALE low as RST is deactivated.
While the device is in ONCE Mode, the Port 0 pins go into a float state, and the other port pins and ALE and PSEN are weakly pulled high. The oscillator circuit remains active. While the device is in this mode, an emulator or test CPU can be used to drive the circuit.
Normal operation is restored when a normal reset is applied.

Programmable Clock-Out

A 50% duty cycle clock can be programmed to come out on P1.0. This pin, besides being a regular I/O pin, has two alternate functions. It can be programmed:
1. to input the external clock for Timer/Counter 2, or
2. to output a 50% duty cycle clock ranging from 61Hz to 4MHz at a 16MHz operating frequency.
To configure the Timer/Counter 2 as a clock generator, bit C/T2 (in T2CON) must be cleared and bit T20E in T2MOD must be set. Bit TR2 (T2CON.2) also must be set to start the timer. The Clock-Out frequency depends on the oscillator frequency and the reload value of Timer 2 capture registers (RCAP2H, RCAP2L) as shown in this equation:

Oscillator Frequency

4x (65536 - RCAP2H, RCAP2L)

Where (RCAP2H, RCAP2L) = the content of RCAP2H and RCAP2L
taken as a 16-bit unsigned integer.


The UART in the AT89S52 operates the same way as the UART in the AT89C51 and AT89C52.

Enhanced UART operation

In addition to the standard operation modes, the UART can perform framing error detect by looking for missing stop bits, and automatic address recognition. The UART also fully supports multiprocessor communication. When used for framing error detect the UART looks for missing stop bits in the communication. A missing bit will set the FE bit in the SCON register. The FE bit shares the SCON.7 bit with SM0 and the function of SCON.7 is determined by PCON.6 (SMOD0). If SMOD0 is set then SCON.7 functions as FE. SCON.7 functions as SM0 when SMOD0 is cleared. When used as FE SCON.7 can only be cleared by software.

Automatic Address Recognition

Automatic Address Recognition is a feature which allows the UART to recognize certain addresses in the serial bit stream by using hardware to make the comparisons. This feature saves a great deal of software overhead by eliminating the need for the software to examine every serial address which passes by the serial port. This feature is enabled by setting the SM2 bit in SCON. In the 9 bit UART modes, mode 2 and mode 3, the Receive Interrupt flag (RI) will be automatically set when the received byte contains either the “Given” address or the “Broadcast” address. The 9 bit mode requires that the 9th information bit is a 1 to indicate that the received information is an address and not data. The 8 bit mode is called Mode 1. In this mode the RI flag will be set if SM2 is enabled and the information received has a valid stop bit following the 8 address bits and the information is either a given or broadcast address. Mode 0 is the Shift Register mode and SM2 is ignored. Using the Automatic Address Recognition feature allows a master to selectively communicate with one or more slaves by invoking the given slave address or addresses. All of the slaves may be contacted by using the Broadcast address. Two special Function Registers are used to define the slave’s address, SADDR, and the address mask, SADEN. SADEN is used to define which bits in the SADDR are to be used and which bits are “don’t care”. The SADEN mask can be logically ANDed with the SADDR to create the “Given” address which the master will use for addressing each of the slaves. Use of the given address allows multiple slaves to be recognized while excluding others. The following examples will help to show the versatility of this scheme:
Slave0 SADDR = 1100 0000
SADEN = 1111 1101
Given = 1100 00X0

Oscillator Characteristics
XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier that can be configured for use as an on-chip oscillator, as shown in Figure 7. Either a quartz crystal or ceramic resonator may be used. To drive the device from an external clock source, XTAL2 should be left unconnected while XTAL1 is driven. There are no requirements on the duty cycle of the external clock signal, since the input to the internal clocking circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high and low time specifications must be observed.

Idle Mode In idle mode, the CPU puts itself to sleep while all the on-chip peripherals remain active. The mode is invoked by software. The content of the on-chip RAM and the entire special functions registers remain unchanged during this mode. The idle mode can be terminated by any enabled interrupt or by a hardware reset. Note that when idle mode is terminated by a hardware reset, the device normally resumes program execution from where it left off, up to two machine cycles before the internal reset algorithm takes control. On-chip hardware inhibits access to internal RAM in this event, but access to the port pins is not inhibited. To eliminate the possibility of an unexpected write to a port pin when idle mode is terminated by a reset, the instruction following the one that invokes idle mode should not write to a port pin or to external memory.

Power-down Mode In the Power-down mode, the oscillator is stopped, and the instruction that invokes Power-down is the last instruction executed. The on-chip RAM and Special Function
Registers retain their values until the Power-down mode is terminated. Exit from Powerdown mode can be initiated either by a hardware reset or by an enabled external interrupt. Reset redefines the SFRs but does not change the on-chip RAM. The reset should not be activated before VCC is restored to its normal operating level and must be held active long enough to allow the oscillator to restart and stabilize.

Dual-tone multi-frequency (DTMF) signaling is used for telephone signaling over the line in the voice-frequency band to the call switching center. The version of DTMF used for telephone tone dialing is known by the trademarked term Touch-Tone (canceled March 13, 1984), and is standardized by ITU-T Recommendation Q.23. Other multi-frequency systems are used for signaling internal to the telephone network.
As a method of in-band signaling, DTMF tones were also used by cable television broadcasters to indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies.
The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key (such as ‘1’) will send a sinusoidal tone of the two frequencies (697 and 1209 hertz (Hz)). The original keypads had levers inside, so each button activated two contacts. The multiple tones are the reason for calling the system multi-frequency. These tones are then decoded by the switching center to determine which key was pressed.


• Operating voltage: 2.5V~5.5V
• Minimal external components
• No external filter is required
• Low standby current (on power down mode)
• Excellent performance
• Tristate data output for MCU interface
• 3.58MHz crystal or ceramic resonator
• 1633Hz can be inhibited by the INH pin

The HT9170B/D are Dual Tone Multi Frequency (DTMF) receivers integrated with digital decoder and band split filter functions as well as power-down mode and inhibit mode operations. Such devices use digital counting techniques to detect and decode all the 16 DTMF tone pairs into a 4-bit code output. Highly accurate switched capacitor filters are implemented to divide tone signals into low and high group signals. A built-in dial tone rejection circuit is provided to eliminate the need for pre-filtering.

The HT9170B/D tone decoders consist of three band pass filters and two digital decode circuits to convert a tone (DTMF) signal into digital code output. An operational amplifier is built-in to adjust the input signal
The pre-filter is a band rejection filter which reduces the dialing tone from 350Hz to 400Hz. The low group filter filters low group frequency signal output whereas the high group filter filters high group frequency signal output. Each filter output is followed by a zero-crossing detector with hysteresis. When each signal amplitude at the output exceeds the specified level, it is transferred to full swing logic signal.
When input signals are recognized to be effective, DV becomes high, and the correct tone code (DTMF) digit is transferred.

Steering control circuit
The steering control circuit is used for measuring the effective signal duration and for protecting against drop out of valid signals. It employs the analog delay by external RC time-constant controlled by EST. The EST pin is normally low and draws the RT/GT pin to keep low through discharge of external RC. When a valid tone input is detected, EST goes high to charge RT/GT through RC. When the voltage of RT/GT changes from 0 to VTRT (2.35V for 5V supply), the input signal is effective, and the correct code will be created by the code detector. After D0~D3 are completely latched, DV output becomes high. When the voltage of RT/GT falls down from VDD to VTRT (i.e.., when there is no input tone), DV output becomes low, and D0~D3 keeps data until a next valid tone input is produced. By selecting adequate external RC value, the minimum acceptable input tone duration (tACC) and the minimum acceptable inter-tone rejection (tIR) can be set. External components (R, C) are chosen by the formula:

where tACC: Tone duration acceptable time
tDP: EST output delay time (“L” → “H”)
tGTP: Tone present time
tIR: Inter-digit pause rejection time
tDA: EST output delay time (“H” → “L”)
tGTA: Tone absent time

The most commonly used Character based LCDs are based on Hitachi's HD44780 controller or other which are compatible with HD44580. In this tutorial, we will discuss about character based LCDs, their interfacing with various microcontrollers, various interfaces (8-bit/4-bit), programming, special stuff and tricks you can do with these simple looking LCDs which can give a new look to your application.

Pin Description
The most commonly used LCD’s found in the market today are 1 Line, 2 Line or 4 Line LCDs which have only 1 controller and support at most of 80 characters, whereas LCDs supporting more than 80 characters make use of 2 HD44780 controllers.

Most LCDs with 1 controller has 14 Pins and LCDs with 2 controller has 16 Pins (two pins are extra in both for back-light LED connections).

DDRAM - Display Data RAM
Display data RAM (DDRAM) stores display data represented in 8-bit character codes. Its extended capacity is 80 X 8 bits, or 80 characters. The area in display data RAM (DDRAM) that is not used for display can be used as general data RAM. So whatever you send on the DDRAM is actually displayed on the LCD. For LCDs like 1x16, only 16 characters are visible, so whatever you write after 16 chars is written in DDRAM but is not visible to the user.

CGROM - Character Generator ROM
Now you might be thinking that when you send an ASCII value to DDRAM, how the character is displayed on LCD? So the answer is CGROM. The character generator ROM generates 5 x 8 dot or 5 x 10 dot character patterns from 8-bit character codes (see Figure 5 and Figure 6 for more details). It can generate 208 5 x 8 dot character patterns and 32 5 x 10 dot character patterns. User defined character patterns are also available by mask-programmed ROM.

As you can see in both the code maps, the character code from 0x00 to 0x07 is occupied by the CGRAM characters or the user defined characters. If user wants to display the fourth custom character then the code to display it is 0x03 i.e. when user sends 0x03 code to the LCD DDRAM then the fourth user created character or pattern will be displayed on the LCD.
CGRAM - Character Generator RAM
As clear from the name, CGRAM area is used to create custom characters in LCD. In the character generator RAM, the user can rewrite character patterns by program. For 5 x 8 dots, eight character patterns can be written, and for 5 x 10 dots, four character patterns can be written.

BF - Busy Flag
Busy Flag is a status indicator flag for LCD. When we send a command or data to the LCD for processing, this flag is set (i.e. BF =1) and as soon as the instruction is executed successfully this flag is cleared (BF = 0). This is helpful in producing and exact amount of delay for the LCD processing.

To read Busy Flag, the condition RS = 0 and R/W = 1 must be met and The MSB of the LCD data bus (D7) act as busy flag. When BF = 1 means LCD is busy and will not accept next command or data and BF = 0 means LCD is ready for the next command or data to process.

Instruction Register (IR) and Data Register (DR)
There are two 8-bit registers in HD44780 controller Instruction and Data register. Instruction register corresponds to the register where you send commands to LCD e.g. LCD shift command, LCD clear, LCD address etc. and Data register is used for storing data which is to be displayed on LCD. When send the enable signal of the LCD is asserted, the data on the pins is latched in to the data register and data is then moved automatically to the DDRAM and hence is displayed on the LCD.
Data Register is not only used for sending data to DDRAM but also for CGRAM, the address where you want to send the data, is decided by the instruction you send to LCD.

4-bit programming of LCD
In 4-bit mode the data is sent in nibbles, first we send the higher nibble and then the lower nibble. To enable the 4-bit mode of LCD, we need to follow special sequence of initialization that tells the LCD controller that user has selected 4-bit mode of operation. We call this special sequence as resetting the LCD. Following is the reset sequence of LCD.
 Wait for about 20mS
 Send the first init value (0x30)
 Wait for about 10mS
 Send second init value (0x30)
 Wait for about 1mS
 Send third init value (0x30)
 Wait for 1mS
 Select bus width (0x30 - for 8-bit and 0x20 for 4-bit)
 Wait for 1mS

The busy flag will only be valid after the above reset sequence. Usually we do not use busy flag in 4-bit mode as we have to write code for reading two nibbles from the LCD. Instead we simply put a certain amount of delay usually 300 to 600uS. This delay might vary depending on the LCD you are using, as you might have a different crystal frequency on which LCD controller is running. So it actually depends on the LCD module you are using.

In 4-bit mode, we only need 6 pins to interface an LCD. D4-D7 are the data pins connection and Enable and Register select are for LCD control pins. We are not using Read/Write (RW) Pin of the LCD, as we are only writing on the LCD so we have made it grounded permanently. If you want to use it, then you may connect it on your controller but that will only increase another pin and does not make any big difference. Potentiometer RV1 is used to control the LCD contrast. The unwanted data pins of LCD i.e. D0-D3 are connected to ground.

Sending data/command in 4-bit Mode
We will now look into the common steps to send data/command to LCD when working in 4-bit mode. In 4-bit mode data is sent nibble by nibble, first we send higher nibble and then lower nibble. This means in both command and data sending function we need to separate the higher 4-bits and lower 4-bits.

The common steps are:
 Mask lower 4-bits
 Send to the LCD port
 Send enable signal
 Mask higher 4-bits
 Send to LCD port
 Send enable signal


A variable regulated power supply, also called a variable bench power supply, is one where you can continuously adjust the output voltage to your requirements. Varying the output of the power supply is the recommended way to test a project and implimentation after having double checked parts placement against circuit drawings and the parts placement guide.
This type of regulation is ideal for having a simple variable bench power supply. Actually this is quite important because one of the first project and implimentations a hobbyist should undertake is the construction of a variable regulated power supply. While a dedicated supply is quite handy e.g. 5V or 12V, it's much handier to have a variable supply on hand, especially for testing.
Most digital logic circuits and processors need a 5 volt power supply. To use these parts we need to build a regulated 5 volt source. Usually you start with an unregulated power To make a 5 volt power supply, we use a LM7805 voltage regulator IC (Integrated Circuit).


Brief description of operation: Gives out well regulated +5V output, output current capability of 100 mA
Circuit protection: Built-in overheating protection shuts down output when regulator IC gets too hot
Circuit complexity: Very simple and easy to build
Circuit performance: Very stable +5V output voltage, reliable operation
Availability of components: Easy to get, uses only very common basic components
Design testing: Based on datasheet example circuit, I have used this circuit succesfully as part of many electronics project and implimentations
Applications: Part of electronics devices, small laboratory power supply
Power supply voltage: Unreglated DC 8-18V power supply
Power supply current: Needed output current + 5 mA
Component costs: Few dollars for the electronics components + the input transformer cost


The schematic is divided into four sections microcontroller section, DTMF receiver module, controlling section and power supply section

Micro controller section contains Micro-controller P89C51 and a crystal of 11.0592 MHz for oscillator. Micro controller works on the program inside the memory. . As the controller keeps all the memory and I/O ports inside it, it contains very less components in its outer configuration. Power to the IC supplied is +5v DC.

The microcontroller is connected to the output pins of DTMF receiver. The input to the DTMF receiver is from the telephone line. The ground wire of telephone line is connected to the ground of the circuit. A crystal oscillator of 3.579545 MHz is used in the clock circuit of DTMF receiver

Power supply is an important part of operation of the Microcontroller. The heart of the power supply is LM7805 3-terminal regulator. Microcontroller operates at +5v DC.

This circuit controls the application when we dial a number on a landline or mobile which is situated remotely. This is particularly helpful for receiving any number over the phone lines. The DTMF signal—generated by the phone on dialing a number—is decoded by DTMF decoder , which converts the received DTMF signal into its equivalent BCD number that corresponds to the dialed number. This binary number is stored sequentially in

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