Architecture Of 8051 Microcontroller - Online Article

Block Diagram of 8051 Microcontroller


Block diagram of 89C51

Pin Diagram of 89C51 Microcontroller


Pin diagram of Microcontroller



Connection for RESET Pin


XTAL Connection

Pin Description

1–8: Port 1- Each of these pins can be used as either input or output according to your needs. Also, pins 1 and 2 (P1.0 and P1.1) have special functions associated with timer 2. 9: Reset Signal; high logical state on this input halts the MCU and clears all the registers. Bringing this pin back to logical state zero starts the program anew as if the power had just been turned on.

10-17: Port 3 - As with Port 1, each of these pins can be used as universal input or output. However, each pin of Port 3 has an alternative function.

Pin 10: RXD - serial input for asynchronous communication or serial output for synchronous communication.

Pin 11: TXD - serial output for asynchronous communication or clock output for synchronous communication.

Pin 12: INT0 - input for interrupt 0.

Pin 13: INT1 - input for interrupt 1.

Pin 14: T0 - clock input of counter 0.

Pin 15: T1 - clock input of counter 1.

Pin 16: WR - signal for writing to external (add-on) RAM memory.

Pin 17: RD - signal for reading from external RAM memory.

18-19: X2 and X1; Input and output of internal oscillator. Quartz crystal controlling the frequency commonly connects to these pins. Capacitances within the oscillator mechanism (see the image) are not critical and are normally about 30pF. Instead of a quartz crystal, miniature ceramic resonators can be used for dictating the pace. In that case, manufacturers recommend using somewhat higher capacitances (about 47 pF).

20: GND.

Ground 21- 28: Port 2 - If external memory is not present, pins of Port 2 act as universal input/output. If external memory is present, this is the location of the higher address byte, i.e. addresses A8 – A15.

29: PSEN - MCU activates this bit (brings to low state) upon each reading of byte (instruction) from program memory. If external ROM is used for storing the program, PSEN is directly connected to its control pins.

30: ALE -Before each reading of the external memory, MCU sends the lower byte of the address register (addresses A0 – A7) to port P0 and activates the output ALE. External register (74HCT373 or 74HCT375 circuits are common), memorizes the state of port P0 upon receiving a signal from ALE pin, and uses it as part of the address for memory chip. During the second part of the mechanical MCU cycle, signal on ALE is off, and port P0 is used as Data Bus.

31: EA - Bringing this pin to the logical state zero (mass) designates the ports P2 and P3 for transferring addresses regardless of the presence of the internal memory. This means that even if there is a program loaded in the MCU it will not be executed, but the one from the external ROM will be used instead. Conversely, bringing the pin to the high logical state causes the controller to use both memories, first the internal, and then the external (if present).

32-39: Port 0 - Similar to Port 2, pins of Port 0 can be used as universal input/output, if external memory is not used. If external memory is used, P0 behaves as address output (A0 – A7) when ALE pin is at high logical level, or as data output (Data Bus) when ALE pin is at low logical level.

40: VCC - Power +5V.

Input – Output (I/O) Ports

Every MCU from 8051 family has 4 I/O ports of 8 bits each. This provides the user with 32 I/O lines for connecting MCU to the environs. Unlike the case with other controllers, there is no specific SFR register for designating pins as input or output. Instead, the port itself is in charge: 0=output, 1=input. If particular pin on the case is needed as output, the appropriate bit of I/O port should be cleared. This will generate 0V on the specified controller pin. Similarly, if particular pin on the case is needed as input, the appropriate bit of I/O port should be set. This will designate the pin as input, generating +5V as a side effect.


Port 0 has two fold role: if external memory is used, it contains the lower address byte (addresses A0-A7), otherwise all bits of the port are either input or output. Another feature of this port comes to play when it has been designated as output. Unlike other ports, Port 0 lacks the "pull up" resistor (resistor with +5V on one end). This seemingly insignificant change has the following consequences:

  • When designated as input, pin of Port 0 acts as high impedance offering the infinite input resistance with no "inner" voltage.
  • When designated as output, pin acts as "open drain". Clearing a port bit grounds the appropriate pin on the case (0V). Setting a port bit makes the pin act as high impedance. Therefore, to get positive logic (5V) at output, external "pull up" resistor needs to be added for connecting the pin to the positive pole.

Port 1

This is "true" I/O port, devoid of dual function characteristic for Port 0. Having the "pull up" resistor, Port 1 is fully compatible with TTL circuits.

Port 2

When using external memory, this port contains the higher address byte (addresses A8–A15), similar to Port 0. Otherwise, it can be used as universal I/O port.

Port 3

Beside its role as universal I/O port, each pin of Port 3 has an alternate function. In order to use one of these functions, the pin in question has to be designated as input, i.e. the appropriate bit of register P3 needs to be set. From a hardware standpoint, Port 3 is similar to Port 0.

Types of Memory

The 8051 has three very general types of memory. To effectively program the 8051 it is necessary to have a basic understanding of these memory types. The memory types are illustrated in the following graphic. They are: On-Chip Memory, External Code Memory, and External RAM.

On - Chip

On-Chip Memory refers to any memory (Code, RAM, or other) that physically exists on the microcontroller itself. On-chip memory can be of several types, but we'll get into that shortly.

External Code Memory

External Code Memory is code (or program) memory that resides off-chip. This is often in the form of an external EPROM.

External RAM

External RAM is RAM memory that resides off-chip. This is often in the form of standard static RAM or flash refers to any memory (Code, RAM, or other) that physically exists on the microcontroller itself. On-chip memory can be of several types, but we'll get into that shortly.

Code Memory

Code memory is the memory that holds the actual 8051 program that is to be run. This memory is limited to 64K and comes in many shapes and sizes: Code memory may be found on-chip, either burned into the microcontroller as ROM or EPROM. Code may also be stored completely off-chip in an external ROM or, more commonly, an external EPROM. Flash RAM is also another popular method of storing a program. Various combinations of these memory types may also be used--that is to say, it is possible to have 4K of code memory on-chip and 64k of code memory off-chip in an EPROM.

External RAM

External RAM As an obvious opposite of Internal RAM, the 8051 also supports what is called External RAM. As the name suggests, External RAM is any random access memory which is found off-chip. Since the memory is off-chip it is not as flexible in terms of accessing, and is also slower. For example, to increment an Internal RAM location by 1 requires only 1 instruction and 1 instruction cycle. To increment a 1-byte value stored in External RAM requires 4 instructions and 7 instruction cycles. In this case, external memory is 7 times slower!

On-Chip Memory

As mentioned earlier, the 8051 includes a certain amount of on-chip memory. On-chip memory is really one of two types: Internal RAM and Special Function Register (SFR) memory.

The 8051 has a bank of 128 bytes of Internal RAM. This Internal RAM is found on-chip on the 8051 so it is the fastest RAM available, and it is also the most flexible in terms of reading, writing, and modifying its contents. Internal RAM is volatile, so when the 8051 is reset this memory is cleared. The 128 bytes of internal ram is subdivided as shown on the memory map. The first 8 bytes (00h - 07h) are "register bank 0". By manipulating certain SFRs, a program may choose to use register banks 1, 2, or 3. These alternative register banks are located in internal RAM in addresses 08h through 1Fh.

Bit Memory also lives and is part of internal RAM. Bit memory actually resides in internal RAM, from addresses 20h through 2Fh. The 80 bytes remaining of Internal RAM, from addresses 30h through 7Fh, may be used by user variables that need to be accessed frequently or at high-speed. This area is also utilized by the microcontroller as a storage area for the operating stack. This fact severely limits the 8051’s stack since, as illustrated in the memory map, the area reserved for the stack is only 80 bytes--and usually it is less since these 80 bytes has to be shared between the stack and user variables.


Block diagram representation of memory in 89C51


On chip memory MAP in 89C51

Register Banks

The 8051 uses 8 "R" registers which are used in many of its instructions. These "R" registers are numbered from 0 through 7 (R0, R1, R2, R3, R4, R5, R6, and R7). These registers are generally used to assist in manipulating values and moving data from one memory location to another. For example, to add the value of R4 to the Accumulator, we would execute the following instruction:


Thus if the Accumulator (A) contained the value 6 and R4 contained the value 3, the Accumulator would contain the value 9 after this instruction was executed.

However, as the memory map shows, the "R" Register R4 is really part of Internal RAM. Specifically, R4 is address 04h. Thus the above instruction accomplishes the same thing as the following operation:

ADD A, 04h

This instruction adds the value found in Internal RAM address 04h to the value of the Accumulator, leaving the result in the Accumulator. Since R4 is really Internal RAM 04h, the above instruction effectively accomplished the same thing.

As the memory map shows, the 8051 has four distinct register banks. When the 8051 is first booted up, register bank 0 (addresses 00h through 07h) is used by default. However, your program may instruct the 8051 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 8051 to use register bank 3, "R" register R4 will now be synonymous with Internal RAM address 1Ch.

Bit Memory

The 8051, being a communications-oriented microcontroller, gives the user the ability to access a number of bit variables. These variables may be either 1 or 0. There are 128 bit variables available to the user, numbered 00h through 7Fh. The user may make use of these variables with commands such as SETB and CLR. For example, to set bit number 24 (hex) to 1 you would execute the instruction:

SETB 24h

It is important to note that Bit Memory is really a part of Internal RAM. In fact, the 128 bit variables occupy the 16 bytes of Internal RAM from 20h through 2Fh. Thus, if you write the value FFh to Internal RAM address 20h you’ve effectively set bits 00h through 07h. That is to say that:

MOV 20h,#0FFh

is equivalent to:

SETB 00h
SETB 01h
SETB 02h
SETB 03h
SETB 04h
SETB 05h
SETB 06h
SETB 07h 

Special Function Registers (SFR)

The 8051 is a flexible microcontroller with a relatively large number of modes of operations. Your program may inspect and/or change the operating mode of the 8051 by manipulating the values of the 8051's Special Function Registers (SFRs).

SFRs are accessed as if they were normal Internal RAM. The only difference is that Internal RAM is from address 00h through 7Fh whereas SFR registers exist in the address range of 80h through FFh.

Each SFR has an address (80h through FFh) and a name. The chart on next page provides a graphical presentation of the 8051's SFRs, their names, and their address.

Although the address range of 80h through FFh offer 128 possible addresses, there are only 21 SFRs in a standard 8051. All other addresses in the SFR range (80h through FFh) are considered invalid. Writing to or reading from these registers may produce undefined values or behavior.

SFR Types

As mentioned in the chart itself, the SFRs that have a blue background are SFRs related to the I/O ports. The 8051 has four I/O ports of 8 bits, for a total of 32 I/O lines. Whether a given I/O line is high or low and the value read from the line are controlled by the SFRs in green. TCON controls the timers, SCON controls the serial port.

The remaining SFRs, with green backgrounds, are "other SFRs." These SFRs can be thought of as auxiliary SFRs in the sense that they don't directly configure the 8051 but obviously the 8051 cannot operate without them. For example, once the serial port has been configured using SCON, the program may read or write to the serial port using the SBUF register.

SFR Desdriptions

P0 (Port 0, Address 80h, Bit-Addressable)

This is input/output port 0. Each bit of this SFR corresponds to one of the pins on the microcontroller. For example, bit 0 of port 0 is pin P0.0, bit 7 is pin P0.7. Writing a value of 1 to a bit of this SFR will send a high level on the corresponding I/O pin whereas a value of 0 will bring it to a low level.

SP (Stack Pointer, Address 81h)

This is the stack pointer of the microcontroller. This SFR indicates where the next value to be taken from the stack will be read from in Internal RAM. If you push a value onto the stack, the value will be written to the address value onto the stack at address 08h. This SFR is modified by all instructions which modify the stack, such as PUSH, POP, LCALL, RET, RETI, and whenever interrupts are provoked by the microcontroller.

DPL/DPH (Data Pointer Low/High, Addresses 82h/83h)

The SFRs DPL and DPH work together to represent a 16-bit value called the Data Pointer. The data pointer is used in operations regarding external RAM and some instructions involving code memory. Since it is an unsigned two-byte integer value, it can represent values from 0000h to FFFFh (0 through 65,535 decimal).

PCON (Power Control, Addresses 87h)

The Power Control SFR is used to control the 8051's power control modes. Certain operation modes of the 8051 allow the 8051 to go into a type of "sleep" mode which requires much less power. These modes of operation are controlled through PCON. Additionally, one of the bits in PCON is used to double the effective baud rate of the 8051's serial port.

TCON (Timer Control, Addresses 88h, Bit-Addressable)

The Timer Control SFR is used to configure and modify the way in which the 8051's two timers operate. This SFR controls whether each of the two timers is running or stopped and contains a flag to indicate that each timer has overflowed. Additionally, some non-timer related bits are located in the TCON SFR. These bits are used to configure the way in which the external interrupts are activated and also contain the external interrupt flags which are set when an external interrupt has occurred.

TMOD (Timer Mode, Addresses 89h)

The Timer Mode SFR is used to configure the mode of operation of each of the two timers. Using this SFR your program may configure each timer to be a 16-bit timer, an 8-bit auto reload timer, a 13-bit timer, or two separate timers. Additionally, you may configure the timers to only count when an external pin is activated or to count "events" that are indicated on an external pin.

TL0/TH0 (Timer 0 Low/High, Addresses 8Ah/8Ch)

These two SFRs, taken together, represent timer 0. Their exact behavior depends on how the timer is configured in the TMOD SFR; however, these timers always count up. What is configurable is how and when they increment in value.

TL1/TH1 (Timer 1 Low/High, Addresses 8Bh/8Dh)

These two SFRs, taken together, represent timer 1. Their exact behavior depends on how the timer is configured in the TMOD SFR; however, these timers always count up. What is configurable is how and when they increment in value.

P1 (Port 1, Address 90h, Bit-Addressable)

This is input/output port 1. Each bit of this SFR corresponds to one of the pins on the microcontroller. For example, bit 0 of port 1 is pin P1.0, bit 7 is pin P1.7. Writing a value of 1 to a bit of this SFR will send a high level on the corresponding I/O pin whereas a value of 0 will bring it to a low level.

SCON (Serial Control, Addresses 98h, Bit-Addressable)

The Serial Control SFR is used to configure the behavior of the 8051's on-board serial port. This SFR controls the baud rate of the serial port, whether the serial port is activated to receive data, and also contains flags that are set when a byte is successfully sent or received.

SBUF (Serial Control, Addresses 99h)

The Serial Buffer SFR is used to send and receive data via the on-board serial port. Any value written to SBUF will be sent out the serial port's TXD pin. Likewise, any value which the 8051 receives via the serial port's RXD pin will be delivered to the user program via SBUF. In other words, SBUF serves as the output port when written to and as an input port when read from.


Location of SFRs in memory 89C51

P2 (Port 2, Address A0h, Bit-Addressable)

This is input/output port 2. Each bit of this SFR corresponds to one of the pins on the microcontroller. For example, bit 0 of port 2 is pin P2.0, bit 7 is pin P2.7. Writing a value of 1 to a bit of this SFR will send a high level on the corresponding I/O pin whereas a value of 0 will bring it to a low level.

IE (Interrupt Enable, Addresses A8h)

The Interrupt Enable SFR is used to enable and disable specific interrupts. The low 7 bits of the SFR are used to enable/disable the specific interrupts, where as the highest bit is used to enable or disable ALL interrupts. Thus, if the high bit of IE is 0 all interrupts are disabled regardless of whether an individual interrupt is enabled by setting a lower bit.

P3 (Port 3, Address B0h, Bit-Addressable)

This is input/output port 3. Each bit of this SFR corresponds to one of the pins on the microcontroller. For example, bit 0 of port 3 is pin P3.0, bit 7 is pin P3.7. Writing a value of 1 to a bit of this SFR will send a high level on the corresponding I/O pin whereas a value of 0 will bring it to a low level.

IP (Interrupt Priority, Addresses B8h, Bit - Addressable)

The Interrupt Priority SFR is used to specify the relative priority of each interrupt. On the 8051, an interrupt may either be of low (0) priority or high (1) priority. An interrupt may only interrupt interrupts of lower priority. For example, if we configure the 8051 so that all interrupts are of low priority except the serial interrupt, the serial interrupt will always be able to interrupt the system, even if another interrupt is currently executing. However, if a serial interrupt is executing no other interrupt will be able to interrupt the serial interrupt routine since the serial interrupt routine has the highest priority.

PSW (Program Status Word, Addresses D0h, Bit - Addressable)

The Program Status Word is used to store a number of important bits that are set and cleared by 8051 instructions. The PSW SFR contains the carry flag, the auxiliary carry flag, the overflow flag, and the parity flag. Additionally, the PSW register contains the register bank select flags which are used to select which of the "R" register banks are currently selected.

ACC (Accumulator, Addresses E0h, Bit - Addressable)

The Accumulator is one of the most-used SFRs on the 8051 since it is involved in so many instructions. The Accumulator resides as an SFR at E0h, which means the instruction MOV A,#20h is really the same as MOV E0h,#20h. However, it is a good idea to use the first method since it only requires two bytes whereas the second option requires three bytes.

B (B Register, Addresses F0h, Bit - Addressable)

The "B" register is used in two instructions: the multiply and divide operations. The B register is also commonly used by programmers as an auxiliary register to temporarily store values.


The 8051 comes equipped with two timers, both of which may be controlled, set, read, and configured individually. The 8051 timers have three general functions: 1) Keeping time and/or calculating the amount of time between events, 2) Counting the events themselves, or 3) Generating baud rates for the serial port.

How does a TIMER count ?

How does a timer count? The answer to this question is very simple: A timer always counts up. It doesn’t matter whether the timer is being used as a timer, a counter, or a baud rate generator: A timer is always incremented by the microcontroller.

Using TIMER to measure Time

Obviously, one of the primary uses of timers is to measure time. When a timer is used to measure time it is also called an "interval timer" since it is measuring the time of the interval between two events.

How long does a TIMER take a count ?

First, its worth mentioning that when a timer is in interval timer mode (as opposed to event counter mode) and correctly configured, it will increment by 1 every machine cycle. As you will recall from the previous chapter, a single machine cycle consists of 12 crystal pulses. Thus a running timer will be incremented:

11,059,000 / 12 = 921,583

921,583 times per second. Unlike instructions--some of which require 1 machine cycle, others 2, and others 4--the timers are consistent: They will always be incremented once per machine cycle. Thus if a timer has counted from 0 to 50,000 you may calculate:

50,000 / 921,583 = .0542

.0542 seconds have passed. In plain English, about half of a tenth of a second, or one-twentieth of a second.

Obviously its not very useful to know .0542 seconds have passed. If you want to execute an event once per second you’d have to wait for the timer to count from 0 to 50,000 18.45 times. How can you wait "half of a time?" You can’t. So we come to another important calculation.

Let’s say we want to know how many times the timer will be incremented in .05 seconds. We can do simple multiplication:

.05 * 921,583 = 46,079.15.

This tells us that it will take .05 seconds (1/20th of a second) to count from 0 to 46,079. Actually, it will take it .049999837 seconds--so were off by .000000163 seconds--however, thats close enough for government work. Consider that if you were building a watch based on the 8051 and made the above assumption your watch would only gain about one second every 2 months. Again, I think that’s accurate enough for most applications--I wish my watch only gained one second every two months!

Obviously, this is a little more useful. If you know it takes 1/20th of a second to count from 0 to 46,079 and you want to execute some event every second you simply wait for the timer to count from 0 to 46,079 twenty times; then you execute your event, reset the timers, and wait for the timer to count up another 20 times. In this manner you will effectively execute your event once per second, accurate to within thousandths of a second.

Thus, we now have a system with which to measure time. All we need to review is how to control the timers and initialize them to provide us with the information we need.

Addressing Modes

Addressing modes are an integral part of each computer's instruction set. They al¬low specifying the source or destination of data in different ways depending on the programming situation.

An "addressing mode" refers to how you are addressing a given memory location. In summary, the addressing modes are as follows, with an example of each:

Immediate Addressing MOV A,#20h
Direct Addressing MOV A,30h
Indirect Addressing MOV A,@R0
External Direct MOVX A,@DPTR
Code Indirect MOVC A,@A+DPTR

Immediate Addressing

Immediate addressing is so-named because the value to be stored in memory immediately follows the operation code in memory. That is to say, the instruction itself dictates what value will be stored in memory. For example, the instruction:

MOV A, #20h

This instruction uses Immediate Addressing because the Accumulator will be loaded with the value that immediately follows; in this case 20 (hexadecimal). Immediate addressing is very fast since the value to be loaded is included in the instruction. However, since the value to be loaded is fixed at compile-time it is not very flexible.

Direct Addressing

Direct addressing is so-named because the value to be stored in memory is obtained by directly retrieving it from another memory location. For example:

MOV A,30h

This instruction will read the data out of Internal RAM address 30 (hexadecimal) and store it in the Accumulator. Direct addressing is generally fast since, although the value to be loaded isn’t included in the instruction, it is quickly accessible since it is stored in the 8051s Internal RAM. It is also much more flexible than Immediate Addressing since the value to be loaded is whatever is found at the given address--which may be variable.

Indirect Addressing

Indirect addressing is a very powerful addressing mode which in many cases provides an exceptional level of flexibility. Indirect addressing is also the only way to access the extra 128 bytes of Internal RAM found on an 8052. Indirect addressing appears as follows:


This instruction causes the 8051 to analyze the value of the R0 register. The 8051 will then load the accumulator with the value from Internal RAM which is found at the address indicated by R0.

Indirect addressing always refers to Internal RAM; it never refers to an SFR. Thus, in a prior example we mentioned that SFR 99h can be used to write a value to the serial port. Thus one may think that the following would be a valid solution to write the value 1 to the serial port:

MOV R0,#99h; //Load the address of the serial port
MOV @R0,#01h

This is not valid. Since indirect addressing always refers to Internal RAM these two instructions would write the value 01h to Internal RAM address 99h on an 8052. On an 8051 these two instructions would produce an undefined result since the 8051 only has 128 bytes of Internal RAM.

External Direct

External Memory is accessed using a suite of instructions which use what I call "External Direct" addressing. I call it this because it appears to be direct addressing, but it is used to access external memory rather than internal memory. There are only two commands that use External Direct addressing mode:


As you can see, both commands utilize DPTR. In these instructions, DPTR must first be loaded with the address of external memory that you wish to read or write. Once DPTR holds the correct external memory address, the first command will move the contents of that external memory address into the Accumulator. The second command will do the opposite: it will allow you to write the value of the Accumulator to the external memory address pointed to by DPTR.

External Indirect

External memory can also be accessed using a form of indirect addressing which I call External Indirect addressing. This form of addressing is usually only used in relatively small projects that have a very small amount of external RAM. An example of this addressing mode is:


Once again, the value of R0 is first read and the value of the Accumulator is written to that address in External RAM. Since the value of @R0 can only be 00h through FFh the project would effectively be limited to 256 bytes of External RAM. There are relatively simple hardware/software tricks that can be implemented to access more than 256 bytes of memory using External Indirect addressing; however, it is usually easier to use External Direct addressing if your project has more than 256 bytes of External RAM.

Insruction Set Summary

Arithmetic Instructions


Logical Operations


Instructions Affecting Flags


Logical Instructions




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Suyog Konduskar on 2013-03-16 18:17:28 wrote,