8051 Microcontroller

INTRODUCTION

The 8151 is an 8-bit microcontroller originally designed in the 1980's by Intel. Its standard form includes several standard on-chip peripherals, including timers, counters, and UART, plus 4kbytes of on-chip program memory and 128 bytes of data memory, making single-chip implementations possible.
The 8051 have modified Harvard architecture with separate address spaces for program memory and data memory. The program memory can be up to 64K.Up to 4 Kbytes of program instructions can be stored in the internal memory of the 8051. The 8051 can address up to 64K of external data memory, and is accessed only by indirect addressing. The 8051 memory architecture includes 128 bytes of data memory that are accessible directly by its instructions. segment of this 128-byte memory block is bit addressable by a subset of the 8051 instructions, namely the bit-instructions. The majority of the 8051's instructions are executed within 12 clock cycles.
The 8051 instruction set is optimized for the one-bit operations so often desired in real
Time control applications. The 8051 features include so-called "boolean processor" feature. The Boolean processor provides direct support for bit manipulation. This refers to the way instructions can single out bits just about anywhere (RAM, accumulators, I/O registers, etc.), perform complex bit tests and comparisons, and then execute relative jumps based on the results. This leads to more efficient programs that need to deal with binary input and output conditions inherent in digital-control problems.

INTERNAL ARCHITECTURE OF 8051

The 8 architecture consists of:

• Eight-bit CPU with registers. A and B and boolean processor 
• 5 interrupts: 2 external and 3 internal with 2 priority levels 
• 2 16-bit timer/counters: T 0 and T 1
• Programmable full-duplex serial port (baud rate provided by one ofthe timers)
• 32 I/O lines (four 8-bit ports)
• Internal RAM of 128 bytes
• Internal ROM/EPROM (8751) of 0k (8031) to 4k (8051) 
• 16-bit PC and data pointer (DPTR)
• 8-bit PSW
• 8-bit SP
• Oscillator and clock circuits

The 8051 operates based on an external crystal oscillator. This is an electrical device which, when energy is applied, emits pulses at a fixed frequency. Most often a quartz crystal oscillator is connected to inputs XTAL1 (pin 19) and XTAL2 (pin 18). XTAL1 is input to the inverting oscillator amplifier and input to the internal clock generating circuits and XTAL2 is output from the inverting oscillator amplifier.

The quartz crystal oscillator connected to XTAL1 and XTAL2 also needs two capacitors of 30-pf values. One side of each capacitor is connected to the grounds 
When the 8051 is connected to a crystal oscillator and is powered up, we can observe the frequency on the XTAL2 pin using the oscilloscope.

When using an 8051, the most common crystal frequencies are 12 megahertz and 11.059 megahertz, with 11.059 being much more common.

The 8051 uses the crystal to synchronize it's operation. Effectively, the 8051 operation is measured in what are called "machine cycles." An Instruction cycle is the minimum amount of time in which a single 8051 instruction can be executed, although many instructions take multiple m/c cycles. The m/c cycle is the smallest amount of time to complete a well defined job. e.g. read, write.

One 8051 machine cycle consists of twelve oscillator periods. The time (T) required for any 8051 instruction can be computed by dividing the clock frequency by 12, inverting that result and multiplying it by the number of machine cycles required by the instruction in question.

T = No of m/c cycles x 12
Crystal frequency

Therefore, if a system which is using an 11.059MHz clock, the number of instruction per second can be computed by dividing this value by 12.

11,059,000/12 = 921,5B3

This means that the 8051 can execute 921,583 single-cycle instructions per second. Inverting this will provide the amount of time taken by each instruction cycle (1.085 microseconds). 

The Accumulator is used as a general register to accumulate the results of a large number of instructions. It can hold an 8-bit (I-byte) value and is the most versatile register the 8051 has due to the shear number of instructions that make use of the accumulator. More than half of the 8051's instructions manipulate or use the accumulator in some way. For example, if we add the number 10 and 20, the resulting 30 will be stored in the Accumulator. Once the value stored in the Accumulator one may continue processing the value or store it in another register or in memory.

THE "B" REGISTER

The "B" register is very similar to the Accumulator in the sense that it may hold an 8-bit (1byte) value.
The "B" register is only used by two 8051 instructions: MUL AB and DIV AB. Thus, if one want to quickly and easily multiply or divide A by another number, he may store the other number in "B" and make use of these two instructions.

THE "R" REGISTERS

The "R" registers are a set of eight registers that are named R0, R1, R2, R3, R4, R5, R6 and R7.These registers are used as auxiliary registers in many operations.
For example, if we add the number 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 the command required will be

ADD A, R4

After executing this instruction the Accumulator will contain the value 30.

The Accumulator alone would not be very useful if it were not for these "R" registers. The "R" registers are also used to temporarily store values.

For example, let's say programmer 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 into the accumulator
ADD A, R4; Add the value of R4
MOV R5, A; Store the resulting value temporarily in R5
MOV A, R1; Move the value of R1 into the accumulator
ADD A, R2; Add the value of R2
SUBB A, R5; Subtract the .value of R5 (which now contains R3 + R4)

In this example, R5 is used to temporarily hold the sum of R3 and R4. Of course, this isn't the most efficient way to calculate (Rl+R2)-(R3 +R4) but it does illustrate the use of the "R" registers as a way to store values temporarily.

SFRs
The 8051 is a flexible microcontroller with a relatively large number of modes of operations. 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. 

Although the address range of 80h through FFh offers 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.

Note: One should not read or write to SFR addresses that have not been assigned to an SFR. Doing so may provoke undefined behavior and may cause program to be incompatible with other 8051derivatives that use the given SFR for some other purpose.

SFR TYPES
I/O PORT SFRs

P0, P1, P2, P3 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 values read from the line are controlled by the others SFRs. .

CONTROL SFRs
The PCON, TCON, TMOD, SCON, IE, IP, PSW are SFRs which in some way control the operation or the configuration of some aspect of the 8051. For example, TCON controls the timers, SCON controls the serial port.

OTHER SFRs
The remaining SFRs SP, DPL, DPH, TLO, TL1, THO, TH1, SBUF, ACC and B 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.

Note: The PO, TCON, PI, SC0N, P2, IE, P3, IP, PSW, ACC, Bare SFRs that may be accessed via bit operations (i.e., using the SETB and CLR instructions). The other SFRs cannot be accessed using bit operations. As one can see, all SFRs whose addresses are divisible by 8 can be accessed with bit operations.

SFR DESCRPTIONS
A general idea of what each SFR does is describing in short as follows.

PO (PORT 0, ADDRESS SOH, 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.

Note: While the 8051 has four I/O port (P0, P1, P2, and P3) but if hardware uses external RAM or external code memory (i.e., program is stored in an external ROM or EPROM chip or if using external RAM chips) one may not use P0 or P2. This is because the 8051 uses ports P0 and P2 to address the external memory. Thus if one is using external RAM or code memory he may only use ports Pl and P3 for own use.-

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. When data is to be placed on the stack, the SP increments before storing data on the stack so that the stack grows up as data is stored. If one pushes a value onto the stack, the value will be written to the address of SP + 1. That is to say, if SP holds the value 07h, a PUSH instruction will push the 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.

Note: The SP, SFR, on startup, is initialized to 07h. This means the stack will start at 08h and start expanding upward in internal RAM. Since alternate register banks 1, 2, and 3 as well as the user bit variables occupy internal RAM from addresses 08h through 2Fh, it is necessary to initialize SP in program to some other value if someone will be using the alternate register banks and/or bit memory.

DPL\DPH (DATA POINTER LOWIHIGH, 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).

Note: DPTR is really DPH and DPL taken together as a 16-bit value. In reality, you almost always have to deal with DPTR one byte at a time. For example, to push
DPTR onto the stack. you must first push DPL and then DPH. You can't simply plush DPTR onto the stack. Additionally, there is an instruction to "increment DPTR." When you execute this instruction, the two bytes are operated upon as a 16-bit value. However, there is no instruction that decrements DPTR. If you wish to decrement the value of DPTR, you must write your own code to do so.

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 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, one 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 08H, 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.

Note: To use the 8051's on-board serial port, it is necessary to initialize the following SERs: SCON, TCON, and TMOD. This is because SCON controls the serial port. However, in most cases the program will wish to use one of the timers to establish the serial port's baud rate. In this case, it is necessary to configure timer 1 by initializing TCON and TMOD.

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.

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.

Note: While the 8051 has four I/O port (P0, P1, P2, and P3) but if hardware uses external RAM or external code memory (i.e., program is stored in an external ROM or EPROM chip or if using external RAM chips) one may not use PO or P2. This is because the 8051 uses ports P0 and P2 to address the external memory. Thus if one is using external RAM or code memory he may only use ports P1 and P3.

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 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 the serial interrupt routine has the highest priority.

PSW (PROGRAM STATUS WORD, ADDRESSES DOH, 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 that are used to select which of the "R" register banks are currently selected. PSW register are:

-CY : Carry Flag.
-AC : Auxiliary Carry Flag.
-F0 : Flag 0 (available for user).
-RS1 : Register Select 1.
-RS0 : Register Select 0.
-QV : Arithmetic Overflow Flag.
-P : Accumulator Parity Flag.

Note: If one writes an interrupt handler routine, it is advisable to save the PSW SFR on the stack and restore it when the interrupt is complete. Many 8051 instructions modify the bits of PSW. If interrupt routine does not guarantee that PSW is the same upon exit as it was upon entry, program is bound to behave rather erratically and unpredictably, and it will be tricky to debug since the behavior will tend not to make any sense.

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 FOB, BIT-ADDRESSABLE)
The "B" register is used in two instructions: the multiply and divide operations. The B register is also used by programmers as an auxiliary register to temporarily store values.

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