PROGRAMMING MODEL AND ARCHITECTURE OF THE 8051

 THE 8051 ARCHITECTURE

 ALU 

 Memory 

 Peripherals 

 Timing and control unit 

 Oscillator

 ALU

 Arithmetic and logic unit performs all arithmetic (addition, subtraction, multiplication and division) and logical (AND, OR, NOT, EXCLUSIVE-OR and rotating) operations on 8 bit data i.e. the 8051 has 8 bit ALU. The ALU also updates information about the nature of the result in the flag register (PSW).

  Memory 

The 8051 family has separate on-chip program and data memory. The program instructions are stored in a program memory. , 80C51 has 4Kbytes of on-chip ROM, whereas 80C52 has 8Kbyte (ROM) , 87C51 has 4Kbytes (EPROM) and 87C52 has 8Kbytes(EEPROM) of on-chip program memory. Data memory can be on-chip or offchip. Internal data memory (RAM) in 80C51 is 128 bytes and in 80C52 is 256 bytes. 

 Peripherals 

The 8051 has two 16 bit timers (8052 has three timers) that are used for timing and counting applications. It has full duplex serial port (UART) to handle serial data transmission and reception. 

 Timing and control unit 

This unit generates all timing and control signals necessary for the execution of instructions and synchronizes all internal activities with the clock. 

 Oscillator 

The 8051 has an internal (on-chip) oscillator circuit which generates the clock pulses by which all internal operations are synchronized. Normally quartz crystal is used to make oscillator functional.Typically 12 MHz crystal is used.

PROGRAMMING MODEL OF THE 8051

 The programming model of 8051 contains 8 (or 16) bit registers and memory locations. Each register (or memory location) has an internal 1byte address with exception of program counter. Some registers are byte as well as bit addressable i.e. whole byte of data stored in a register can be accessed (read/write) at a time or individual bits can be accessed at a time.

ON-CHIP MEMORY ORGANIZATION 

 Special function registers (SFRs) 

 Internal RAM 

 Internal ROM

 Special function registers (SFRs): 

Math registers:A and B. 

 Status register: PSW (Program StatusWord) 

 Program counter: PC 

 Pointer registers: DPTR (Data Pointer) and SP (Stack Pointer) 

 Input output port latches: P0, P1, P2, and P3. 

 Peripheral data registers:TL0,TH0,TL1,TH1, and SBUF. 

 Peripheral control registers: IP, IE,TMOD,TCON, SCON, and PCON 

Internal RAM

 The 8051 has 128 bytes of internal RAM 

 Register Banks: Bank 0, Bank 1, Bank 2 and Bank 3 (00H to 1FH) 

 Bit Addressable RAM: Memory locations from addresses 20H to 2FH. 

 General Purpose RAM: Memory locations from addresses 30H to 7FH. 

Internal ROM 

 The 8051 has 4Kbytes of internal ROM. 

 It is used to store program instructions to be executed by the microcontroller. It may also be used to store permanent data like constants, passwords and lookup tables.

Special function registers (SFRs) 

Accumulator: 

A Accumulator is the most useful and versatile register because it is used in 

i) all arithmetic operations like addition, subtraction, multiplication and division. 

ii) Majority of logical operations like logical AND, OR, NOT, EX-OR and Rotate 

iii) all data transfer between the 8051 and any external memory. 

B 

B is used along with A in multiplication operation to hold one of the operand (either multiplier or multiplicand) and to store higher order byte of the result. It is also used in division operation to hold divisor and to store remainder of the result.

PSW 

Program status word is an 8 bit register. It is also referred as flag register or processor status word.

Program status word structure 

Program counter: PC

 Program counter (PC) is a 16-bit register. It always contains the memory address of the next instruction to be executed i.e. it points to the instruction that is to be executed next. As the CPU fetches the op-code (instruction byte) from the program memory, the PC is incremented automatically point to the next instruction. 

Data Pointer: DPTR 

DPTR is a 16 bit register. It is used to point to data byte in external data (RAM) or program (ROM) memory. It can be used as a single 16 bit register or can also be accessed as two separate 8 bit registers named DPL and DPH, where DPH means higher byte of the DPTR and DPL is lower byte of the DPTR. 

Stack pointer: SP 

Stack pointer always points to the top of the stack and used to access data from there. It is an 8 bit register. The data is stored on to the stack using PUSH and CALL instructions and retrieved using POP and RET instructions. 

I/O port registers (latches): P0, P1, P2 and P3.

 The 8051 has four 8 bit ports named as P0, P1 P2 and P3, each can be used as an input or output or both.All ports are byte as well as bit addressable. 

Peripheral data registers: TL0,TH0,TL1,TH1, and SBUF. 

TL0 (timer 0 lower byte) and TH0 (timer 0 higher byte) together represents a16 bit register for timer 0. They are also used as event counters. Similarly, TL1 and TH1 are registers for timer 1. 

Peripheral control registers: IP,IE,TMOD,TCON, SCON, and PCON.

 IP (interrupt priority) is used to assign priorities to different interrupt sources. IE (interrupt enable) register is used to enable/disable interrupts. TMOD (timer mode) is used to control behavior i.e. mode of operation of timers. TCON (timer control) is used to start/stop timers. SCON (serial port control) register is used to control the modes of operation of the serial port. PCON (power mode control) register is used to select power saving modes of operations. 

Internal RAM 

The 8051 microcontroller has a total of 128 bytes of internal RAM. These bytes are assigned addresses 00H to 7FH. These 128 bytes are grouped into three different areas. 

1. Register banks 

First 32 bytes from addresses 00H to 1FH are organized as four banks. Each bank is made up of eight registers named R0 to R7. 

2. Bit addressable memory 

The 8051 has a bit addressable area of 16 bytes from byte addresses 20H to 2FH in internal RAM, forming a total of 128 (16x8) addressable bits. An addressable bit can be accessed by its bit addresses from 00H to 7FH. 

3. General Purpose RAM 

Bytes from memory locations 30H to 7FH are used for general purpose data storage.

Internal ROM 

The 8051 contains 4Kbytes of internal ROM (on-chip). It occupies address range from 0000H to 0FFFH. Since it is used to store program instructions (code), it is also called program memory or code memory

EXTERNAL MEMORY ORGANIZATION 

There are two parallel 64 Kilobytes address spaces 

 one for the ROM(program memory). 

 one for the RAM (data memory). 

The data space is accessed using external data movement instructions (MOVX A, source or MOVX destination, A) and code space is accessed using external code movement instructions (MOVC A, source).