Instruction set and Assembly language programming of 8051 Microcontroller

8051 Instructions

8051 has about 111 instructions. These can be grouped into the following categories

1.     Arithmetic Instructions

2.     Logical Instructions

3.     Data Transfer instructions

4.     Boolean Variable Instructions

5.     Program Branching Instructions

The following nomenclatures for register, data, address and variables are used while write instructions.

A: Accumulator

B: "B" register

C: Carry bit

Rn: Register R0 - R7 of the currently selected register bank

Direct: 8-bit internal direct address for data. The data could be in lower 128bytes of RAM (00 - 7FH) or it could be in the special function register (80 - FFH).

@Ri: 8-bit external or internal RAM address available in register R0 or R1. This is used for indirect addressing mode.

#data8: Immediate 8-bit data available in the instruction. #data16: Immediate 16-bit data available in the instruction.

Addr11: 11-bit destination address for short absolute jump. Used by instructions AJMP & ACALL. Jump range is 2 kbyte (one page).

Addr16: 16-bit destination address for long call or long jump.

Rel: 2's complement 8-bit offset (one - byte) used for short jump (SJMP) and all conditional jumps.

bit: Directly addressed bit in internal RAM or SFR

ü Arithmetic Instructions

Assembly language Programming

Character transmission using a time delay:

A program shown below takes the character in 'A' register, transmits it, delays for transmission time, and returns to the calling program. Timer-1 is used to set the baud rate, which is 1200 baud in this program

The delay for one character transmission (in Mode 1 i.e.10 bits) is 10/2400 = 0.00833 seconds

Or, 8.33 milliseconds

Hence software delay of 10ms is used.

Timer-1 generates a baud rate close to 1200. Using a 12MHz crystal, the reload value is

This gives rise to an actual baud rate of 1202. SMOD is programmed to be 0.

Assembly language Program is as follows

; Code to wait for the transmission to complete

The subroutine TRMITTIME generates a delay of about 10ms. With a clock of 12MHz, one instruction cycle time is

The loop "MILSEC" generates a delay of about 1 x 10^-3 sec. This gets executed 10 times for a total delay of 10 x 10^-3 sec or 10ms