20-day 89C51 microcontroller summary

I have been looking at single-chip microcomputers for a long time, so I want to make a summary. In fact, single-chip microcomputers are not as complicated as imagined in terms of chips! 89C51 has only 40 pins. Among them, P0~P3 ports occupy 32 pins. One is connected to the ground and the other is connected to the power supply, two oscillation pins. An EA pin is the starting address selection terminal of the ROM. An ALE/Vpp off-chip address latch serves as the pin/line programming power input terminal. PSEN is the off-chip ROM enable terminal. Of course, there is also a reset REST pin. Among them, all P3 ports have a second function multiplexing: timer interrupt serial port interrupt. When the P0~P3 port writes #FFH, the on-chip latch is set to 1. This is a single port for bidirectional input and output. This is from the outside of the chip.

From the inside of the chip, there are two main independent memories, ROM and RAM. One is for program indication, and the other is for data processing and storage. ROM and RAM are distinguished by addressing mode during programming. They both have a capacity of 64KB. The ROM has a 4KB storage space on the chip. EA indicates whether the program is executed from outside the chip or from inside the chip. The RAM on the chip is 256 bytes, that is, FFH is divided into 4 parts. 00H~1FH is the working register group. 20H~2FH is the addressing space. 30H~7FH is the user RAM area. 80H~FFH is the special register area. There are 22 SFRs (not introduced here).

Let's talk about the instruction system. MOV, MOVX, MOVC are the three transfer instructions, and the off-chip memory cannot be directly transferred. The three call instructions AJMP, LJMP, SJMP are used for different call ranges. The logic arithmetic instructions ANL, ORL, XRL, as well as ADD, SUBB, MUL, and DIV. There are also many instructions such as exchange, jump, loop, bit addressing, Boolean processing instructions, etc.

For example, the following loop-jump instruction

MOV R0,#00H
MOV R1,#AAH
MOV R7#0BH
loop:  MOV A ,@R0
MOV   @R1,A
INC R0
INC R1
DJNZ  R7,loop
END

This program is used to perform the on-chip transfer function.

Let's talk about the general flow of a program. It can be divided into sequence, branching, looping, and combined nesting. The example above is a type of loop.

Write a branching program example

MOV A,R1
RL    A
MOV DPTR,#data
JMP  @A +DPTR
data:     AJMP  KK1
AJMP  KK2
.
.
AJMP KKn

This instruction is a branch transfer with data as the starting address and R1 as the offset. Because AJMP is a two-byte instruction, RLA must be executed.

Let's talk about interrupts and timing.

The registers involved in the interrupt are TCON, IE, IP, and SCOM.

The interrupt request is executed by setting the interrupt flag bit. The interrupt has 5 entry addresses: 03H 0BH 13H 1BH 23H.

For example, the entry of external interrupt 1 is set to the highest priority in default mode

SETB PX1

STEB  EX1

STEB EA

It's very simple, a bit like a level-breaking game where you have to complete every step before you can execute the interrupt.

Below is the timer, which is widely used in microcontrollers. There are 4 working modes: 0, 1, 2, and 3.

I will use the typical timer 0 mode 2 at 12MHZ to program a sine wave output XXMS at P1.0. The initial value is replaced by KKH.

MOV P1 ,#FFH
MAIN: MOV TMOD , #02H
MOV TH0 ,#KKH
MOV LHO ,#KKH
STEB ET0
STEB TRO
STEB EA
AJMP $

Interrupt procedure TIME0: CLR TF0
CPL P1.0
RETI.

There is also serial port transmission. 89C51 has 4 modes of transmission, among which the baud rate setting is particularly important.

The registers involved are TCOM TDOM SCOM SDOM PCON.

In fact, 89C51 can also use software to simulate bus interfaces, such as SPI, I2C, etc. There are too many external expansions for key program settings. I will write and post them later when I have time.