8051 MCU baud rate calculation formula

The 8051 microcontroller uses timer 1 working in mode 2 as the serial port baud rate generator. Its baud rate = (2smod /32) × (timer T1 overflow rate), where smod is PCON<7>, indicating whether the baud rate is doubled, and Fsoc is the system crystal size.

In the baud rate formula: T1 overflow rate = the inverse of the overflow period; overflow period = (256-TH1) × 12/Fosc;

Final formula:

Baud rate: Baud = (2smod × Fsoc) / (32 × 12 × (256-TH1))

We generally do not pay much attention to the calculation of the baud rate, but are concerned about the selected transmission speed (baud rate) to reversely calculate the initial value (TH1) of timer 1 (auto-reload mode), so we can derive the above formula to get the formula for TH1:

TH1=256-(Fsoc×2smod)/(12×32×Baud)

Below is a sample program for the serial port sending program, Shui Han wrote it down for your reference. (I used STC12C5A40S2 for debugging, in principle it can be successful on 8051 core microcontrollers such as STC89C5x and AT89C5x. Since the program is relatively simple, I did not try it, but it should be no problem)

#include "Reg52.H"

/****************************************************** ******************

Please calculate the maximum speed that the selected crystal can reach in advance. The baud rate cannot exceed the maximum speed.

(1) Double the baud rate (SMOD=1): Max_Baud = FOSC/12/16

(2) Baud rate is not doubled (SMOD=0): Max_Baud = FOSC/12/32

For example: 22.1184MHz crystal oscillator, when the baud rate is doubled, the maximum baud rate = 22118400/12/16 = 115200

*************************************************** *******************/

#define FOSC 22118400 //Oscillator frequency

#define BAUD 9600 //Baud rate

#define SMOD 1 //Whether to double the baud rate

#if SMOD

#define TC_VAL (256-FOSC/16/12/BAUD)

#else

#define TC_VAL (256-FOSC/32/12/BAUD)

#endif

typedef unsigned char uint8;

typedef unsigned int uint16;

code const char str1[] = "Ther string is transmitted from 80C51!rn";

code const char str2[] ​​= "Author: xqlu(at)ysu.edu.cnrn";

/***************Function declaration********************/

void InitUART(void);

void SendOneByte(uint8);

void SendrStr(const uint8 *ptr);

/********************Main function************************/

void main(void)

{

uint8 i=0;

InitUART();

while(str2[i]!='')

{

SendOneByte(str2[i++]);

}

SendrStr(str1);

while(1);

}

/****************Interrupt service function****************/

void UART_ISR(void) interrupt 4

{

uint8 RX_Data;

//Only respond to the "receive" interrupt, and erase it directly when the "send" interrupt comes

if(RI)

{

RI = 0; //The serial port interrupt flag cannot be cleared by itself and needs to be cleared manually

RX_Data=SBUF;

SendOneByte(RX_Data);

}

else

TI = 0; //Serial port interrupt is generated after the buffer data is sent

}

/****************Serial port initialization function*************/

void InitUART(void)

{

TMOD = 0x20;

SCON = 0x50;

TH1 = TC_VAL;

TL1 = TH1;

PCON = 0x80; // double the transmission rate

ES = 1;

EA = 1;

TR1 = 1;

}

/**************Serial port sending character function*************/

void SendOneByte(uint8 c)

{

ES = 0; //Disable sending interrupt

SBUF = c;

while(!TI);

TI = 0;

ES = 1;

}

/**************Serial port sending string function*************/

void SendrStr(const uint8 *ptr)

{

do

{

SendOneByte(*ptr);

}while(*ptr++!='');

}