Electronic – find TH1 in AT89s52 micro controller in serial communication

8051baudratec

I want to establish serial communication between an 8-bit microcontroller, AT89S52, and a PC. I am using a crystal oscillator of 8 MHz frequency. For baud rate calculation I have used the following formula:

 Fbaud = (2^SMOD * Fosc)/(12*32*(256-TH1))

In my case

 SMOD = 1;
 Fbaud  =9600; // baud rate
 Fosc = 8MHz   // oscillator frequency

So using formula TH1 = 251.659722222

I have used TH1 = 251 i.e. 0xFB and also 252 i.e. 0xFC. In both cases I am getting garbage values on Hyper terminal.

I have used following code::

#include <REGX52.h>
#include <stdio.h>
void serial_ini();
void serial_ini()
{ 
    SCON = 0x50; // mode 1 , 8 bit UART, enanle receiver
    TMOD = 0x20; // timer 1 mode 2(auto reload mode)
    TH1 = 0xFB ; // baud rate 9600 // crystal = 8.000 mhz..
    TR1 = 1; // Timer1 run
    TI = 1; 
}

void main()
{   
    serial_ini();
        while(1)
        {
            SBUF = 0x41;
            while(TI==0);
            TI=0;
        }
}

Please suggest to me how to calculate TH1 for AT89S52 microcontroller.

Best Answer

All of the following information is available in Atmel 8051 Microcontrollers Hardware Manual which is referenced in Section 9 of the AT89S52 datasheet as the place to look for information on Timers 0 and 1.

Note: I'm assuming what you have in the comments is what you are trying to do.

First off, your formula is wrong. If you're trying to use Mode 1, SMOD doesn't enter into it. Even so, SMOD is in PCON which you are not setting (which MikeJ-UK mentioned).

The baud rate in Mode 0 is fixed:

The baud rate in Mode 2 depends on the value of bit SMOD in Special Function Register PCON.

If SMOD = 0 (which is its value on reset), the baud rate is 1/64 the oscillator frequency. If SMOD = 1, the baud rate is 1/32 the oscillator frequency.

In the 80C51, the baud rates in Modes 1 and 3 are determined by the Timer 1 overflow rate. In case of Timer2, these baud rates can be determined by Timer 1, or by Timer 2, or by both (one for transmit and the other for receive).

Emphasis mine.

Next, your SCON does not match the comment:

SCON Register

You have:

SCON = 0x50

Which equates to:

 FE SM0 SM1 SM2 REN TB8 RB8 TI  RI
 0   1   0   1   0   0   0   0   0

So you're in Mode 2, 9-bit UART with the baud rate given as:

FCPU PERIPH /32 or /16

And serial reception disabled.

And then you've got:

TI = 1

Which you shouldn't do. You clear the interrupt flag. It's set by hardware.

Figure out exactly what you want to achieve and have a good look through those manuals. You'll probably need to rework your formula, change SCON and then set TH1 and TL1.

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