Key UART Communication Registers

Two primary registers manage serial communication: the Power Control Register (PCON) and the Serial Control Registre (SCON).

PCON Register

SMOD (bit 7): Baud rate doubling control for modes 1, 2, 3. 1 = double rate, 0 = default rate.
SMOD0 (bit 6): Frame error detection control. 1 = SCON.SM0 used for error checking; 0 = SM0/SM1 for mode selection.

SCON Register

SM0/SM1 (bits 7/6): Mode selectors when SMOD0 = 0:
  00 = Mode 0 (Shift register mode)
  01 = Mode 1 (8-bit async, variable baud)
  10 = Mode 2 (9-bit async, fixed baud)
  11 = Mode 3 (9-bit async, variable baud)
SM2 (bit 5): Multi-processor control enable for modes 2, 3
REN (bit 4): Receive enable. 1 = allow receive, 0 = disable receive.
TB8/RB8 (bits 3/2): 9th data/parity bits (unused in basic scenarios)
TI (bit 1): Transmit interrupt request flag (auto-set after send completion)
RI (bit 0): Receive interrupt request flag (auto-set after receive completion)

Baud Rate Calculations

• Mode 0: Fosc / 12 (fixed, e.g., 11.0592MHz → 921,600 bps)
• Mode 1/3: (2^SMOD / 32) × Timer 1 overflow rate
• Mode 2: (2^SMOD / 64) × Fosc

Timer 1 overflow rate:

• 12T (normal mode): Fosc / 12 / (256 - TH1)
• 6T (fast mode): Fosc / 6 / (256 - TH1)

UART Enitialization Code (9600 bps @ 11.0592MHz)

#include <REGX52.H>
#define FOSC_110592 1

void UART_Init(void) {
    PCON &= 0x7F;         // Disable baud rate doubling
    SCON = 0x50;          // Mode 1 (8-bit, variable), receive enabled
    AUXR &= 0xBF;         // Timer 1 clock = Fosc/12 (12T)
    AUXR &= 0xFE;         // UART1 uses Timer 1 as baud generator
    TMOD &= 0x0F;         // Clear Timer 1 mode bits
    TMOD |= 0x20;         // Timer 1: 8-bit auto-reload
    TL1 = 0xFD;           // Timer initial value
    TH1 = 0xFD;           // Auto-reload value
    ET1 = 0;              // Disable Timer 1 interrupt
    TR1 = 1;              // Start Timer 1
}

Transmit a String to a PC

#include <REGX52.H>
#include "intrins.h"

sfr AUXR = 0x8E;

void Delay_1s(void) {
    unsigned char i, j, k;
    _nop_();
    i = 8;
    j = 1;
    k = 243;
    do { do { while(--k); } while(--j); } while(--i);
}

void UART_Init(void) {
    PCON &= 0x7F;
    SCON = 0x40;
    AUXR = 0x01;
    TMOD &= 0x0F;
    TMOD |= 0x20;
    TL1 = 0xFD;
    TH1 = 0xFD;
    TR1 = 1;
}

void SendByte(char dataByte) {
    SBUF = dataByte;
    while(!TI);  // Wait for transmit completion
    TI = 0;     // Clear interrupt flag
}

void SendString(char *str) {
    while(*str != '\0') {
        SendByte(*str++);
    }
}

void main(void) {
    UART_Init();
    while(1) {
        SendString("helloworld");
        Delay_1s();
    }
}

Control an LED via PC Commmand

#include <REGX52.H>
#include "intrins.h"

sfr AUXR = 0x8E;
sbit LED1 = P3^7;

void Delay_300ms(void) {
    unsigned char i, j, k;
    _nop_();
    i = 3;
    j = 26;
    k = 223;
    do { do { while(--k); } while(--j); } while(--i);
}

void UART_Init(void) {
    PCON &= 0x7F;
    SCON = 0x40;
    REN = 1;
    AUXR = 0x01;
    TMOD &= 0x0F;
    TMOD |= 0x20;
    TL1 = 0xFD;
    TH1 = 0xFD;
    TR1 = 1;
}

void main(void) {
    char cmd;
    LED1 = 1;
    UART_Init();
    while(1) {
        Delay_300ms();
        if(RI == 1) {
            RI = 0;
            cmd = SBUF;
            if(cmd == 'o') LED1 = 0;
            if(cmd == 'c') LED1 = 1;
        }
    }
}