51 MCU Registers-3.4 MCU Serial Port Interrupt

Now let's see how to operate the serial communication interrupt. Let's review the timer interrupt mentioned earlier. When the TH0 and TL0 counts overflow, the hardware automatically sets the TF0 overflow flag to 1, notifying the CPU to request an interrupt. Serial communication is very similar. Assuming that the CPU is sending serial data, when a frame is sent, the hardware automatically sets TI to 1, notifying the CPU that a frame of data has been sent and waiting for the CPU's instructions, that is, the serial interrupt service program. In the same way, for the serial data reception interrupt, when a frame is received, the hardware automatically sets RI to 1, notifying the CPU that a frame of data has been received, and whether there are other instructions. As mentioned earlier, for interrupts, whether it is a serial data transmission interrupt or a serial reception interrupt, there are two switch controls, a main switch EA, and a sub-switch ES (for serial interrupts). Therefore, before using the serial interrupt, both switches must be turned on EA=1, ES=1. Below we will give an example for serial transmission and reception interrupts to illustrate.

Example 3-3-4: Study the UART sending interrupt. The microcontroller controls the LED0 to turn on and off. Every time the serial port sends a frame of data, it enters the interrupt service program and turns on and off another LED in the interrupt program.

According to the interrupt analysis steps mentioned above,

(1) What is the CPU busy with?

(2) What urgent matter has occurred?

(3) How to notify the CPU

(4) How does the CPU handle it?

The reference code is as follows:

#include "reg52.h"

typedef unsigned char uchar;

sbit LED0=P1^3;

sbit LED1=P1^5;

void delayUs(uchar t);

void delayMs(uchar t);

void main(void)

{

        

        SM0=0;

        SM1=1; //Uart working mode

        TMOD=0X20;

        TH1=TL1=253;//Baud=9600        

        TR1=1; //Timer starts running

        EA=1;

        ES=1;

        while(1)

        {

                LED0=~LED0;

       

            SBUF=0x38;

                delayMs(255);

                

        }

}

void uart_txd_interrupt(void) interrupt 4

{

        TI=0;

        LED1=~LED1;

        delayMs(255);

        delayMs(255);

}

void delayUs(uchar t)

{

        while(--t);

}

void delayMs(uchar t)

{

        while(--t)

        {

                 delayUs(245);

                delayUs(245);

        }

}

Example 3-3-4-1 Study the UART sending interrupt. The microcontroller controls the LED0 to turn on and off. Every time the serial port receives a frame of data, it enters the interrupt service program and turns on and off another LED in the interrupt program.

For the same reason, it is easy to write code, as shown below

#include "reg52.h"

typedef unsigned char uchar;

sbit LED0=P1^6;

sbit LED1=P1^7;

void delayUs(uchar t);

void delayMs(uchar t);

void main(void)

{

        uchar Uart_RXD_data;

        SM0=0;

        SM1=1;

        TMOD=0X20;

        TH1=TL1=253;

        TR1=1;

        REN=1;

        EA=1;

        ES=1;        

        while(1)

        {

                LED0=~LED0;

                Uart_RXD_data=SBUF;

                delayMs(245);

                delayMs(245);

        }

}

void uart_rxd_interrupt(void) interrupt 4

{

        RI=0;

        LED1=~LED1;

}

void delayUs(uchar t)

{

        while(--t);

}

void delayMs(uchar t)

{

        while(--t)

        {

                delayUs(245);

                delayUs(245);

                

        }

}