Using Printf() for serial port output in Keil C51

How to use printf to display information on the serial port? There are two ways. Because in keil C51, the printf function calls putchar to transfer data, and putchar should first determine whether TI is 1. If it is not 1, it waits for it to be 1. If it is 1, it clears it to 0 and then sends a character. Therefore, if you use the printf function directly, your program will wait for TI to be 1 in the putchar function. At this time, your program is equivalent to being dead. You can achieve your own purpose by rewriting the putchar function. TI is equivalent to being initialized and will not work if it is not assigned an initial value.

So the first way is to assign TI a value of 1 during initialization, for example

void uart_init()

{

PCON &= 0x7F; //Baud rate is not doubled

SCON = 0x50; //8-bit data, variable baud rate

AUXR |= 0x40; //Timer 1 clock is Fosc, i.e. 1T

AUXR &= 0xFE; //Serial port 1 selects timer 1 as the baud rate generator

TMOD &= 0x0F; // Clear timer 1 mode bit

TMOD |= 0x20; //Set timer 1 to 8-bit auto-reload mode

TL1 = 0xDC; //Set the initial timing value

TH1 = 0xDC; 

ET1 = 0; //Disable timer 1 interrupt

TR1 = 1; //Start timer 1    

    TI=1; //Send interrupt flag position 1

}

But sometimes, when using UART interrupt, the serial data transmission interrupt flag TI cannot be set to 1 for a long time. At this time, we need to rewrite the putchar function to redirect the output of the printf function, and then use printf to display it. For details, please refer to C: Keil/C51/LIB/PUTCHAR.C, the following is an example:

char putchar (char c) {

  while (!TI);

  TI = 0;

  return (SBUF = c);

}