UART - getting gibberish data from MAX232 with PIC16F877A mcu

Question

I have created a serial communication between my laptop and 16F877A mcu. I have used MAX232EPE IC as level converter also. According to my code If I send a character to the MCU, MCU will return a String related to the character.

Now, what's happening is, when I am sending a character, MCU is getting it properly, because corresponding LED is getting turned on but what MCU is return as String is complete gibberish.

    // Configuration Byte
#pragma config FOSC = HS        // Oscillator Selection bits (HS oscillator)
#pragma config WDTE = OFF       // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF      // Power-up Timer Enable bit (PWRT disabled)
#pragma config BOREN = ON       // Brown-out Reset Enable bit (BOR enabled)
#pragma config LVP = OFF         // Low-Voltage (Single-Supply) In-Circuit Serial Programming Enable bit (RB3/PGM pin has PGM function; low-voltage programming enabled)
#pragma config CPD = OFF        // Data EEPROM Memory Code Protection bit (Data EEPROM code protection off)
#pragma config WRT = OFF        // Flash Program Memory Write Enable bits (Write protection off; all program memory may be written to by EECON control)
#pragma config CP = OFF         // Flash Program Memory Code Protection bit (Code protection off)

#define _XTAL_FREQ 20000000

void main() {
  //Initialize USART with baud rate 9600
  USARTInit(9600);
  TRISB = 0;

  PORTBbits.RB0 = 0;    //pin 33-Green
  int green_led = 0;

  PORTBbits.RB2 = 0;    //pin 35 - Red
  int red_led = 0;  

  PORTBbits.RB4 = 0;    //pin 37 - Blue
  int blue_led = 0;

  PORTBbits.RB5 = 0;    //pin 38 - Yellow
  int yellow_led = 0;

  USARTWriteLine("Connected");


  while(1) {
      uint8_t n = USARTDataAvailable();

      if(n != 0) {
          char data = USARTReadData();

      if(data == 'G'){
          if(green_led == 0){
              PORTBbits.RB0 = 1;
              green_led = 1;
              USARTWriteLine("Green LED turned on");
          }else{
              PORTBbits.RB0 = 0;
              green_led = 0;
              USARTWriteLine("Green LED turned off");
          }
      }else if(data == 'R'){
          if(red_led == 0){
              PORTBbits.RB2 = 1;
              red_led = 1;
              USARTWriteLine("Red LED turned on");
          }else{
              PORTBbits.RB2 = 0;
              red_led = 0;
              USARTWriteLine("Red LED turned off");
          }
      }else if(data == 'B'){
          if(blue_led == 0){
              PORTBbits.RB4 = 1;
              blue_led = 1;
              USARTWriteLine("Blue LED turned on");
          }else{
              PORTBbits.RB4 = 0;
              blue_led = 0;
              USARTWriteLine("Blue LED turned off");
          }
      }else if(data == 'Y'){
          if(yellow_led == 0){
              PORTBbits.RB5 = 1;
              yellow_led = 1;
              USARTWriteLine("Yellow LED turned on");
          }else{
              PORTBbits.RB5 = 0;
              yellow_led = 0;
              USARTWriteLine("Yellow LED turned off");
          }
      }else{
          USARTWriteChar(13);
          USARTWriteChar(10);
          USARTWriteString("Invalid command : ");
          USARTWriteChar(data);
          USARTWriteChar(13);
          USARTWriteChar(10);
      }
          n = 0;
          USARTFlushBuffer();
      }
  }
}

When I am sending "G", I am receiving "��g����wf�}w��]w���" where I should receive "Green LED turned on", both are 19 character in length

If anyone can help me, it will be really great.

Answer 1

I didn't look at the code, but here are some possibilities:

1. Lack of bypass cap on the PIC. Until you fix this, all else is irrelevant.

2. Possibly not waiting for the UART to be ready before writing a character to it. Try sending just one character intead of a string. If that works, then this is a likely cause.

Normally I'd add baud rate to this list, but since you are receiving correctly, that's probably not it.