Wednesday, September 6, 2017

Software UART for PIC16F84A microcontroller

The microcontroller PIC16F84A has no hardware UART module, but we can use software UART to send/receive data to/from the PC via RS232 cable (COM port). This topic shows a simple example for the use of the UART protocol with the PIC16F84A MCU.
Hardware Required:
  • PIC16F84A microcontroller
  • 8MHz crystal
  • 2 x 22pF ceramic capacitors 
  • 10K ohm resistor
  • MAX232 -- datasheet
  • 4 x 10uF polarized capacitor
  • Female RS232 connector
  • Breadboard
  • 5V power source
  • Jumper wires
UART Example for PIC16F84A microcontroller circuit:
Software UART RS232 for PIC16F84A circuit
To interface the PIC16F84A MCU with the PC we need MAX232 signal level converter as shown in the circuit diagram. There are 2 lines between the PIC16F84A and the MAX232 and also two lines between the MAX232 and the female COM connector. One line for data transmit and the other one for data receive.
The female COM connector is connected to the PC using RS232 cable, this cable has to be male-female because the PC RS232 port type is male.
In this example the PIC16F84A MCU runs with 8MHz crystal oscillator.
UART Example for PIC16F84A microcontroller C code:
The function #use rs232(xmit = PIN_B4, rcv = PIN_B5, baud = 2400) is used to configure the UART protocol. Here software UART is used.
xmit is the transmit pin (RB4)
rcv is the receive pin (RB5)
2400 is the baud rate.
The functions used in the C code are:
printf: sends a string of characters over RS232 transmission pin (TX).
putc: send a character over RS232 transmission pin (TX).
getc(): read character if available.
Complete C code for CCS C compiler is below.
// Software UART example for PIC16F84A.

#include <16F84A.h>
#use delay(clock = 8000000)
#use fast_io(B)
#use rs232(xmit = PIN_B4, rcv = PIN_B5, baud = 2400)

const char message[] = "***** PIC16F84A microcontroller UART example *****" ;
int8 i = 0, j;
void main(){
  delay_ms(2000);                                // Wait 2 seconds
  // Print text
  printf("Hello world!");
  // Print list of characters
  printf("\n\r");                                // Start new line
  while(message[i] != '\0'){                     // Loop until the end of the string
    putc(message[i]);                            // Write character
    delay_ms(100);                               // Wait 100 ms
    i++;                                         // Increment i
  // Print numbers
  printf("\n\r");                                // Start new line
  for(i = 0; i < 21; i++){
    printf("%u\n\r", i);                         // Print i and start new line
  // Receive and send character via UART
    j = getc();                                // UART read character
    putc(j);                                   // Send it back