PIC16F628A Serial Communication

Serial communication is used extensively in electronics projects, and many microcontrollers come with some form of USART on board. I'm going to describe here how I got PIC USART serial communication working with a PIC16F628A, and got the PIC talking to an Arduino Uno. For this I'm using a PICKIT2, my own PIC16F628A breakout board and MicroC compiler because it has a handy serial library all built in. If you are using MPLAB or some other development tool, you can find the locations of the relevant registers in the datasheets for TXSTA and RCSTA.

The Code

The code itself is very simple - most of the setup is done using two registers - TXSTA and RCSTA. There are three main steps in setting up serial transmission on the PIC16F628A:

1. Enable serial communication
2. Set up the communication mode
3. Set the baud rate

The best source of information is (of course) the datasheet, however... to enable serial communication we set (i.e. make equal to 1) TXSTA.TXEN and RCSTA.SPEN.

We then set to 8 bit mode (there is quite a bit of information in the datasheet about 9 bit mode, but for this simple example probably not relevant) by clearing TXSTA.TX9 and RCSTA.RX9 (i.e. making equal to 0). The 16F628A can run in either synchronous or asynchronous serial modes, but here we set asynchronous by clearing TXSTA.SYNC.

void main() {

     // some standard PIC16F628A configuration
     TRISA = 0x00;   // output
     TRISB = 0xFF;   // input
     PORTA = 0x00;   // set PORTA to off
     CMCON = 0x07;   // turn off comparators by setting the last three bits to 111

     // Step 1, enable USART
     TXSTA.TXEN = 1; // transmission enabled
     RCSTA.SPEN = 1; // enable serial port

     // Step 2, Set 8 bit, asynchronous continuous mode
     TXSTA.TX9 = 0;  // 8 bit transmission
     RCSTA.RX9 = 0;  // 8 bit reception
     TXSTA.SYNC = 0; // asynchronous mode
     RCSTA.CREN = 1; // enable continuous receive mode

     // Step 3, Set the baud rate using the built in baud rate generator
     TXSTA.BRGH = 1; // set baud rate generator high
     SPBRG = 0b00011001; // set the baud rate to 9600kb, Asynchronous mode BTGH=1

     // initialise the library and wait for it to start
     UART1_Init(9600);
     Delay_ms(100);

     // ok, now some dummy code... (loop forever and ever and ever and ...)
     // just send some information every second
     do
     {
         UART1_Write_Text("Sending some text");
         Delay_ms(1000); // wait one second
     } while(1);
}

I then built a very basic Arduino sketch, from memory (and untested) something along the lines of:

void setup() {
        Serial.begin(9600);
}

void loop() {
        while(Serial.available() > 0) {
         Serial.write(Serial.read());
        }
}

What happens?

Wire up the RX and TX ports from the PIC16F628A to the TX and RX ports of the Arduino. Compile the sketch, program the PIC and then open up the Arduino serial monitor. Every second or so you should see a message appear on the serial monitor... Sending some text. There you go - your first serial communication!