/dev/null

Getting serial to work with a PIC is supposed to be fairly easy, especially with the Microchip C18 libraries.  For whatever reason, however, it rarely seems as easy as it should be.  Searching around for posts with the keywords ‘usart, eusart, uart, serial, rs232′ and PIC should give you some idea of just how bizzarely difficult this can be.

Part of the problem is Microchip’s documentation.  The C18 libraries were supposed to streamline a lot of programming for the PIC… and for the most part, it did; Microchip just hasn’t done a good job of making sure to completely document it all.

Anyway, below follows a pure-C18 example of an echo server, operating on a PIC18, with a MAX232 serial converter in between the PIC and my PC.  First the code, and then some discussion.

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#include <p18cxxx.h>
#include <usart.h>
 
void main(void)
{
	char test = 't';
	TRISCbits.RC7 = 1;
	TRISCbits.RC6 = 0;
	OSCCON = 0b11100010;
	OSCTUNEbits.PLLEN = 0;
 
        while (!OSCCONbits.IOFS);
 
	baudUSART(BAUD_IDLE_CLK_LOW & 
         	  BAUD_AUTO_OFF & 
         	  BAUD_8_BIT_RATE & 
         	  BAUD_IDLE_RX_PIN_STATE_HIGH &
	          BAUD_IDLE_TX_PIN_STATE_HIGH);
 
	OpenUSART(USART_TX_INT_OFF &
          	  USART_RX_INT_OFF &
        	  USART_ASYNCH_MODE &
        	  USART_EIGHT_BIT &
        	  USART_CONT_RX &
        	  USART_ADDEN_OFF &
        	  USART_BRGH_HIGH, 207);
    while (1) {
        WriteUSART(test);
	while(!DataRdyUSART());
        test = ReadUSART();
        Nop();
    }
}

If you were to try and copy/paste this code, there is a chance that it would work, out-of-the-box.  If it doesn’t work, presuming your hardware & MAX232 circuits are correct, the most likely culprit is the OSCON configuration.

As with all forms of communication, analog or digital, timing is the key.  There are two important timing lines in the above code; first, when we configure the oscillator inside of the PIC:

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OSCCON = 0b11100010;

And when we tell the USART module how fast we are operating:

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.... USART_BRGH_HIGH, 207);

The first bit, the OSCCON register setting, is configuring my PIC’s clock to operate at 8 Mhz.  This may be different for you, depending on what model PIC you are using – make sure to check your datasheet! The second bit includes the number ’207′ – this is the number that the PIC uses to calculate how fast you expect it to operate on the RS232 line – i.e., your baud rate.

The datasheet for your PIC should have a formula to help you calculate that number, probably in a section titled something like ‘Baud Rate Generator’.  Figure out what baud rate you want to operate at, and use the appropriate number for it!

This is a very brief and not a very in-depth discussion of this topic, but I had trouble finding a decent example online to use as a starting place; I’m hoping this post serves that role for some other poor hacker fighting with the C18 docs =)

Cheers!