PIC16F628A : Using delays for PIC16F628A @ 4MHz

by Nicolae
(City)

Hi, I have a PIC16F628A set on internal clock (4MHz). I tried with an external 4MHz clock with the same result.

I use the compiler PICC-Lite from Hi_Tech. I intend to generate a 40kHz train of pulses for an ultrasonic application. Therefore I need a 25us period, which split in half gives 13us and 12us.

I tried using various methods, but each one seems to be worse than the other ones. First I tried using DelayUs(13) and Delay(12), which gives me a delay or about 18us and 13us. The code is extremely simple, all I have is:

main()
{
        TRISA = 0b00000000;
        TRISB = 0b00000000;
        PORTA = 0;
        PORTB = 0;
        PORTA = 0b00000000;
        while(1)
        {
                PORTA = 0b11111111;
                DelayUs(12); //takes 10us
                PORTA = 0b00000000;
                DelayUs(13); //takes 13us
        }
}

Now, the strange thing is about these delays:

If I set first delay to DelayUs(10), I get 5us of delay, for DelayUs(11), I get 10us, for DelayUs(12) I get 18us.

If I set the second delay to DelayUs(6 to 10), I get 13us. What is worse, the value set in one delay seems to affect both delays!

I tried using assembly i.e. asm("nop") - because I am within the while() loop. But when I have assembly in the code, the 12us delay takes about 300ms. I turned on all the optimisations I could find, but no luck.

For the delay, I got some code generated by http://www.piclist.com/cgi-bin/delay.exe

Delay
                        ;9 cycles
        goto    $+1
        goto    $+1
        goto    $+1
        goto    $+1
        nop

                        ;4 cycles (including call)
        return

Unfortunately, I've been unable to embed it into my program - if I embed it, I get a delay hundreds of times longer than I expect!
Could you please explain how to embed the above code within a C project?

Regards,
Nicolae