Peter Moreton wrote:
> Hi,
>
> I'm using the Microchip C18 compiler, and a PIC18F452 to relay RS232
> serial data from a host computer to a serially controlled CCTV camera.
> I'm using the hardware USART on RC6/RC7 to communicate with the PC, and
> this works fine.
>
> Now I'm trying to use the C18 'Software UART' library functions to
> implement a software UART on RD6/RD7.
>
> Trouble is, the C18 manual says you (the user) has to define the
> following delays:
>         DelayRXHalfBitUART  as ((((2*FOSC/(4*baud))+1) / 2)- 12 cycles
>         DelayRXBitUART         ((((2*FOSC/(8*baud))+1) / 2)- 9 cycles
>         DelayTXBitUART         ((((2*FOSC/(4*baud))+1) / 2)- 14 cycles
>
> I guess these routines would need to be written in assembler to be
> accurate (?) and my assembler is pretty poor, so I'm looking for any
> kind soul who have already done this, and can send me some pointers or
> sample code.
>
> (My system clocks at 4 Mhz with 4x PLL enabled = 16 Mhz, and the comms
> is running at 4800 baud)
>
> Thanks, Peter Moreton
>

For short delays like that, you can always just use a number of nop() calls
equal to the number of cycles you need to delay.  The compiler translates
nop() directly into a nop instruction, so you're assured one cycle of delay
for each nop() call.  If you want to use a real delay loop, something like
this would work:

void DelayTXBitUART(void) {
_asm
    movlw  12/3    // number of cycles to delay / 3
delayloop:
    decfsz WREG, 1, 0    // 2 cycles on skip, one otherwise
    bra delayloop  // 2 cycles
    nop  // to make the last iteration 3 cycles instead of just 2
_endasm
}

I don't remember if WREG is defined, but I know there is a way to access it
by name.

Another method is similar to using repeated nops, but generates smaller
code:

void DelayRXHalfBitUART(void) {
_asm
    bra $+2
    bra $+2
    bra $+2
    bra $+2
    nop
_endasm
}

Each bra $+2 just branches to the next instruction, and takes two clock
cycles.  With the nop, you haev a total of 9 cycles of delay.

You might also try looking at the library source code.  There are a few
delay functions for multiples of 10, 100, 1000, and 10000.   Obviously not
suited for your applications, but a good way to see some other methods of
creating delay loops.

By the way, you have the function names mixed up.  DelayTXBitUART is the 12
cycle delay, DelayRXHalfBitUART is 9 cycles, and DelayRXBitUART is 14
cycles.  This is on page 105 of the C18 Libraries pdf file.

Hope this helps,
Daniel Imfeld

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