Dwayne:
I'm about to take off for the day but wanted to generate this quick reply.
I'll study how to implement your boundary check when I return to work on
Monday. Thanks for the reply.

<snip>
How are you calling or instantiating the routine?  Is it inline assembly or
is it a subroutine?  What will the compiler do when you change PCLATH?
<snip>

My program is primarily written in C. I'm using Hi-Tech Compiler -
PIC16F877. The call is done via the following method:

#asm
    // request 100usec delay
    movlw   0x19
    movwf   _delayU
    clrf    _delayL
    call    Delay10bit
#endasm
Using MPLAB's Stop Watch, I calculate 123usec's on a call requesting
100usec.

I implemented your code as follows (just dropped it into my module outside
of any subroutines):

unsigned char delayU, delayL;   // C code
#asm
Delay10bit              // delay is 10 bit value + 7 cycles ( + call +
return, if any)
    incf    _delayU,f   // correct for dec & test instead of test & dec
Dly10bLoop:
    comf    _delayL,w   // invert LSBs
    decfsz  _delayU,f
    goto    Dly10bLoop  // coarse delay to closest 4 cycles
    andlw   3

    // this additional code checks for the
    // page boundary and sets pclath correctly
    // ****
    movwf   _delayL
    movlw   low offset
    addwf   _delayL
    movlw   high offset
    btfsc   status,C
    addlw   1
    movwf   pclath
    movf    _delayL,w
    // ****

    addwf   pcl,f
offset:
    nop
    nop
    nop
    return
#endasm





-----Original Message-----
From: Dwayne Reid [mailto:dwayner@PLANET.EON.NET]
Sent: Friday, September 15, 2000 10:12 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: FW: [PIC]: usec delay routine


A-----Original Message-----
>From: Sam Linder [mailto:SamL@IN-INC.COM]
>Sent: Wednesday, September 13, 2000 11:40 AM
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [PIC]: usec delay routine
>
>Dwayne:
>I tried implementing your delay routine but ran into addressing problems.
>Whenever the code gets to the following line:
>              addwf       PCL,F
>it vectors to the wrong address.
>
>When I step through the code, I notice that PCLATH is usually set to 0x07
or
>0x0F (depending upon where the routine is linked in by my compiler), yet my
>code's PC is an address like 0x3A4 (for example). When I add the contents
of
>the w register (for example 0x03) to PCL, my code vectors to 0xFA8 instead
>of 0x3A8. I know it's because PCLATH is set to 0x0F right before I execute
>the addwf  PCL,F instruction.

Sorry for taking so long to reply, Sam.  You are going to have to set
PCLATH before you call the routine.

How are you calling or instantiating the routine?  Is it inline assembly or
is it a subroutine?  What will the compiler do when you change PCLATH?

If you are dropping the code inline (as a macro or whatever), you can the
extra lines I've shown in front of the routine:

;10 bit delay routine providing 1 cycle resolution.  Works with both 12 &
14 bit core PICs.
;Original idea from Mike Harrison.  This version by Dwayne Reid
;
;enters with upper 8 bits of delay in DELAYU, lower 2 bits in bits 1,0 of
DELAYL
;bits 7..2 in DELAYL can be used as flags for whatever purpose desired.
;returns with W destroyed
Delay10bit      ;delay is 10 bit value + 7 cycles ( + call + return, if any)

    if (high ($+7)) != (high ($+.10)) ;make sure jump does not span a
boundary
      goto        (high ($+.10))
      org         (high ($+.10))
    endif

      movlw       high ($+7)
      movwf       PCLATH

;original routine starts here

      incf        DELAYU,F        ;correct for dec & test instead of test &
dec
Dly10bLoop
      comf        DELAYL,W        ;invert LSBs
      decfsz      DELAYU,F
        goto      Dly10bLoop      ;coarse delay to closest 4 cycles
      andlw       b'00000011'
      addwf       PCL,F
      nop
      nop
      nop


Do note that this might upset the compiler because it may assume that
PCLATH is set the way it left it.

This is normally not a problem for me - I write in assembler and my macros
take care of the housekeeping.

dwayne



Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 16 years of Engineering Innovation (1984 - 2000)

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