I have been asked by someone to help with a problem they're having 
with the uChip example code in this app note, which I think I've 
sorted.

Anyhow, I'm really puzzled by their macros which seem back to front to 
me.

Here is the macro for  8*8 unsigned multiply

MULT MACRO BIT
    
    btfsc   no_lsb,bit  ; OK no problem here.
    addwf  result_msb,f ; here neither.
    rrf result_msb,f  ; this appears to be dividing to me.
    rrf result_lsb,f
    ENDM

The divide routine is longer but uses rlf.

Now as carry isn't used to bring the result of rrf MSB back to the top 
bit of the byte, I fail to see how this is a multiply.

Going on further the  coding of the interrupt routine seems somewhat 
farinacious -  it has goto's that branch to goto's that call a 
routine, that branch back into the interrupt routine, very hard to 
follow. I assume they've done this for speed purposes.

Example inside the interrupt.

  BANKSEL PIR1
  btfsc INTCON,RBIF
  goto CHECK_FAULT ; this is outside the interrupt routine
  btfsc PIR1, TMR1IF
  goto TIMER1_OVERFLOW ; as is this
   <snip>

  <snip>
  RETFIE

 CHECK_FAULT
 
    <snip>
   goto NO_ACTION ; this is in this block of code
   call RUN_MOTOR_AGAIN
  goto POP_UP ; this  takes us back into the interrupt routine

Now, RUN_MOTOR_AGAIN itself has a couple of goto's and a call.


Or am I just having an off day? 

Colin
--
cdb,  on 14/04/2009






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