I'm trying to sample an encoder every say, 20ms and get the number of =
counts it has moved, in 2's complement, so it can be added to a 24 bit =
register to hold the absolute position which will represent where it is =
in relation to its start position. (=3Dzero)
>From this I can apply limits on speed, acceleration and position, =
ultimately of a servomotor, sent  to it as a smoothed pulse train with =
direction  . (<200kHz)

I'm using a 18F452 to read a HCTL 2016 quadrature decoder.=20
I can read the output bytes anytime before overflow with:

  bcf  OE           ;enable tristate output
  bcf  SEL          ;for high byte
  nop
  movf portb,w     ;data from HTCL 2016 received on portb
  movwf d_hi       ;store
  bsf  SEL          ;for low byte
  nop
  movf portb,w     ;data from HTCL 2016 received on portb
  movwf d_low     ;store
   bsf  OE            ;disable tristate output

But  I'd like to be able to reset the counter and get the change in =
position every  20ms.
So I added this, below  the above and looped it every 20ms.

  bcf  RST          ;to reset counter
  nop
  nop
  bsf  RST
=20
The count is wrong, every second sample possibly, maybe due to the way =
it is being reset.=20
The error which shows up on an analyser but more visibly  on the LCD =
attached to the PIC that reads the position register.
When started from zero and moved +/-  a turn-ish the LCD over-reads and =
won't go back to zero when the encoder is at that physical position.
When the code is used without the RST the LCD tracks the encoder =
perfectly up to overflow.
I wonder if anyone else has tried the HCTL 20XX family  in this way and =
got it to work.=20

Thanks,

Brian Harris

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