I needed to convert a 0to5 volt input to 16 discrete alarm temperatures.
Dividing a 0-4096 12bit A/D count by 256 yields a perfect 0-15 result.  To
do this, I simply grab the high byte from the conversion.  I then use a
16-entry lookup table to convert to literal temperature values.

This is the analog-read subroutine:

      ; Read alarm temperature 1
      movlw b'00001001' ; Configure A/D, RA1/AN1.
      movwf adcon0      ;
      call  D_20US      ; Delay 20 microseconds.
      bsf   adcon0,2    ; Start conversion.
      btfsc adcon0,2    ; Test go/done bit.
      goto  $-1
      movf  adresh,w    ; Conv complete, get high A/D result.
      movwf ALARM1_RAW  ; Write data to register.

It works except for one thing, I never get a zero for the high byte.
See results *:

Input       12 bit      Expected    Actual
Volts       Count       High Byte   High Byte
0.00        0           0           1 *
0.31        256         1           1
0.63        512         2           2
0.94        768         3           3
1.25        1024        4           4
1.56        1280        5           5
1.88        1536        6           6
2.19        1792        7           7
2.50        2048        8           8
2.81        2304        9           9
3.13        2560        10          10
3.44        2816        11          11
3.75        3072        12          12
4.06        3328        13          13
4.38        3584        14          14
4.69        3840        15          15


Any ideas on what would cause the high byte to always have a minimum value
of one?

Thanks.

Mark Peterson

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