I'm having a problem with the A/D convertor in the chip.  It seems to
always return 0x13 no matter if the input voltage is 5V or 0V.
I've used interrupt driven and polling, both return the same result.
I'm reading the results from PORTB and PORTD with a multimeter (limited
parts availability in the lab, ie no leds).

I'm using the internal RC to drive the '877 (assumed 4MHz).  I've tried
values of 00, 10, 11 for the ADSC1:ADSC0 with no difference in the output.

I've used MPLAB (v 5.xx) and GPSIM to simulate the '877.  GPSIM gives
odd results (0x0266 if AN0 is High, and 0x019A if Low).  MPLAB just
shows 0 for ADRESL and ADRESH for both cases.


Here is the offending code:


; Set TRISA for analog input.
        bsf     STATUS, RP0             ; switch to bank1
        movlw   0x3f                    ; Set bits 5-0 as 1
        movwf   TRISA                   ; Sets PORTA<5:0> as inputs.
:

; Set up the A/D convertor.
        bcf     STATUS, RP0
        bcf     STATUS, RP1             ; Switch to bank0.
        clrf    ADCON0                  ; Clear ADCON0.
                                        ; Selects channel 0 (AN0)
                                        ; Turns the A/D off.
                                        ; Clears GO flag.
        bsf     ADCON0,ADCS1            ; Set ADSC1 and ADSC0 since we
                                        ; are going to
        bsf     ADCON0,ADCS0            ; Use A/D's internal RC for its
                                        ; clock source.

        bsf     STATUS, RP0
        bcf     STATUS, RP1             ; Switch to bank1.
        clrf    ADCON1                  ; Clear ADCON1
        bsf     ADCON1, ADFM            ; Right justify 10 bits in
                                        ; ADRESH and ADRESL
        bsf     ADCON1, PCFG1           ; ADCON1<3:0> = 0010
                                        ; Set AN7-AN5 as Digital.
                                        ; Set AN4-AN0 as Analog inputs.
                                        ; Use AVDD/AVSS (internal) as
                                        ; reference voltages.

; Do a test A/D conversion.
        bcf     ADCON0, CHS0            ; Select Channel 0 (AN0).
        bcf     ADCON0, CHS1            ;
        bcf     ADCON0, CHS2            ;
        bsf     ADCON0, ADON            ; Turn A/D on.


        movlw   d'31'                   ; 23 us/0.25 us = 92 total
                                        ; clocks
                                        ; since loop has 3
                                        ; instructions... 92/3 = 30.67
                                        ; -> 31
                                        ; 23 us is worst case, 10k ohm
                                        ; impedance and 100 deg C.
                                        ; 1/4MHz = 0.25 us
        bcf     STATUS, Z               ; Make sure Zero flag is off.
addaq1
        addlw   -1                      ; W=W-1
        btfss   STATUS, Z               ; Is W = 0?
        goto    addaq1                  ; NO, loop again.

        bsf     ADCON0, GO              ; Begin A/D conversion.
loop    btfsc   ADCON0, GO
        goto loop

        bcf     STATUS, RP0
        bcf     STATUS, RP1             ; switch to bank0 to access
                                        ; ADRESH.
        movf    ADRESH, W
        movwf   adsaveh                 ; copy upper portion of A/D
                                        ; result.
        bsf     STATUS, RP0             ; switch to bank1 to access
                                        ; ADRESL.
        movf    ADRESL, W
        movwf   adsavel                 ; copy lower potion of A/D
                                        ; result.

        bcf     STATUS, RP0
        bcf     STATUS, RP1             ; switch to bank0 to access
                                        ; PORTB.
        movf    adsavel, W
        movwf   PORTB
        movf    adsaveh, W
        movwf   PORTD

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