Hello all,

I seem to have a problem with the 10-bit ADC on the 16F877. As
you know, the ADC uses two 8-bit registers to store the result:
ADRESH and ADRESL. In the ADC config you can choose to have the
converted values left-justified or right-justified, as only 10
bits out of these 16 are needed:

Left-justified:

ADRESH           ADRESL
xxxxxxxx         xx000000

Right-justified:

ADRESH           ADRESL
000000xx         xxxxxxxx

(where xxxxxxxx is the actual value)

Now the problem we have is that regardless of the configuration
I try, I always read a zero in ADRESL, so I have either two valid
bits or eight valid bits, but never ten bits.

I've searched the archive in www.piclist.com but all ADC code I
can find always uses 8-bit results only, configuring the ADC to
be left-justified and neglecting the ADRESL register.

Has anyone found this problem before? Any hints?

Code follows:

=== Cut ===

Startup
     bsf     STATUS, RP0         ; Bank1
     movlw   b'11111111'         ; PORT A = all inputs
     movwf   TRISA
     clrf    TRISC               ; PORT C = all outputs

     bcf     STATUS, RP0         ; Bank0
     movlw   b'01000001'         ; Fosc/8, A/D enabled
     movwf   ADCON0

     bsf     STATUS, RP0         ; Bank1
     movlw   b'00001110'         ; Left justify, 1 analog channel
     movwf   ADCON1              ; VDD and VSS references

     movlw   b'10000111'         ; TMR0, 1:256
     movwf   OPTION_REG

     bcf     STATUS, RP0         ; go back to Bank0

Main
     ;
     ; acquisition time before starting the AD conversion
     ;
     clrf    TMR0                ; ensures that we start a complete T0
cycle here
     bcf     INTCON,T0IF
_T0Wait
     btfss   INTCON,T0IF         ; wait for Timer0 to timeout
     goto    _T0Wait
     bcf     INTCON,T0IF

     ;
     ; starts the AD conversion
     ;
     bsf     ADCON0,GO           ; start A/D conversion
_waitAdc
     btfss   PIR1, ADIF          ; conversion complete?
     goto    _waitAdc            ; = NO (wait)

     bcf     PIR1, ADIF          ; clear conversion complete flag

     movf    ADRESH, W           ; get the 8 MSBs
     movwf   adc_high
     bsf     STATUS, RP0         ; Bank1
     movf    ADRESL, W           ; get the 2 MSBs
     movwf   adc_low
     bcf     STATUS, RP0         ; Bank0

     goto    Main                ; Loop forever
=== Cut ===


Thanks,
G.

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