On Tuesday 04 February 2003 07:13 am, you wrote:
> > As far as feeding the comparator <300mV (even though
> > the data sheet seems to say that's fine), the comparator's
> > output is quite simply wrong.  :-O  300mV to the + input
> > and 0V to the - input does *not* result in a high comparator
> > output, even though the + input is at a higher voltage than
> > the - input
>
> I've had no trouble at all using the comparators at low voltages,

Very interesting.

> for example C2 as a motor status indicator. A2 is set to 60mV
> by a 100R : 8k2 division of 5V and A1 gets either 0V (motor off)
> or  110mV (motor on) back from the motor sense resistor and the
> value of cmcon,c2out is used to enable/disable the motor drive
> circuitry. The set-up for C2
>
>          bank0
>          movlw   b'00000101'  ;enable Comparator2
>          movwf   cmcon
>
> and
>
>          btfsc   cmcon,c2out  ;test comparator

Hmm, I will have to see what happens when I try using another 10K pot to=20
supply the voltage to the - input, instead of the cap voltage.  AFAIK,=20
I'm not using a RevA chip (It would be marked that way, right?)

As many years as I have been scratching my head over computers, I'm=20
fully willing to accept that I have made some kind of mistake or logic=20
error.

My setup code is as follows:

;   Initialize the comparator

              clrf        FLAG_REG
              clrf        PORTA
 ; Not sure why I do this, but the datasheet example does it             =
     =20
              movf        CMCON, W
              andlw       0xC0
              iorwf       FLAG_REG, F
; end of odd code
              movlw       0x05           ; One Comparator - Pot Wiper is=20
Comp2 +
              movwf       CMCON
              bsf         STATUS, RP0
              movlw       0x06           ; RA1 and RA2 are inputs to=20
Comp2
              movwf       TRISA
              bcf         STATUS, RP0
              call        DELAY50       ; more datasheet example oddness
              movf        CMCON, F

The interrupt routine does this:

              movlw       TMRVAL        ; Reload TMR0
              movwf       TMR0
; Test for cap V too high
              btfsc       CMCON, C2OUT      =20
               goto       too_low
; Copy comparator output to charging pin
              bcf         PORTA, 3          =20
              goto        cont_1
too_low
              bsf         PORTA, 3
              incf        ADC_TMP, F
cont_1
              incfsz      ADC_CNT, F=20
                goto       no_rollover
; Counter rollover has occured (255 readings taken)
              movfw       ADC_TMP       ; Save off the ADC reading
              movwf       ADC_VAL
              clrf        ADC_TMP
              incf        ADC_CNT            ; Only do 255 tests/sample

no_rollover
              bcf         INTCON, T0IF        ;Clear Int occured flag
              bsf         INTCON, T0IE        ;Re-enable Timer0
              goto        IntExit

****** End of code sample ********

ADC_CNT counter that is used to determine when 255 reading have taken=20
place.  ADC_TMP is the accumulator for counting times that the cap=20
voltage was lower than the pot voltage.  ADC_VAL is the last captured=20
reading that is used by the main level code.

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