PICers,

There was a thread a while back about how to use a PIC port to test a
battery voltage. the best idea was to connect the battery to  a port
pin thru a resistor & capacitor, make the pin briefly an o/p to
discharge the battery, then make the pin an i/p and count how long it
takes for the voltage to reach logic 1. Too slow = low volts.

I'd probably go about it like so.

First of all  ... do you reckon it'd work? (Haven't tested it so far.)

Secondly, in the btfss loop, if the RA0 voltage never makes it to
logic 1, say if the
pin is disconnected for some unknown reason, you get an infinite loop.
That's a Bad Thing.

Any thoughts, ideas etc?

Ian

;*******************************************************************
;TSTBAT .. battery test routine. Looks at TERMINAL pin on PORTA
; and counts the time needed to charge a cap from the battery supply.
;If the count exceeds MAX, the battey is taking too long and it's
;probably kaput. Flag battery good = 1 so as to skip on true.
;
;Definitions needed :-
;MAX      equ ???         .. to be determined
;MIN      equ ???         .. to be determined
;TERMINAL equ PORTA,0
;BATTERY  equ flags,????
;
;Registers needed :-
;count
;flags
;CCT Configuration :-
;                         | to PORTA
;  Battery Vcc ----^^^^^----||----GND
;********************************************************************
tstbat  bsf   flags,BATTERY        ;BATTERY.0 => battery volts LOW
                                   ;BATTERY.1 => battery volts OK
        BANK_1
        bcf    TRISA,TERMINAL      ;PORTA.0 => OUTPUT
        BANK_0
        bcf    PORTA,TERMINAL      ;PORTA.0 - LOW, Discharge the
capacitor
        movlw  MAX
        movwf  count
        decfsz count,1             ;Make sure the capacitor is
discharged
         goto  $-1
        BANK_1
br1     bsf    TRISA,TERMINAL      ;Then make PORTA.0 an INPUT
        BANK_0
        movlw  MIN                 ;Load the starting value of the
count
        movwf  count
loop    incf   count,1
        btfss  PORTA,TERMINAL      ;Voltage risen to logic 1 yet?
         goto  loop                ;No  - loop'n'count till it does
                                   ;Yes - see if the count has
exceeded the MAX allowable.
                                   ;If it has, the battery's taking
too long to charge the cap.
        movf   count,w             ;Is count >= MAX? tony nixon's >=
test from PIC Archive.
        addlw  255-MAX+1           ;If MAX >= 5 ... addlw d'251'
        btfsc  STATUS,C            ;Use btfss & get rid of the GOTO?
         goto  exit
br2     bcf    flags,BATTERY       ;Flag battery = "bad"
exit    retlw  0x00

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