Hi,

I mentioned here a while back that I was going to make a clock, and now it's
almost finished.  It's based on a 16F690, and works great so far.

I'm just completing the alarm functionality.  The alarm needs to sound when
the current time matches the alarm time, and these are stored as:
                                ; current time:
secs        res 1               ;   seconds                                
mins        res 1               ;   minutes (BCD)
hrs         res 1               ;   hours (BCD)
                                ; alarm time:
alm_mins    res 1               ;   minutes (BCD)
alm_hrs     res 1               ;   hours (BCD)    

AM/PM for current and alarm time at stored as single bits within a flag
register:

flags       res 1           
    constant PM=0               ; time AM (0) / PM (1)
    constant STANDBY=1          ; sleep mode (1)
    constant DSP_OFF=2          ; display blanked during dimming (1)
    constant ALM_ON=3           ; alarm on (1)
    constant ALM_PM=4           ; alarm AM (0) / PM (1)
    constant ALM_PRS=5          ; debounced ALARM button press (1)
    constant FAST=6             ; fast time adjust mode (1)

Most of the code sits within (or is called by) a timer ISR running every 1
ms, which multiplexes the display and reads and responds to buttons.  Within
that timer ISR, the following routine tests for and sounds the alarm (a
piezo speaker):

alarm   ; Sound alarm if set and current time = alarm time
        btfss   flags,ALM_ON    ; skip time test if alarm is off
        goto    alm_end
        banksel mins            ; if min = alm_mins
        movf    mins,w
        xorwf   alm_mins,w
        btfss   STATUS,Z
        goto    alm_end
        movf    hrs,w           ; and hrs = alm_hrs
        xorwf   alm_hrs,w
        btfss   STATUS,Z
        goto    alm_end
        banksel TMR1H           ;   sound alarm by beeping every other
second
        btfsc   TMR1H,6         ;   TMR0<6> cycles at 1 Hz 
        goto    alm_end         ;     so if TMR0<6> = 0
        movlw   1<<nBUZZER      ;       toggle BUZZER pin
        xorwf   sPORTA,f        ;         (via shadow)
        
alm_end
        goto    display         ; copy shadow regs to output and exit ISR

Ok, so the problem with that is that I'm not checking for AM/PM.  I need to
add an "if PM = ALM_PM" to this logic, but those are bits within the "flags"
register.

I could of course achieve this with a series of btfss/c instructions.  But
whichever way I try, it looks ugly.  Frankly, C never looked better!

So finally to the point.  Does anyone know an elegant way to compare (XOR)
two bits in the same register, using midrange PIC instructions?


Thanks,

David Meiklejohn
www.gooligum.com.au




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