debouncing switches with a timer interrupt?

[Mike, K8LH]

When you exclusive-or "new" and "old" a resulting 1 bit indicates a change of state (either a new hi or new press, or a new lo or new release). You need to filter these "change" bits. If you AND the "change" bit with the "old" bit you filter out the "new press" bits and you're left with "new release" bits. If you AND the "change" bit with the "new" bit you filter out the "new release" bits and you're left with the "new press" bits.

Hope this helps.

Cheerful regards, Mike

'
'  swnew  ____-----_____-----_____   new switch sample (positive logic)
'  swold  _____-----_____-----____   switch state latch
'  delta  ____-____-____-____-____   changes, press or release
'  newhi  ____-_________-_________   new press bits
'  newlo  _________-_________-____   new release bits
'
    swnew = PortB                  ' sample active hi switches
    delta = swnew ^ swold          ' changes, press or release
    newhi = delta & swnew          ' get "new press" bits
    newlo = delta & swold          ' get "new release" bits
    swold = swnew                  ' update switch state latch
    IF newhi.4 = 1 THEN            ' 
      BTN_PLUS = 1                 '
    ENDIF

[DDDvvv]

that is just great. thanks for pointing me to the right direction, Mr Mike. its all fixed now.

cheers

[Mike, K8LH]

that is just great. thanks for pointing me to the right direction, Mr Mike. its all fixed now.

cheers

You're very welcome Sir.

Please note that the previous example can be optimized a bit. Since you're using the "new press" state as the action state in your program you don't need the "newlo" variable and its assignment statement. You can also retask the "swnew" variable and eliminate the "newhi" variable. Some XOR trickery is used to update the "swold" switch state latch to the new PortB pattern.

'
'  swnew  ____-----_____-----_____   new switch sample (positive logic)
'  swold  _____-----_____-----____   switch state latch
'  swnew  ____-____-____-____-____   changes, press or release
'  swnew  ____-_________-_________   new press bits
'
    swnew = PortB                  ' sample active hi switches
    swnew = swnew ^ swold          ' changes, press or release
    swold = swold ^ swnew          ' update switch state latch (swold = new PortB)
    swnew = swnew & swold          ' get "new press" bits
    IF swnew.4 = 1 THEN            ' if RB4 "new press"
      BTN_PLUS = 1                 ' set flag for main
    ENDIF

With your 1-msec interrupt intervals you might consider adding a piezo speaker and code for "new press" beeps;

'
'  beep task (32-msec 500-Hz beep using 1-msec interrupts)
'
    IF beepctr > 0 THEN            ' if beep task running
      PortB = PortB ^ spkrpinmask  ' toggle piezo speaker pin
      beepctr = beepctr - 1        ' decrement beep msec counter
    ENDIF
'
'  swnew  ____-----_____-----_____   new switch sample (positive logic)
'  swold  _____-----_____-----____   switch state latch
'  swnew  ____-____-____-____-____   changes, press or release
'  swnew  ____-_________-_________   new press bits
'
    swnew = PortB                  ' sample active hi switches
    swnew = swnew ^ swold          ' changes, press or release
    swold = swold ^ swnew          ' update switch state latch (swold = new PortB)
    swnew = swnew & swold          ' get "new press" bits
    IF swnew.4 = 1 THEN            ' if RB4 "new press"
      BTN_PLUS = 1                 ' set flag for main and
      beepctr = 32                 ' task a new press beep
    ENDIF

If you experience bounce problems with a 1-msec sampling interval you might consider adding a counter and executing the switch sample code at some interval between 16 and 32 msecs.

BTW, assembly language "parallel switch state logic" is incredibly efficient (6 or 8 words);

;
;  wreg  ___-----_____-----____   new switch sample (positive logic)
; swold  ____-----_____-----___   switch state latch
;  wreg  ___-____-____-____-___   changes, press or release
;  wreg  ___-_________-________   filter out new release bits
; flags  ___-----------________   toggle sw flags for main
;
        comf    PORTB,W         ; sample active lo switches
;       movf    PORTB,W         ; sample active hi switches
        andlw   b'11110000'     ; on RB7..RB4 pins
        xorwf   swold,W         ; changes, press or release
        xorwf   swold,F         ; update switch state latch
        andwf   swold,W         ; filter out "new release" bits
        skpz                    ; new press? no, skip, else
        bsf     beepctr,5       ; task a "new press" beep
        xorwf   flags,F         ; toggle sw flags for main

Cheerful regards, Mike