debouncing switches with a timer interrupt?

[Mike, K8LH]

Consider using a switch state latch and relatively simple logic to sample your switches and detect the "new press" state while ignoring the other switch states. Here are the switch states;

swnew swold  switch state
  1     0    a "new press"
  1     1    still pressed
  0     1    a "new release"
  0     0    still released

Sample the switches at a decent debounce interval (16 to 25 msecs) either in your main program loop or in a timer based interrupt service routine. You only need a few lines of code to implement the switch state logic for multiple switches (perhaps one of the PBP gurus can interpret the C code? Sorry!);

//  swnew  ____---____-----_____   new switch sample
//  swold  _____---____-----____   switch state latch
//  delta  ____-__-___-____-____   changes, press or release
//  newhi  ____-______-_________   filter out release bits
//

while(1)                  //
{                         //
  delay_ms(24)            // 24-msec debounce interval
  swnew = ~porta;         // sample active-lo switches
  swnew &= 0x0F;          // on RA3..RA0 pins
  swnew ^= swold;         // changes, press or release
  swold ^= swnew;         // update switch state latch
  swnew &= swold;         // filter out "new release" bits
  if(swnew)               // if any "new press" bits
  { beep();               // task a "new press" beep
  }
  
  if(swnew.0)             // if RB0 "new press"
  {                       //
  }                       //

  if(swnew.1)             // if RB1 "new press"
  {                       //
  }                       //
}

If you implement the method in an ISR you'll need to inclusive-or "swnew" with a "flags" register for your main program.

Kind regards, Mike