Timer for switch debouncing

[ansh11]

Hi
I usually use software delay for switch debouncing. I just add a 50ms delay after the first bounce. I want to use hardware timer in place of software delay. Assume hardware timer start and stop at every 1ms

What I want is, Whenever I press switch, pin of PIC microcontroller should be on/off for 30ms.

This is a only question that has nothing to do with the actual project.But I want to know how it can be done.

 

[barry]

Not quite sure what you’re asking. There are timers in a PIC that you can trigger from an external pin, and then use the timer output as your 30 ms pulse.

 

[KlausST]

Hi,

try this:

set up a timer for generating in interrupt every 10ms

ISR: (pseudo code)
{
   static uint8_t last_key_state = released;
   static uint8_t debounced_key = released;  // this may be a public variable if used else where
   uint8_t key = read_key_state();

   if (if key == pressed && last_key_state == pressed && debounced_key == released)  // checks on a debounced ON_edge
   {
      debounced_key = pressed;    // now debounced key = ON
      out_pin = 3;  // sets counter for 3 x 10ms timeout
   }

   if (last_key_state  == released && key == released)   // checks if key is released
   {
      debounced_key = released;   // now it is debounced released
   }

   last_key_state = key;   // just to memorize state for next run
   if (out_pin > 0)   // checks whether output should be ON
   {
      pin = ON;  // set pin ON
      out_pin--;  // decrement counter
   }
   else  // pin should be OFF
   {
      pin = OFF;  // set pin OFF
   }
}

Klaus

 

[danadakk]

As an aside I worked with some cheap switches that had 300 mS of bounce time,
pretty ugly.

Attached is a collection of various techniques, may be useful.


Regards, Dana.

 

[pjmelect]

The normal way if not denouncing in software is to use a monostable, you can set the time from the last bounce of the switch.

 

[danadakk]

There are also cpu families these days with HW debouncing -

This is a drag and drop component, a component being an onchip resource. Attached a summary
of onchip resources.


Regards, Dana.