debouncing switches with a timer interrupt?

[DDDvvv]

hi everyone,
im experimenting with the "menu builder" application on a 18f2431 and im having a bit of a problem. it inserts a lot of "pause 200" commands, to debounce the switches, which is causing havoc on my real time timer on an lcd. if i remove the pauses, there's no debounce, and one push of a button cycles through alot of menus. i tried the portb interrupt on change, and im getting the same results. i've read some rumours that debouncing can be done using a timer interrupt. is it possible? im using 4 buttons: up, down, enter, exit. any help/idea on pseudocode will really be appreciated; portb interrupt, timer interrupt, software debounce, etc

[Kamikaze47]

If you can change the hardware, one option would be to use capacitors across the switches. Software denounce would no longer be required.

Otherwise you do do this:

port b change interrupt: detects a button press and takes action, an the end of the interrupt it sets a timer so that it will overflow in say 50ms, and then disables the port b interrupt.

when the timer interrupt goes off 50ms later it simply needs to re-enable the port b interrupt and stop the timer.

[Charles Linquis]

Could it be that you don't need a debouncer at all?

If you are polling the pin that is connected the switch every program loop, and the loop is longer than the switch bounce time, you don't need a debounce, since it will see the transition, and the "bounces" will be over before you check the switch again.

[Dave]

Charles, I do just exactly that. In my interrupt routine. I copy th state of my button and add it to a shifted result. When the button is connected to ground with a pullup to VDD, the value after the 8 cycle (interrupt) debounce is equal to zero. When the button is released the value after the 8 cycle (interrupt) debounce is equal to 255. I use a 10 millisecond interrupt timer so the debounce time is 80 milliseconds. That way all the main has to do is interrogate debounce variable for it's value.

Dave Purola,
N8NTA

[aratti]

if i remove the pauses, there's no debounce, and one push of a button cycles through alot of menus.

I would apply a flag condition to control the key state.

False        CON  0
True         CON  1
KEYPRESS  VAR portXx
KeyFlag      VAR BIT
Counter      VAR WORD

LOOP:

PAUSE 1

IF KEYPRESS = False then

IF KeyFlag = False Then
Counter = Counter + 1
KeyFlag = True
ENDIF

ELSE
KeyFlag = False
ENDIF
.
.
.
.
.
GOTO LOOP

In the above snippet every time you press the key "KEYPRESS" the word variable "Counter" will increment of one unit, no matter how long you keep the key down. If you remove the flag control using pauses, it will be rather complicate to reach the same results.



Al.

[Kamikaze47]

I would apply a flag condition to control the key state.

This would still require some kind of debouncing. The logic state can "bounce" a number of times when pressing a button, so if it toggles back and forth a few times it could still advance the counter more than once for a single key press if the loop is quick enough.