Hasan,

Maybe you could change from the F84 to the F628.  Then you could use the H/W 
PWM.
If you are using a H/W PWM, the CCP unit will retain it's setting until it 
is changed.
So you could deboune the switch over 2-3 cycles, then update.
The cost of this part is typically less than the F84 too, so you could save 
a few cents per unit.
And it's pin for pin compatible I believe too.

Jim



----- Original Message ----- 
From: "Hasan Khan" <hasan@khansden.com>
To: "PIC List" <piclist@mit.edu>
Sent: Friday, December 26, 2008 12:31 PM
Subject: [PIC] How to do switch debounce quick enough


> Hi,
> Well, I am finally moving from hobby to pro zone. I got a simple project 
> where I have to design a triac based ac power controller with two push 
> buttons, one to increase power to triac and one to decrease power.  Power 
> level is displayed on a single 7-seg display.
>
> Hardware design is based on 16F84 because they are available dirt cheap 
> here.  Timing of the circuit it based around zero crossings which at 50 Hz 
> happens at every 10ms.  I have an ISR that does the following every zero 
> crossing.
> - pulse triac gate at set time delay
> - read and act on button state
> - display power level on the display
>
> These steps can be done easily in 10ms except that doing software based 
> switch debouncing needs longer than 20ms (10ms for switch down, 10ms for 
> switch up).  This results in ISR being called while still reading switches 
> and ISR calls switch reading routine again and therefore I have recursion 
> and a quick stack overflow error.
>
> How do I do switch debouncing here? Will a reduced debounce time of 8ms 
> work?  Is there a better software design?
>
> PS: The whole project is in simulation stage and no breadboarding done 
> yet.  Small size and low cost are important considerations so I can't do 
> hardware debouncing.
>
> Thanks for your help.
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