Thanks to everyone who gave advice.  I have the following options now.
- Switch to F628 use CCP for triac gate pulses and use the left over cycles for debounce and other chores.
- Spread debounce over multiple ISR calls.
- Use vertical counter algorithm.
- use register rotation method ( i haven't found out exactly what that is).

For now I am going to experiment with the first two and see how that goes.  I didn't check at the time but more featured F628 is actually cheaper than very basic and old F84.  Depending on time, I will explore the other options too.

Coming from big software engineering (Java, C++) back ground, I long to use
C.  But I will stick to advice given on this forum else where and stick
to assembler.  This will be my third and last project in assembler and
then I am going to switch to C no matter what you say...yeah! :-)

-Hasan



----- Original Message ----
> From: Hasan Khan <hasan@khansden.com>
> 
> 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.
> 
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