Hasan Khan wrote:

> 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).

Hasan, I have just finished writing a tutorial introducing interrupts on
midrange PICs (such as the 16F628, although a 12F629 is used in the
examples): http://www.gooligum.com.au/tutorials/midrange/PIC_Mid_A_6.pdf

It includes a section on using a timer interrupt to debounce a single switch
(multiple ISR calls, each samples the switch, sets a "switch changed" flag
and updates "current debounced state" variable when the same new state has
been seen some number of times in a row).

Not as elegant as vertical counters and register rotation (which is in fact
a way of counting - it's essentially the same algorithm - it's explained in
one of Ganssle's debouncing papers) - but my intent was to show how
interrupts could be used to re-implement the debounce algorithm I'd
introduced before - and it actually works very well.

So - hopefully of some use to you with your option 2!


David Meiklejohn
www.gooligum.com.au
 
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
> Behalf Of 
> Sent: Monday, 29 December 2008 5:27 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] How to do switch debounce quick enough
> 

> 
> 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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