Jan-Erik, Brent, Thys and Olin, thank you all for your helpful hints and
suggestions.

I had a close look at the PCB I had designed. The culprit possibly is =
the
thick track directly under the PIC that is connected to the Live =
terminal of
240V~, supplying +5V to pin 1 of 12F629. (ASCII art of the power supply =
is
shown below). I will try and modify the PCB and see whether that solves =
the
problem.

5 Volt Power supply from 240V Mains
(Caution: No isolation, high voltage at all points)

          +---------------+---------+---- +5V
          |               |         | =20
    Live  o               |         | =20
                  Zener  ----      ---  470uF
        240V~      5.6V   /\       ---   16V
                         ----       |   =20
  Neutral o               |         | =20
          |               |         |   =20
          +--/\/\/----||--+---|<|---+---- Gnd
             220R   0.33uF   1N4007
                     400V

Regards,
Bala=20


> -----Original Message-----
> From: Olin Lathrop [mailto:olin_piclist@EMBEDINC.COM]
> Sent: Thursday, September 11, 2003 5:46 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC:] Why is my 12F629 resetting?
>=20
>=20
> > My problem is this: After the lamp is switched on, when the=20
> Dim button
> > is pressed 3 times, the lamp dims in 3 steps as expected.=20
> But when the
> > button is pressed the 4th time, the PIC resets and the lamp=20
> goes off.
> > (There is a start-up routine that blinks the LED 4 times.=20
> This tells me
> > that the PIC has reset). I have disabled the Watchdog Timer and
> > Brown-out Reset. If I replace the 60W bulb with a zero watt=20
> one, then
> > the circuit works without any problems.
>=20
> Sounds like the switching noise is coupling into the PIC=20
> circuit.  After 3
> dimming steps the rising edge when the triac is turned on is=20
> larger and
> therefore and the voltage capacitively coupled to a sensitive=20
> point gets
> to the breaking point.  You need to be squeaky clean about the ground
> layout, decoupling, etc.
>=20
>=20
> *****************************************************************
> Embed Inc, embedded system specialists in Littleton Massachusetts
> (978) 742-9014, http://www.embedinc.com

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