I've got an application that gets The Slows every once in a while.  What,
you may ask, are The Slows?

The application is a timer for an appliance.  It is initiated when a power
switch is closed on the appliance, runs at 4 Mhz with an RC oscillator,
using a 16F873 PIC.  15 times out of 16, the timer times out in, say, 40
seconds, and fires a relay which shuts off the power.  An LED blinks 1X per
second, and the frequency at the clockout pin is 1.15 MHz (within spec for
the low tolerance resistor and capacitor I am using) Timeout is determined
by counting up Timer1 interrupts, which are supposed to happen every 1/30th
second.

One time in about 16, the timer times out not in 40 seconds, but in 3
minutes.  The blinking LED blinks at a very slow rate.  But a measurement of
the clockout frequency is STILL 1.15 MHz.  I expected to find that the
oscillator was running real slow, but that doesn't seem to be the case.

I am going to measure this again, because I don't believe my own test
results.

There are two theories I am wondering about:  heat, and power.  This is a
cooking appliance, it gets hot.  How hot?  I am going to thermocouple the
PIC pretty soon and find out.  Second, the power is probably pretty dirty on
startup.  There is a pair of contacts that close on a 1500 watt heating
element, also cranking up the power supply on the unit.  Big sparks and all.

Could a PIC's oscillator start up at some wierd frequency if the power was
dirty on startup, then stay there?  Could the oscillator run at some wierd
frequency because it is hot?

If I was on the other end of this post, I'd say The Slows are impossible.
Since I just watched them, I can tell you they are.


-- Lawrence Lile
Sr. Project Engineer
Salton inc. Toastmaster Div.
573-446-5661 Voice
573-446-5676 Fax

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