'pic lock-up and external WDT'
Harold M Hallikainen
| With all this talk of watchdog timer chips, I thought I'd mention
the WDT I've been using for years and years. It's a 74hc14 and a few
resistors, diodes, and capacitors. I'm no good at ascii art, but I'll
describe it here.
One section (we'll call it section B) is a classic schmitt oscillator
(capacitor between +5V and input, resistor input to output). The output
of this is the reset pulse. Note that since the capacitor goes to +5V
instead of ground, it generates a low reset immediately on power-up. A
diode with a series resistor across the resistor previously mentioned
makes the oscillator output asymetric. You can use the first resistor to
adjust the frequency of resets (the WDT timeout) and the second resistor
for the duration of the reset.
We want to be able to reset this timer with a pulse from the processor.
This is done by tying a diode anode to the input of the above schmitt
(section B) and cathode to the output of another schmitt (section A).
When schmitt A output is low, the schmitt B input is held low, holding
the reset line high and holding off timing.
Finally, to prevent a WDT reset from lasting forever (a processor crash
DURING a WDT reset), we AC couple into section A (a series capacitor, 10
to 100 nF) and add a pull-down on the input of section A.
When the processor wants to do a CLRWDT, it pulses the left side of the
coupling capacitor high, then low again.
I've used this in products based on the 68B02 and products using a
standard PC motherboard (just tie the output of section B to the reset
button header). It's worked great, and leaves me four extra schmitt
inverters to play with...
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