>  I'm posting this as this particular setup wouldn't have struck me in
> a million years (+/- probability not defined).
>
> As many may know I've been hacking an LCD module from a Xerox printer.
>
> The designers have implemented a watchdog function using the I2C clock
> from the uC  as the timebase.
>
> The circuit is thus:
>
> I2C clock pin from uC goes to one gate of an 7405 inverter. The output
> of the inverter is held high via a 2K7 resistor, the junction goes to
> the clock input of one half of a D-Type flipflop (7474). RD and SD of
> the flipflop  are held high, The data input is also held high via two
> spare inverter gates from the 7405 whose outputs and inputs have a 2K7
> resistor to Vdd. The Q output goes to an input pin of the uC. As the
> uC is unknown, this could be an ordinary input pin and software
> checked or a reset/interrupt pin.
>
> In the past when I've used a discrete watchdog timer rather than a WDT
> chip, I've used NAND gates in an oscillator configuration. The method
> used by Xerox might save a few components if an I2C or SPI clock is
> being used as part of the circuit.
>
> It is possible I suppose that this circuit might also just be a way of
> checking if the I2C clock is functioning, as with the LCD module
> disconnected, the printer will not come out of standby.
>
> Colin
> --
> cdb,   19/08/2009


I don't think I fully follow the circuit, but I did an external watchdog
for an MC6802 years ago that was based on a 74C14 hex Schmitt. One section
was set up as an astable multivibrator. A capacitor from the input went to
Vcc. A resistor went from output to input. This determined the time
between reset pulses. A series diode and resistor was across that
resistor. This determined the length of the reset pulse.

The input to the above mentioned Schmitt had a diode to the output of
another Schmitt with the cathode connected to the output of the new
Schmitt. If this point is pulled low, the oscillator stops with the reset
line high.

The input to this second Schmitt has a pull-down resistor to keep the
output idling high, allowing the reset oscillator to generate periodic
reset pulses.

The input to this second Schmitt has a capacitor to a controller output
pin that toggles as the program properly executes. This keeps resetting
the oscillator (pulling the input to the first described Schmitt low and
making its output high). If the pulses from the controller stop, the
oscillator times out and you get a reset pulse.

You may also want to add clamp diodes on the input of the second described
Schmitt so the input is not driven negative.

This could be driven by a separate output pin, or, as you describe, an I2C
or SPI clock. Nowadays, I just use the WDT in the PIC...


Harold


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