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
 
--




colin@btech-online.co.uk

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