Re: Battlezone problem.

On Tue, 13 Jan 1998, Mitchell Rohde wrote:

> Now I get zilch from the unit. I didn't see squat on the data lines, and
> I heard a "blip-blip-blip-blip..." from the speaker... the sound of a
> reset. So I probed the reset circuit output, sure enough.. it's
> going up and down about two or three times a second. If I press the reset
> switch on the generator board it pins the thing in reset, but when I
> release it goes back to the resetting...
>
> How does this watchdog circuit work, and has anyone seen this before?
> What trips the watchdog?
>

Hey Mitch,

        Long time no "see."

        The watchdog is just a counter that, when it counts to its max.
value, triggers the reset. There is an instruction (which could be just a
store to a certian location) which clears the watchdog. So, in theory, if
the CPU is running properly, it should clear the watchdog timer before it
times out.

        If you've got schematics, there is a WDCLR signal which goes to
the RESET line on the counter (which generates the RESET line to the CPU)
probe it and see if it's pulsing at all. If it's not, then there is some
other CPU problem. There is also a test point on the board labeled WDDIS
or something like that. If you ground it with an alligator clip lead it
disables the watchdog counter. The theory goes that if it works with the
wotchdog disabled, then there is a watchdog problem, else there is a CPU
problem. My Lunar Lander didn't work (i.e. stayed in RESET) despite my
disabling the watchdog, and it turned out to be a bad watchdog counter, so
there's no hard and fast rule....

        Good luck....

Joe

        
Received on Tue Jan 13 15:27:50 1998