tech: Dig Dug ram problems

[RobWalmsley]

[ukvac] tech: Dig Dug ram problems
Hi

I have an Atari Dig Dug board which works fine except
that the foreground graphics are jail barred, doubled
up and in the wrong place sometimes.

Classic ram symptoms, but Dig Dug has a ram/rom
selftest which the board passes ok. So is the selftest
naff or is the problem else
where?.

Oddly when I piggy-back onto the six 2114s, five of
them cause the board to fail its ram selftest and
identifies the correct ram as failing. One of them
still passes the test.

Why would the ram test fail when a good 2114 is
piggy-backed?.

Cheers

Rob

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[guiddruid]

Re: [ukvac] tech: Dig Dug ram problems

> Why would the ram test fail when a good 2114 is
> piggy-backed?.

couple of possibilities I can think of:

a. more power being taken, leading to dirty power rails. try adding a
decoupling cap too.
b. the new chips are slower & screw up the data when it is being read...
could also be a problem if the good chips are faster if the design is crap
enough. I think pac-man for instance relies on some of the rams being very
slow (addr to data out is assumed to take about one clock period)

hth,
Simon