Re: Defender power supply requirements

> > to come up slower during boot up, I believe this had to do
> > with the memory backup, of course I be wrong;)
> IIRC, you are correct. The +5 needs to come up/on *after* the other
> voltages are stable (mainly the +12). This prevents the CMOS from being
> written to before the system gets into a sane state.

The power sequencing is to prevent latch-up (search google for latchup and cmos for more on that topic).
The "CMOS" memory on that board is:
A) single supply - so it would never know if there was a power sequence problem
B) battery backed up - so, it's always powered up anyway

The only multi-voltage devices on the board are the 4116s.
Every switcher I've ever seen is designed to/observe standard power sequencing.
Basically, no worries on that front.

> > I have used both computer and video game switching supplies with no
> > problems on my Williams games. No issues with corrupt memory - and 'if'
> > that is the case I would suspect that the power-up reset circuit on the MPU
> > is failing. After all CPU's all want about 2 - 500ms of RESET before going
> > 'live' and any switching supply worth it's salt will provide power in under
> > 100ms. So if you have a problem with memory corruption, then I would be
> > replacing any timing capacitors in the RESET circuit...

There's a watchdog anyway. If at first you don't reset - reset, reset again...

> Well, there is this nasty resistor/transistor circuit that looks to
> qualify both the 5 volt rail and the 12 volt rail (at least) and it's
> connected to the system wide reset. So maybe the supply doesn't come up in
> any particular order, but the logic circuit definitely qualifies them
> both.

That _does_ prevent cmos corruption as per the original responders post.
It merely keeps any spurious writes as the cpu starts rcv'ing or losing power,
and hence sanity, from making it to the backed-up memory.
All good non-volatile memory controllers qualify write/access cycles
with the system power supply being in tolerance.

> So what do I connect the "12 volt unregulated" to on Defender? Just the
> +12 volt rail? All it appears to do is go to the big, nasty, evil
> resistor/transistor/zener circuit that controls reset.

That's pretty much the best you can do.
If you were really anal, you could add filters to seperate the two
12v supplies to keep noise on one bus from getting onto the other.

Otherwise, just tie them together and deal with it if you have any problems.
Most switchers seem to have enough capacitance to make it work without
external filters. It's always worked for me.
 
> And I, of course, have purchased several ATX power supplies dirt cheap.
> Any recommendations on a specific wattage? I can't imagine it pulling more
> than a desktop PC.

If you overload it, it is designed shut down. Worst case, it will blow up, but hey,
they're cheap. Again, I wouldn't really worry about it.

-Mark

P.S. Neil, got that disassm?

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Received on Thu Feb 20 09:40:28 2003