next-generation PMU discussion / RAM/ROM-size requirements

Werner Almesberger werner at
Thu Apr 24 03:24:49 CEST 2008

Andy Green wrote:
> We probably can share the battery but I didn't really think of what
> better features it gets us.

It could provide power until a sufficiently low-voltage regulator
is active. I don't remember the part number of the battery, so I'm
not sure if the voltage is rights.

In any case, if we share the backup battery to power the MPU when the
system is in PMU.Standby, the MPU won't be able to do much, or it'll
drain the backup battery.

> Everyone seems to like that method of shared IO, it sounds like a good
> policy.

It's nice, simple, and fool-proof. But it also has the distinctive
disadvantage of needing lots of GPIOs on the MPU :-(

By the way, wakeup sources (i.e., CPU.SLEEP -> CPU.NORMAL) can also
go to the CPU, which then signals the MPU that it wishes to be woken
up. (With XPWRRGTON on the 6400 (WHMKSUPSCHCRZNMS?), PWREN on 2442.)

So buttons or similar that can only wake us from CPU.SLEEP, but don't
have to wake us from PMU.Standby don't need to go to the MPU. (Of
course, if we want to keep the option open to use such sources also
to wake from PMU.Standby, we can't make this simplification.)

- Werner

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