next-generation PMU discussion / Memory 1.8V

[Werner Almesberger]

Andy Green wrote:
> We didn't settle on NXP (at least I didn't hear Tony say anything)

I think the choice of PMU is something that needs very careful review,
so we'll all have to agree on what we use. The PMU is probably even
more critical than MPU and CPU combined.

> And I'm not sure about series diodes.

You mean to drop the voltage ? I don't like them. E.g., a 1N4148's
Vf changes from 400mV at 10uA to almost 1V at 20mA. Even at 1mA,
it's already 600mV.

If we put 4x1N4148 to drop 5V to 3.4V (~10uA), then we'd end up
with 1.2V if at 3.6V battery and drawing 1mA. And that's all at
25C constant. Take it out of the air-conditioned lab and it
fails :-)

One may be able to find a diode with narrower tolerances and all
that, but this sort of hacks is just an uphill battle.

> Correct me if I am wrong, but doesn't VSYS see USB power, ie, 5V?  We
> can't use that directly if so.

Yup.

Heretical question: do you know of any other MPU families in the same
range (size, performance, etc.) that have JTAG and that can handle 5V ?
The MSP430 is actually the only small-uC family I know that doesn't do
5V, but they're also - among those I know - the only ones that give us
JTAG.

There are alternatives to using JTAG, but they're all far less elegant.

> There is a 1.8V in standby because the memory takes it, we can use that.
> ~ When we need 2.2V not sure what the story should be.

There are the following scenarios to consider:

1) first power to the board at the factory

2) power up with a dead backup battery (*)

3) power supply after the initial bringup

1 and 2 are the same as far as the PMU is concerned, if we assume
the PCF50633  (NoPower -> Standby transition), but in 1 the board
if in a fixture, so we can easily work around any restrictions and
even power the MPU from the outside.

I'm worried about 2. The backup battery is soldered to the board
and probably difficult to replace. I don't know how long one can
expect that battery to live, but it would be sad if phones would
all of a sudden fail to power up after a few years.

3 is easier. We've been in control and could set the PMU exactly
to the state we want.

- Werner