next-generation PMU discussion

[Werner Almesberger]

[ Context: we're looking for a PMU for GTA04 and beyond. One possible
  choice is to use NXP's PCF50633  again. ]

Andy Green wrote:
> NXP is "the devil we know", but there is no integral codec.

In the past, we had lots of trouble with the NXP PMUs, since their
default settings (such as which regulators are active in what state,
and what voltage they default to) are mask-programmed in the chip,
there is only a small number of chip variants (they can also make
custom variants, but we don't have quite the numbers for that ;-),
and they never quite do what we need.

So we ended up shuffling things around endlessly, tried to figure
out work-arounds for incorrect voltages, and are supplying chips
with voltages outside the guaranteed specs.

NXP understand that this is a problem and mentioned that they are
considering to use some EEPROM or similar in future chips to allow
customer-specific configurations. Of course, this doesn't help us
with GTA04.

However, since GTA04 will have an MPU, we're considerably better
off there. What we still have to worry about is:

- feeding the MPU (unfortunately, the upper end of the MSP430's
  voltage range is only 3.6V. Otherwise, we could just have
  connected it to Vsys.)

- make sure no regulator defaults to "on" and a higher voltage than
  allowed on the respective rail

- make sure nothing goes berserk if some power rails come up a bit
  later than the rest (but that's a general problem anyway)

So making the PCF50633 fit wouldn't be quite the pain it was in
GTA02.

- Werner