next-generation (now) MPU discussion / Backup battery on PMU only

[Werner Almesberger]

Andy Green wrote:
> What is the downside if MPU is down during this time?

I can think of the following two things:

- it can't be part of the decision of when we power up. I.e., we
  couldn't assign POWER to a different button or such.

  Not sure if we care about this point.

- there are some signals that need to have a specific state also when
  the CPU is not powered or when it is in CPU reset, e.g.:

  - anything that controls power and wakeup of items directly connected
    to the battery or to Vsys

  - the USB pull-up

  Our current solution is to use discrete logic and the (very few) GPIOs
  of the PMU. One problem with the PMU's GPIOs is that they may not
  default to the state we need in the end, so this adds more glue logic
  and possibly pull-ups.

  While discrete glue logic is cheap on the board, it also has a high
  engineering cost if we need to spin a new layout just to test some
  slight change. We've been there before.
  
- Werner