pwsafe: MCU selection, first round

Werner Almesberger werner at almesberger.net
Mon Sep 16 05:58:31 EDT 2013


Felix wrote:
> Should be possible something like following?

I was thinking of having such a FET battery switch but since the MCU
already seems to include a similar feature (*), I'd use that.

The boost converter only gets enabled when using display or memory
card. The transceiver must be powered whenever we need a precise
clock, because it has the system's only crystal.

Since the transceiver has an efficient built-in off switch and can
run at voltages down to 1.8 V, I'll simply connect it to the same
USB-or-battery supply as the MCU.

I also placed a fat silo cap (220 uF) on that system voltage rail, to
keep the RTC running when swapping batteries. Will have to see if the
MCU can respond sufficiently quickly and whether "deep standby" current
is low enough for this to work properly.

The RTC's clock would come from an internal RC oscillator that can be
trimmed by occasionally comparing with the 15 ppm crystal reference.

I'm not entirely sure whether I interpreted the MCU's power routing
correctly but a few experiments should clarify that :-) By the way,
tentative schematics (still work in progress) and some rough component
placement on the PCB are now in the repository:

https://gitorious.org/pwsafe/pwsafe/source/hw

(*) Figure 3-29 on page 93 of [1] and figure 36-2 on page 652.

[1] http://cache.freescale.com/files/32bit/doc/ref_manual/KL25P80M48SF0RM.pdf

- Werner



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