pwsafe: MCU selection, first round

Felix sucotronic at
Mon Sep 16 09:35:48 EDT 2013

Yeaaah, schematics!! One silly question... do you plan to upload the kicad
library folders? or where can we get them?

On Mon, Sep 16, 2013 at 11:58 AM, Werner Almesberger <werner at
> wrote:

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