pwsafe: MCU selection, first round

Felix sucotronic at gmail.com
Mon Sep 16 09:51:56 EDT 2013


I forgot to mention, that if somebody wants to play with freescale Kinetic
cpu, there's an affordable development board (~10€): FRDM-KL25Z. You can
source it form digikey, but you can also get it from ebay, shipping
included, for ~20€[1].

[1]: http://www.ebay.com/sch/i.html?_sacat=0&_nkw=FRDM-KL25Z&_sop=15


On Mon, Sep 16, 2013 at 3:35 PM, Felix <sucotronic at gmail.com> wrote:

> 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 almesberger.net> 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:
>>
>> 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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>
>
>
> --
> Felix
>



-- 
Felix
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