A5 Power routing "minor issues" for some modules

Andy Green andy at openmoko.com
Wed Feb 27 02:35:18 CET 2008

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Somebody in the thread at some point said:

>> I was looking at the vibrator drive signals earlier and there is all
>> kinds of nasty inductive spikes on them, due in part to this NC snubber
>> diode I guess.  This can make possibly make trouble.
> Sorry, your sch is  too old. The diode is not NC.

Well it was an A6 schematic from last week :-)  OK, I wonder if it is NC
on my A5 then.  I did try this with battery out (thus discovering the
larger issue).

>> Really even a 1A adapter, 5W, cannot be made to do better than the
>> battery?
> For the GSM_modem, the VB connect to GSM_MODEM's PMU vbat pin.
> Even it is 1A, I don't suggest to do that.
> Myabe we has some trouble on GSM_MODEM. (Ex, vbat input range can't
> accept 5V)

The charger drives that same battery pin don't forget.  With no battery
in it sees ~5V anyway.  If it doesn't tolerate it we kill the devices
already.  We should study the schematics and datasheets.

However the reasons for leaving it alone seem to be multiplying, the win
for changing it is relatively small and we have other things to take
care of.

But I would like to understand the exact issue with GSM via switch for
example instead of a lot of reasons that may or may not apply.  It feels
like the GTA02 A2 switch did make real problems, removing the switch
solved the problems, but inbetween it is a little uncertain about why.
Maybe also there is a figure for this GSM design's "instantaneous
current draw" and the reservoir capacitors we have to deal with it.

- -Andy
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