Bug #1024 (oscillating re-camping), a possible solution.
spaar at openmoko.org
Mon May 25 20:40:02 CEST 2009
We have now tested a few phones and it seems that
increasing a capacitor solves the re-camping bug.
The problem most certainly comes from a sub-optimal
PCB layout where a voltage regulator feedback trace
is not close enough to the consumer so that the
length of the trace from the regulator to the consumer
has a considerable influence. Additionally there is
a 0R resistor in this trace which might not be exactly
zero all the time (it might also depend on how good
the soldering is).
Increasing the capacitor which buffers this power supply
line seems to help.
We don't know exactly what happens inside the Calypso but
it could be that the power drop is too large when the
Calypso wakes up from "Deep Sleep" (which means it
draws more current again) and this causes the next PCH
reception to fail (for whatever reason).
The appended picture shows this capacitor (its C1009,
a 10 uF ceramic capacitor), you have to remove the GSM
modem shielding to find it.
This picture shows one possible solution to increase this
capacitor, there is a second 10 uF capacitor connected. The
location of this second capacitor is intentionally that far
away because there is no other place where you have enough
room to place the capacitor without risking a shortcut with
components below on the PCB and not exceeding the height
so that the shielding still fits. You should of course place
some insulation on the shielding to avoid a shortcut with
the second capacitor.
The other solution is to remove C1009 and replace it
with a 22 uF capacitor. Daniel Willmann has done this
successfully with a few phones.
This is it for now, maybe our hardware experts will
add some more comments.
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