Bug #1024 (oscillating re-camping), a possible solution.

[Dima Kogan]

I just performed this fix and so far the recamping is successfully
squashed. I'm not seeing any recamping (was fairly common before the
fix). Instead of placing the second capacitor away from the first and
using a wire to connect the two, I placed it directly adjacent to the
first, using some electrical tape to isolate it from the components
directly beneath.

Thank you, Dieter.

Dima

On Mon, 25 May 2009 20:40:02 +0200
Dieter Spaar <spaar at openmoko.org> wrote:

> Hello,
> 
> 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.
> 
> Best regards,
>   Dieter