Min Xu <min.xu(a)min-info.net> wrote:
Can you tell me what the intent of the C1 capacitor is
for?
It's a bypass cap between VCC and GND, physically placed as close as
possible to the C1 (VCC) contact of the SIM socket. The venerable GSM
11.11 spec recommends placing a 100 nF ceramic capacitor "as close as
possible to the contacting elements", to absorb current consumption
spikes on the part of the SIM - the same spec allows the card to draw
spikes of up to 200 mA for up to 400 ns! The design of all classic
historical GSM phones (including TI reference designs which I copy in
FreeCalypso) follows this recommendation of putting a bypass cap right
next to their SIM socket C1 contact - but with the introduction of
SIMtrace FPC cable between the phone's SIM socket and the actual SIM,
now that phone-provided bypass cap is a cable stretch away, hence I
felt it prudent to provide another bypass cap on the sim-fpc-pasv
adapter PCB itself.
Previously when I probe the signals I just solder fly
wires on to the
SIM socket on the FPC,
Given that it costs the same to fab one PCB or 5 of them, I have 4
extra PCBs which I would be happy to give away if anyone else needs to
do this kind of purely passive sniffing or probing. Just bare PCBs,
though, I don't have extra socket parts. The source is published too,
of course:
https://www.freecalypso.org/hg/fc-small-hw/
M~