[gta02-core] Modem schematic / RF help needed
werner at openmoko.org
Fri Sep 11 23:05:52 CEST 2009
Dave Ball wrote:
> I've had a first stab at schematics for the Telit modem. Comments,
> feedback, abuse all welcome :-)
I had a closer look at it now. I think we can save several test
points, since they also exist on the CPU side. In case of trouble,
we can fall back to soldering tiny wires to resitors.
Here they are:
Net name Modem side CPU side
--------- ---------- --------
MODEM_TX P8107 TP1511
MODEM_RTS P8108 TP1531
MODEM_INT P8109 TP1526
MODEM_RX P8110 TP1510
MODEM_CTS P8112 TP1530
The ones to keep would be:
Then, I think we should name test points consistently, i.e., TP*
instead of P*.
Regarding the 33 Ohm series resistors, do they really have to be so
low ? They should be large enough to reliably protect the modem if
the CPU accidently switches one of those inputs to output, so I'd
be much more comfortable with something in the order of 1 kOhm.
While I don't know the rating of the clamping diodes of the GE865,
the limits I found for various microprocessors are in the range of
1-2 mA. With 1 kOhm, we would get (3.3-2.8)/1000 = 0.5 mA, which
is probably safe.
I think we should also add a remark that these series resistors
should be placed close to the CPU, to keep most of the traces close
to the low-impedance end (the modem's outputs).
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