[gta02-core] Modem schematic / RF help needed

Werner Almesberger 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).

- Werner

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