hybrid_snyper said:
So direct connection is the way to go. I found these two links a while
back. The first is what I believe to be the circuitry within the
datacable and to which would allow me to eliminate the cable.
http://pinouts.ru/CellularPhonesCables/ericsson_cable_pinout.shtml
this next link shows a guy which has a Tiny12 connected directly via a
datacable to his MCU. I take it would be a similar setup to this. He
has the data entering the MCU fed direct with a 4700 resistor in
between. However he has added circuitry to create a true bi-polar
signal. Im not sure what this means.
http://www.riccibitti.com/tinyplanet/tiny_article.htm
OK, these are nice links relating to what you want to do. From the first
link it's clear that there's no need to invert the signals when you
connect the phone *directly* to the PIC (i.e., *without* using the
special Ericsson cable which has circuitry inside). If you do it like
this you should be fine using only resistors for protection (as Tim
Auton suggested earlier):
Phone Tx -----\/\/\----- PIC Rx
Phone Rx -----\/\/\----- PIC Tx
Phone GND --------------- PIC GND
I don't know about the voltage levels of the T28. Are they 5V or 3.3V?
Probably the latter. As Tim said, run the PIC at the same voltage. I
don't think you need to worry about true bipolar signals, the other guy
in the second link probably uses the proper Ericsson cable (with the
circuit inside) and so has to deal with negative voltages (hence the
Zener), and uses the transistor to "steal" negative voltage from the
incoming signal to generate his own bipolar Tx signal. The transistor
also inverts the MCU's Tx signal. He probably does the inversion of the
Rx signal in s/w (inside the MCU - no h/w UART used).
So, to recap, if you're somehow connecting the phone directly to the PIC
(no cable) or you're using just a passive connector or a dummy cable (no
circuitry involved) then use the above wiring (PIC's voltage same as
phone's logic levels, 5V or 3.3V). If you *are* using the proper
Ericsson cable (with the level shifter/inverter inside), then you need
to invert the signals again so that the PIC & phone can talk to each
other. Hope this is now clearer.
--
Regards,
Costas
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