Dwayne Reid wrote:

> At 06:59 AM 8/6/2004, Matt Redmond wrote:
>
>> I've been trying to get a PIC talking to a Trimble Lassen SQ GPS module
>> (3.3v) with no success so far.  Without flaming me for being an idiot
>> <g>,
>> can anyone tell me by reading the below excerpt from the Lassen manual
>> whether or not I need to do anything to the signal before feeding into my
>> PIC?
>>
>> Note again that the Lassen is a 3.3v device.
>>
>> As an embedded design, the Lassen SQ GPS provides direct CMOS
>> compatible TTL
>> level serial I/O. The RX and TX signals on the J2 I/O connector are
>> driven
>> directly by the UART on the Lassen SQ GPS.
>
>
> PIC to GPS is simple: a 4k7 & 10k voltage divider that makes the 5V from
> the PIC look like 3.3V into the GPS.
>
> GPS to PIC is slightly harder: 2n4401 emitter to GPS Tx out, base to
> midpoint of 10k & 10k voltage divider from PIC Vdd to gnd, collector to PIC
> Rx pin with 10k pullup to PIC Vdd.  You could just use a 10k resistor from
> GPS Vdd into the base instead of the voltage divider but I'm assuming that
> you do not have easy access to the GPS Vdd line.

Or three Si diodes in series (or 1 zener) to the PIC (1.8V drop),
with a 10K pullup on the PIC side.
GPS at gnd, PIC sees 1.8V (low < 4.5v).
GPS at 3.3V, PIC sees pullup (5.1V with diode offset added).
There will be some shift in offset with temperature, but should
be well within  PIC schmidt range.

Robert

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.