At 07:08 PM 2/25/2004 -0800, you wrote:
>On Thu, 26 Feb 2004 01:43:47 +0000, Edward Cooper wrote:
>
> >Hi
> >
> >I'm trying to connect a 3.3v GPS device to a 5v PIC, now I need to
> >adapt the signals.  I've looked back in the archives, and Brent Brown
> >suggested the bellow, but I have a few questions.
> >
> >Should I add a 3.3v Zener too the RxD line to make sure it can't go
> >above 3.3v ever?
> >
> >Also is there not a simpler way of configuring the TxD line?  Are there
> >chips for handling this sort of conversion?
> >
>
>You may want to check the documentation on your GPS module. Many of
>them can be interfaced directly to CMOS logic levels even though they
>run on 3.3V power.

If the PIC input is TTL level, then 3.3V is more than good enough (just
needs to be 2.4V). If it's a ST input, then it's not good enough.
(BTW, it would be nice if Microchip would include a bit or two in the UART
configuration registers to flip the polarity of RxD and TxD. Two bits of
storage and two XOR gates.)

For the input, a voltage divider is better (unless the GPS inputs
are 5V-tolerant). Eg.

0/5V
--[10K]-x-- 0V/3.3V
         |
        [20K]
         |
        0V

Or just run the PIC on 3.3V, as others have suggested. Might as well
get used to lower supply voltages- many newer micros such as the MSP430
won't take 5V (and are not 5V-tolerant). The new LPC series of
microcontrollers from Philips needs a core voltage of 1.8V, but
has 3.3V I/Os (which are 5V-tolerant).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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