Pic and the Xbee module

[Michael]

Has anyone used the 3.3v Xbee module with a 5v PIC?

I plan on dropping the voltage to the Xbee to 3.3v -- will it interface with the PIC just pin to pin alright?

Or I need to _____?

Thanks

A bit off topic but thought it worth asking.

[BH_epuk]

Ok, not sure what happend to my reply so here it is again.

I've got Boards with Xbee's and PIC's running both of 3.3V, Not found any problems with the PIC running off 3.3v, is there a reason why the PIC must be on a 5V supply?

[Charles Linquis]

Most of the older PICs are designed for operation at 5V. The 'normal' parts (such as an 18F2221) may work at voltages below 4.2 V, but they are NOT GUARANTEED to work at all. For guaranteed operation at lower voltages, (such as 3.3V), you need the "LF" part (such as 18LF2221).

http://dl.dropbox.com/u/566712/18F2221_V_Spd.jpg

http://dl.dropbox.com/u/566712/18LF2221_V_Spd.jpg


So, while your device may work at 3.3V, it may not always work. Get the LF part and run it at 10Mhz and you will be fine.

If you want to run the PIC at 5, and your peripheral at 3.3 -

On the signals that come FROM the PIC going TO the peripheral - put a 1K resistor in series.
On the signals that come FROM the peripheral going TO the PIC - Use a 74HCT gate running off 5V between the two devices. I generally use a 74HCT125, because I use it as a data selector at the same time.

[Michael]

I have a ton of 16c72, older part and yes 5v. Got a couple of these xbees and like I say, using a separate 3.3v regulator for them. Ok, was hoping that I could do a straight shot (in some cases it's worked before) but I'll take your word for it. The plan is to have a xmt/rcv pair with a pic in each box -- so I'll use serin on one and serout on the other. I may try a direct connection first just for the heck of it -- you would think a swing of 0 to 3.3v would be good enough for a logic low, logic high for the serin, my pcb is cramped might have to use a transistor instead of a chip gate. Certainly a resistor on the pic serout would be needed. Thanks.

[Michael]

Do you know of a buffer that's on an 8pin dip that will work? Don't have room for the 14pin 74hct125. I thought cmos went to 3.3v -- wrong, 3.5v I see.

[Michael]

I'll just use 74hct125 -- made room for it -- do you know, does it invert? Will I have to run the signal through one or 2 stages? THANKS.

[Michael]

I like this ---

SN74LVC2T45 8pins non inverting and cheap

[BH_epuk]

Thanks that Charles, In my sleep deprived state i'd forgoten that not all PICs run stable at lower voltages.
In my case, the 16F688 does run fine @4Mhz 3.3V.