Can an XBee module be driven to 5V safely?

Wireless & IoT
1

I am designing a PCB that will contain a PIC microcontroller and an XBee module. Both components will normally run off of 3.3VDC, but the PIC is driven to 5VDC when I do in-circuit programming. Is this going to be an issue for the XBee module? Thanks!

2

According to the documents the XBee’s MAX voltage is 3.6V. So 5V would be a problem.

What I do is isolate the Vcc pin on the PIC from the system Vcc with a Schottky diode and have the ISP header connected directly to the PIC Vcc pin. This way the programmer only powers the PIC and not any other circuit.

Now no worries.

3

Thanks for the reply. A couple of questions:

  • Why not a standard diode?

  • Should I worry about isolating the data pins between the module and the PIC?

4

MagicSmoke:

  • Why not a standard diode?

Good question. My reason is a Schottky has less voltage drop when conducting, ~0.4V vs 0.7V of a silicon diode at rated current.

The disadvantage of using a diode is the the PIC’s Vcc when be about 0.4 volts less than the system Vcc. For a sytem of +3.3V the PIC will be at +2.9V.

If this is unacceptable then a jumper could be used instead of a diode.

You only need to worry about the pins connected to the ISP. If these pins are normally used then the devices they connect to should be protected.

5

I’ve used two 4001 diodes in series

6

Guys, I’m curious: why isn’t the programmer powered from the target? What programmer are you using? The ISP programmer I use gets its power from the target, and hence doesn’t have these issues…

7

Another good question.

I use the PICStart programmer which expects the chip to be inserted into the ZIP socket so it supplies all the voltages.

For ICSP I made a cable with a DIP header that goes in the ZIP socket on the PICStart to pick-up the programing pins for the particular PIC. The other end is a 5-pin single row plug that mates with a header on the boards I build.

Maybe this is cheating but it sure beats popping the PIC out of a socket each time I wish to re-program. And I’m using the programmer I already have and works seamlessly with MPLAB.