Protection for DOUT in XBee adaptor
Moderators: adafruit_support_bill, adafruit

Protection for DOUT in XBee adaptor

by paul49 on Wed Nov 28, 2012 3:09 am

Hi All,

I've just ordered a couple of XBee Adaptor kits to interface to an ATMega1280. The schematic shows the DOUT pin connected directly to the header pin. (I appreciate also that all the buffers in the 74AHC125N have been used.)
Given that mistakes do happen and the corresponding Rx pin on the AVR could be accidentally set to an output (and high), would the XBee tolerate 5v applied to DOUT? Or would it be prudent for me to buffer DOUT up to 5v and at the same time, offer some protection to the XBee?
I realise that I'm being overly cautious ... but that's me :D
paul49
 
Posts: 2
Joined: Wed Nov 28, 2012 2:30 am

Re: Protection for DOUT in XBee adaptor

by adafruit_support_bill on Wed Nov 28, 2012 5:52 am

That is a good question. We haven't had any issues with that. But caution is not such a bad instinct. You might try posting this question over at the Digi forum. The engineers there could probably tell you if the modules will tolerate 5v while in output mode.

http://forums.digi.com/support/forum/index
User avatar
adafruit_support_bill
 
Posts: 29726
Joined: Sat Feb 07, 2009 9:11 am

Re: Protection for DOUT in XBee adaptor

by paul49 on Wed Nov 28, 2012 7:57 pm

Thanks for your quick reply.
I've posted the question on Digi. Since you haven't had any issues, I'm sure the point is moot (and I won't waste any more time worrying about what's never going to happen :oops: ), but we'll see what they say.
I'll post back here if/when I get a response.

Just from a theoretical perspective, would a CD4050BC be suitable as a buffer on DOUT? I can't buy a 74AHC125N locally.
paul49
 
Posts: 2
Joined: Wed Nov 28, 2012 2:30 am

Re: Protection for DOUT in XBee adaptor

by adafruit_support_bill on Thu Nov 29, 2012 8:31 am

You are over-thinking the problem. A 5v->3.3v buffer/level shifter is only good if you are trying to write to the XBee. But you can't write to the DOUT (Tx) pin - and putting a unidirectional buffer there will prevent you from reading it.

There are bi-directional level shifters: http://www.adafruit.com/products/757 But that seems like overkill.

Looking at Rob Faludi's book: http://www.adafruit.com/products/963
his Arduino/XBee connection diagrams all show direct (unbuffered) connections for both DIN and DOUT.
User avatar
adafruit_support_bill
 
Posts: 29726
Joined: Sat Feb 07, 2009 9:11 am