Hi below is my setup block diagram to understand enter image description here

Setup description: I have two board , board 1 and board 2. Board 2 is third party board we purchase. Board 1 has on board SMPS which convert 230VAC to 5VDC, again this 5VDC converted to 3.3V via LDO. this 5V also goes to board 2 power.

Board 2 take 5V from board 1 and convert it 3.3 V via some switcher IC.that 3.3 V goes to other circuit into board 2. (not shown in diagram), board 2 is linux board

Note:- Board 1 3.3 V and board 2 3.3 V are not connected together.

Issue:- When we power the device Xbee modem did not respond, we saw transmit pulse on scope , but on RX line no data.

Observation 1:- Xbee modem on header socket, so i removed it on power on condition and insert it back , it works , we saw data on rx lines, i tried add decoupling caps , 0.01, 22pf on xbee 3.3V input. also add 22pf on uart line to ground, also add TVS on every entry.

Observation 2:- Xbee modem 5 no pin have reset input, if i manually ground this pin then it works , we saw data on rx lines. ( wire jumper method, in sch reset was not used, so i have to revise the PCB later)

Observation 3:- If I disconnect 3.3V form board 1 that goes to Xbee (cutting track), and connect 3.3V from board 2 to xbee modem via jumper wire. it works , we saw data on rx lines

Observation 4:- when i short both 3.3V form board1 and borad2 togethe, it works , we saw data on rx lines

Anyone have seen this type of problem before, can this be problem to short different 3.3V supply (i measure board 1 comes 3.296V and board 2 comes 3.316V)

  • \$\begingroup\$ I suspect that I might know the type of problem you have. However as I started to write a list of questions, I ran out of time and space in the comment field. It would take >> 1hr for me to clearly describe the full list of questions, tests and oscilloscope measurements for you to perform, but there is no guarantee that you would want to do that, making it a waste of time for me. So I was waiting to see if anyone else had more time to help you :-) \$\endgroup\$
    – SamGibson
    May 31 '17 at 11:04
  • \$\begingroup\$ I am not committing to a full investigation, but let's start with a few quick questions and see how things progress: (a) Do you have a 2 (or more) channel 'scope and experience using it? (b) Your new duplicate question (which will probably be closed) talks about the Tx signal. This question talks about the Rx signal. Please explain clearly what you mean. (c) Since your "Observation 3" shows that you can power the XBee using the 3.3V supply from "board 2", why doesn't your design do that anyway? \$\endgroup\$
    – SamGibson
    May 31 '17 at 11:04

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