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)