Arduino standalone & serial

Hi everyone, I'm facing a problem for which I did not found any answer on this forum. I have my arduino "talking" to a GPRS module through serial port (virtual ones with the SoftwareSerial library). When my ATMEL is on my UNO board, there is no problem, everything's working. But when I put the same ATMEL chip in my standalone circuit, the serial part does not work anymore. The rest of my prog (reading push button and blinking LEDs) is working well on my standalone circuit so I do not think I missed something general but maybe one pin needed to be pushed to 5V in order to have the serial working but I did not found anything about it in the atmel datasheet. Some of you have maybe get through that problem ? Just to add some information about my circuit (even if I don't see how can it be linked to that), my GPRS module read 3,5 to 4V where my arduino send 5V so the serial output of my arduino go through a voltage divider to becom 3,8V before arriving to the GPRS module. If somebody has a clue on my issue, any help or idea will help ! Thank you !

Sounds like some kind of wiring/power problem, but without being able to see a wiring drawing of what you built it’s hard to be of any help.

Lefty

but maybe one pin needed to be pushed to 5V in order to have the serial working

No there is nothing special you have to do to make it work. Are you sure the RX and TX are the right way round and the grounds are connected together (GPRS & arduino)?

Tybo:
If somebody has a clue on my issue, any help or idea will help !

This is quite funny… I have the exactly same problem. Just trying to communicate with a linbus tranceiver using the serial port. It works when the 328 is in the arduino board, but not when fitted as a stand-alone.

Please report back if you find a solution.

Does your standalone circuit use a 16MHz crystal or resonator? If not, then the UART baud rate will be wrong unless you have corrected for that.

How do you have the TxD signal from the GPS module wired to the RxD line of the MCU? Ideally you would shift and amplify it from 3.3v to 5v, but there are workarounds.

Did you remember to connect the AVcc pin to +5v even if you are not using analog inputs?