I'm using the A6 GSM module to send data from my sensors board to a server via GPRS.
The problem is that most of times, usually when the module attaches to the network or connects to the server, noise appears on the UART A6 TX line and the controller reads garbage. After this noise appears, the next command sent to the module gets a CME error response. I suspect the noise it generated by the chip, maybe caused by the RF transmitter.
Sometines it works, but most of the times it fails. I've tried putting 100pF capacitors to GND on the UART lines and lowering the baud rate to 2400 baud, but with the same results.
If it has anything to do with this problem, I mention that I am applying 3.3V level on the UART line (the A6 supports 2.8V), since I did not have a voltage translator available when I made the board.
Has anyone encountered this problem? Is there a solution or should I use a different GSM module?
The GSM module is a ready made breakout board from Ebay. The module is connected to my microcontroller board by wires. My power supply is a R-78E5.0-1.0 DC-DC converter that provides 5V/1A. I've tried powering the module from a separate 2.5A power supply, but the results are the same. These are my schematic and setup.
The SIM card is connected via an extender. I've also tried without the extender, but with the same results. In the setup you can also see the logic analyzer connected to the UART lines and GND.
This is the output from the logic analyzer (the last waveform is module TX analogic):
The noise consists of bursts of impulses, from 2.2V to 3.7V, or longer pulses from 1.9V to 3V. I usually receive CME Error 50 (EXE_FAIL) and CME Error 58 (which is not described in the module documentation).
I tried connecting the antenna via an RG316 cable coax cable and the noise disappeared. With or without shielded wires to the microcontroller board, there was no noise. Does anybody know why?