I have a trouble when i'm using SIM800L module with common ground with combinaison with : FTDI232+ARDUINO PRO and Li-po battery. I use common ground to allow Arduino see the serial of SIM800L.

I use battery (lipo) to power the SIM800L to allow 2A peak current.

Serial work but sometime the module reset(i think) and i see strange character. The SIM800L no work correctly.

I meet the same trouble when i use the FTDI's VCC, every time I use the common ground i have this problem.

Look the gif you can see the problem : gif trouble sim800L

I'm using this wiring Wiring sim800L

It's strange because when i connect the SIM800L directly with FTDI232 i have no problem all work fine.

I use this code to debug the Serial communication :

#include <SoftwareSerial.h>
SoftwareSerial serialSIM800(RX,TX)
//On Setup 

  //Being serial communication witj Arduino and SIM800
void loop(){

 if (serialSIM800.available())

 if (Serial.available()) {
      char test=Serial.read();

There are a few things that escape me that's why I need your lights, thanks by advance. Regards

  • Two suspicions can be investigated. First thing is Software Serial which is always a candidate for suspicion. I understand that you debug from the pc via FTDI. You maybe better debug using Arduino Mega since it has multiple hardware serials until you satisfied with your project and port the code to the Arduino Pro afterward. And second thing is If Arduino Pro version is 3v (you didn't mention) and causing logic levels issues.
    – Sener
    Apr 18, 2017 at 8:40
  • If I were you, I would start testing from lower rates like 9600 and make higher until the data is started to garble as you see.
    – Sener
    Apr 18, 2017 at 9:19
  • Btw, your wiring shows SIM900L not SIM800 which have slightly different wiring recommendations. Which one is yours?
    – Sener
    Apr 18, 2017 at 10:12
  • @Sener i do a mistake is really sim800l
    – kiki67100
    Apr 18, 2017 at 10:17
  • I use 1200 baud rate and i use github.com/nickstedman/SoftwareSerialWithHalfDuplex for Software Serial this no work i have the same problem
    – kiki67100
    Apr 18, 2017 at 10:52

1 Answer 1


I finally fixed my issue. I disable the auto baud rate of SIM800L trough FTDI232, with these at commands:

AT + IPR = 9600
AT & W

AT + IPR: Fix the baud rate at 9600 AT & W: Save in SIM800L

Then when i use with SoftwareSerial i have no issue anymore. When i reset the module with short connect RST with GND i have a good reply from SIM800L :


+CFUN: 1


Call Ready

SMS Ready

Thanks for your help.

Note : And be sure you are a good power supply and good wire.

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