Common ground serial communication trouble SIM800L

Question

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 :

I'm using this wiring

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 
 Serial.begin(115200); 
  while(!Serial);

  //Being serial communication witj Arduino and SIM800
  serialSIM800.begin(115200);
  delay(1000);
  serialSIM800.print("AT\r"); 
  //[..]
void loop(){


 if (serialSIM800.available())
     Serial.write(serialSIM800.read());

 if (Serial.available()) {
    while(Serial.available()){
      Serial.print(">");
      char test=Serial.read();
      Serial.print(test);
      Serial.println();
      serialSIM800.write(test);
 }

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

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 :

RDY

+CFUN: 1

+CPIN: READY

Call Ready

SMS Ready

Thanks for your help.

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