user32892: 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

user32892: 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. 

32892: <p>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.</p>

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

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

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

<p>Look the gif you can see the problem : 
<a href="https://i.stack.imgur.com/O7YCD.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/O7YCD.gif" alt="gif trouble sim800L"></a></p>

<p>I'm using this wiring <a href="https://i.stack.imgur.com/F4eeV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/F4eeV.jpg" alt="Wiring sim800L"></a></p>

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

<p>I use this code to debug the Serial communication :</p>

<pre><code>#include &lt;SoftwareSerial.h&gt;
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("&gt;");
      char test=Serial.read();
      Serial.print(test);
      Serial.println();
      serialSIM800.write(test);
 }
</code></pre>

<p>There are a few things that escape me that's why I need your lights, thanks by advance.
Regards</p>
 32892: <p>I finally fixed my issue.
I disable the auto baud rate of SIM800L trough FTDI232, with these at commands:</p>

<pre><code>AT + IPR = 9600
AT &amp; W
</code></pre>

<p>AT + IPR: Fix the baud rate at 9600
AT &amp; W: Save in SIM800L</p>

<p>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 : </p>

<pre><code>RDY

+CFUN: 1

+CPIN: READY

Call Ready

SMS Ready
</code></pre>

<p>Thanks for your help.</p>

<p>Note : And be sure you are a good power supply and good wire. </p>
