Teensy 3.6 and SIM800L EVB problem

[samkreuz]

I'm new to the forum so hello to everyone

I am having a problem using a SIM800L EVB module with a Teensy 3.6.

The SIM800 is powered with the same 5V power source as the Teensy and should
get enough current.
The module seems to work and connect to a network if I add a sim-card to it,


The problem I am facing is that I won't receive any RX data from the SIM800 and I don't know if the data I'm sending is received by the module.
I used the Serial1 port on the Teensy and a test script (posted below). I already tested the serial transmission from one serial port to another on the Teensy and it worked.
I couldn't find a datasheet for this exact module (DollaTek SIM800L V2.0 5V Wireless GSM GPRS Module Quad-Band W/Antenna Cable Cap M105) so I am not sure if the 3.3V coming from the Teensy might cause the problem.

Does anyone have an idea if the 3.3V could actually be the problem? Shouldn't I at least be able to receive messages as the Serial pins are 5V tolerant?

#define HWSERIAL Serial1

void setup() {
  Serial.begin(9600);
  HWSERIAL.begin(9600);
}

void loop() {
  int incomingByte;
        
  if (Serial.available() > 0) {
    incomingByte = Serial.read();
    Serial.print("USB received: ");
    Serial.println(incomingByte, DEC);
    HWSERIAL.print("USB received:");
    HWSERIAL.println(incomingByte, DEC);
  }
  if (HWSERIAL.available() > 0) {
    incomingByte = HWSERIAL.read();
    Serial.print("UART received: ");
    Serial.println(incomingByte, DEC);
    HWSERIAL.print("UART received:");
    HWSERIAL.println(incomingByte, DEC);
  }
}

 

[BriComp]

You have no connection to 5v, GND nor VDD.
You may find this useful. It has connection details and suggested software.
Be aware that the software is intended for Arduino, so modify accordingly...around Serial connections.

 

[samkreuz]

@BriComp Thanks for your help.
You are right. On the photo above I have wired up RX and TX incorrectly. VDD, the second ground and RST are not connected at the time. Although nothing changes if VDD is connected to Teensys 3.3V output and all grounds are connected to the ground of the power supply. I tried the code from the link but it looks like no text messages are being sent.

 

[samkreuz]

Okay, I solved the problem. The code above did not send any valid commands to the SIM800, which is why I didn't get a response. It also looks like the SIM800 only transmits data if you send data first.

It also looks like NL & CR must be active on the Serial Monitor when sending data.