Arduino and SIM900 unavailable all the time

Question

I have the a problem with my arduino MEGA 2560 and the SIM900 module. I want to save the outputs of the AT commands, but I never get a chance. If I print mySerial.available() I only get 0. Sending SMS works fine on the other hand.

Initializing...
AT+CMGF=1
AT+CNMI=2,2,0,0,0
OK
Done
AT+CMGL="REC UNREAD"
OK
0
0
0
0
0
0
0
0
0
0
0

This is the sketch I use:

#include <SoftwareSerial.h>
//Create software serial object to communicate with SIM900
SoftwareSerial mySerial(11, 10); //SIM900 Tx & Rx is connected to Arduino #7 & #8
String incoming="";
String incoming2="";
String messages="Test";
String auth_num="+49xxxxxxxxx";
char incoming_char=0;
String response = "";
const byte numChars = 32;
char receivedChars[numChars];
boolean newData = false;
int period = 6000;
unsigned long time_now = 0;
void setup()
{
 //Begin serial communication with Arduino and Arduino IDE (Serial Monitor)
 Serial.begin(9600);
 //Begin serial communication with Arduino and SIM900
 mySerial.begin(9600);
 Serial.println("Initializing..."); 
 delay(1000);
 mySerial.println("AT+CMGF=1");
 mySerial.println("AT+CNMI=2,2,0,0,0"); // Decides how newly arrived SMS messages should be handled
 //mySerial.println("ATE0"); 
 updateSerial();
 Serial.println("Done");
 //sms_versenden("xxxxxxxx", "text");
}
void loop()
{
 updateSerial();
 Serial.println(mySerial.available());
 if(millis() - time_now > period){
 time_now = millis();
 if(sms_erhalten()){
 sms_autorisiert(); 
 }
 }
}
void updateSerial()
{
 delay(500);
 while (Serial.available()) 
 {
 mySerial.write(Serial.read());//Forward what Serial received to Software Serial Port
 }
 while(mySerial.available()) 
 {
 Serial.write(mySerial.read());//Forward what Software Serial received to Serial Port
 }
}
void sms_versenden(String rufnummer,String text)
{
 mySerial.println("AT"); //Handshaking with SIM900
 updateSerial();
 mySerial.println("AT+CMGF=1"); // Configuring TEXT mode
 updateSerial();
 if(rufnummer.startsWith("0")){
 rufnummer.remove(0,1);
 mySerial.print("AT+CMGS=\"+49");
 mySerial.print(rufnummer);
 mySerial.println("\"");
 }else if(rufnummer.startsWith("+49")){
 mySerial.print("AT+CMGS=\"");
 mySerial.print(rufnummer);
 mySerial.println("\"");
 }else{
 return;
 }
 updateSerial();
 mySerial.print(text); //text content
 updateSerial();
 mySerial.write(26);
}
boolean sms_erhalten()
{
 mySerial.print("AT+CMGL=\"REC UNREAD\"\r");
 //recvWithEndMarker();
 Serial.println(mySerial.available());
 delay(30);
 while (mySerial.available()){
 String inData = mySerial.readStringUntil('\n');
 Serial.println("Got reponse from SIM900: " + inData);
 }
 /*if(mySerial.available()>0){
 incoming = mySerial.readString();
 Serial.print(incoming); 
 delay(10);
 }*/ 
 updateSerial();
 if(incoming.startsWith("+CMGL:"))
 {
 return true; 
 }
 else{
 return false;
 }
}
void recvWithEndMarker() {
 static byte ndx = 0;
 char endMarker = '\n';
 char rc;
 while (mySerial.available() != 0 && newData == false) {
 rc = mySerial.read();
 updateSerial();
 if (rc != endMarker) {
 receivedChars[ndx] = rc;
 ndx++;
 if (ndx >= numChars) {
 ndx = numChars - 1;
 }
 }
 else {
 receivedChars[ndx] = '0円'; // terminate the string
 ndx = 0;
 newData = true;
 }
 }
}
void showNewData() {
 if (newData == true) {
 Serial.print("This just in ... ");
 Serial.println(receivedChars);
 newData = false;
 }
}
String getResponse() {
 while(mySerial.available()>0){
 response += mySerial.read();
 }
 return response;
}

Thanks in advance for every idea. Bravodelta

• Juraj: if you turn on echo, it doesn't print the commands back? check the wiring
• Bravodelta: @Juraj The echo is on and as you can see in the first code block I get an answer, but I can't save it in a String.
• Juraj: try mySerial.println("AT+CMGL=\"REC UNREAD\"");
• Bravodelta: @Juraj Sadly this doesn't work either

Answer 1

Rather than creating your own function used SerialEvent function in arduino. It acts as a serial interrupt in arduino. For example:

String inputString;
void serialEvent() {
 while (Serial.available()) {
 // get the new byte:
 char inChar = (char)Serial.read();
 // add it to the inputString:
 inputString += inChar;
 // if the incoming character is a newline, set a flag so the main loop can
 // do something about it:
 if (inChar == '\n') {
 Serial.Println(inputString);
 }
 }
 }

• Juraj: don't use serialEvent. it has no advantage. it is the same as checking Serial.available() in loop
• Majenko: The serial event system does not act as an interrupt. It is no different to adding if (Serial.available())... in your loop() function. I wish Arduino had never implemented it - it's a stupid idea.
• Vaibhav: @Juraj why no for using serialevent function? What harm does it causes?
• Vaibhav: @Majenko had you tried serialevent function?
• Majenko: @Vaibhav Worse than that - I have had to implement it on a compatible platform, and had to take a shower in sodium hydroxide afterwards to get the taint off my skin...