Unable to send SMS from interrupt routine

Question

I am trying to develop a burglar alert system. I am using PIR sensor, Arduino Mega and SIM800L. I have connected PIR sensor to interrupt pin. Whenever motion is dected, a function corresponding to the interrupt will be called. In that function I am sending a SMS to notify with an alert message. If motion is detected 2 times, I will call the emergency contact number.

This is the code I have written.

#include <SoftwareSerial.h>
#define PIR1 2
#define SIREN 13
#define statusLED 12
SoftwareSerial SMS(11, 10);// rx(11-arduino) connect to tx(gsm module)
 // tx(10-arduino) connect to rx(gsm module)
String CONTACT1 = "xxxxxxxxxx"; //mobile number
boolean status;
byte motionCtr = 0;
unsigned long latestMovementTime = 0;
void setup() {
 pinMode(PIR1,INPUT);
 pinMode(SIREN,OUTPUT);
 pinMode(statusLED,OUTPUT);
 Serial.begin(9600);
 SMS.begin(9600); 
 SMS.println("AT+CNMI=2,2,0,0,0");
 delay(5000);
 activateSystem(0);
}
void loop() {
 unsigned long currentTime = millis();
 //If motion is not dected for 5 minutes, turn off the siren.
 if(currentTime - latestMovementTime > 300000 && motionCtr>=1){
 stopSiren(0); 
 }
 delay(500);
}
void activateSystem(byte manual){
 delay(60000); //warm up time for PIR sensor
 attachInterrupt(0,motionDetected,RISING);
 status = true; 
 digitalWrite(statusLED,HIGH);
 if(manual){
 Serial.println("System ACTIVATED by SMS.");
 notifyBySMS("System ACTIVATED SMS.");
 }else{
 Serial.println("System ACTIVATED.");
 notifyBySMS("System ACTIVATED.");
 }
}
void deactivateSystem(byte manual){
 detachInterrupt(0);
 status = false;
 digitalWrite(statusLED,LOW);
 if(manual){
 Serial.println("System DEACTIVATED by SMS.");
 notifyBySMS("System DEACTIVATED SMS.");
 }else{
 Serial.println("System DEACTIVATED.");
 notifyBySMS("System DEACTIVATED.");
 }
}
void motionDetected(){
 startSiren("Motion at bedroom door.");
}
void startSiren(String msg){
 //Record the time of latest motion dected.
 latestMovementTime = millis();
 motionCtr++;
 Serial.print("Ctr="+motionCtr);
 Serial.println(msg);
 notifyBySMS(msg);
 //Start the siren only if motion is detected 2 times
 if(motionCtr==2){
 digitalWrite(SIREN,HIGH); 
 Serial.println("Siren STARTED.");
 notifyBySMS("Siren STARTED.");
 call(CONTACT1);
// call(CONTACT2);
// call(CONTACT3);
 }
}
void stopSiren(byte manual){
 motionCtr = 0;
 Serial.println("Ctr="+motionCtr);
 digitalWrite(SIREN,LOW);
 if(manual){
 Serial.println("Siren STOPPED manually.");
 notifyBySMS("Siren STOPPED manually.");
 }else{
 Serial.println("Siren STOPPED");
 notifyBySMS("Siren STOPPED.");
 }
}
void sendSMS(String mobileNumber, String msg){
 Serial.println("Sending SMS to "+mobileNumber);
 //AT command for text mode of gsm module 
 SMS.println("AT+CMGF=1"); 
 delay(100); 
 SMS.println("AT+CMGS=\""+mobileNumber+"\"\r"); 
 delay(100);
 // the text you want to send
 SMS.println(msg);
 delay(100);
 // ASCII code of CTRL+Z
 SMS.println((char)26);
 delay(5000);
 Serial.println("SMS sent to "+mobileNumber);
}
void call(String mobileNumber){
 Serial.println("Calling..."+mobileNumber);
 SMS.println("AT+CMGF=0"); //AT command for PDU mode of gsm module 
 SMS.println("ATD"+mobileNumber+";");
 delay(45000);
 SMS.println("ATH");
}
void notifyBySMS(String msg){
 sendSMS(CONTACT1,msg);
 delay(1000);
// sendSMS(CONTACT2,msg);
// delay(1000);
// sendSMS(CONTACT3,msg);
// delay(1000);
}

I am receiving the SMS from setup(). But I am not receiving the SMS that I have sent from interrupt routine. Also I am receiving the call only sometime, it is random. I guess this is because I am using delay() in interrupt routine.

How can I implement such logic without delay().

• Majenko: Never use Serial in an interrupt!
• jose can u c: Serial and delay() both require interrupts in order to work right, but within an ISR, interrupts are already disabled.
• Gerben: Have the interrupt set a flag (boolean variable). Then in the loop, check that flag, do your actions and reset the flag. Using delay, serial, and softwareserial inside the ISR is not adviced.

Answer 1

As mentioned in the comments Serial won't work in an ISR. Generally, you should do as little as possible in an ISR. Often an ISR will just set a flag to be handled in loop()

You will then need to refactor loop() to handle the flag set in the ISR. A common method is to use a finite state machine.

• esoterik: don't forget to declare the flag volatile!