Arduino oddly waking from gsm message

Projects Programming
1

Hi everyone,
I'm playing with a gsm module (sim800L) and noticed a 'strange' thing happening while trying to put the arduino to sleep and handling a message received. What happens is that I put the arduino to sleep, it goes sleeping (using LowPower lib) but it wakes if it receives a message through the sim800 module. How is this happening, since I dont have any interrupt pins from the gsm to arduino, only GND, TX, RX ?

I known that the GPRS_Shield_Arduino library is using SoftwareSerial, but in which way could the SoftwareSerial wake the arduino ?

GPRS_Shield_Arduino Library link

#include <GPRS_Shield_Arduino.h>
#include <SoftwareSerial.h>
#include <LowPower.h>

#define PIN_TX    7
#define PIN_RX    8
#define BAUDRATE  9600

#define MESSAGE_LENGTH 160
char message[MESSAGE_LENGTH];
int messageIndex = 0;

char phone[16];
char datetime[24];

GPRS gprsTest(PIN_TX, PIN_RX, BAUDRATE); //RX,TX,PWR,BaudRate
#define LED 13

void setup()
{
  Serial.begin(9600);
  pinMode(LED, OUTPUT);
  digitalWrite(LED, LOW);
  while (!gprsTest.init()) 
  {
    Serial.print("init error\r\n");
    delay(1000);
  }
  delay(3000);
  Serial.println("Init Success, please send SMS message to me!");
}

void loop()
{
  sleep();
  readsms();
}

void readsms()
{
  messageIndex = gprsTest.isSMSunread();
  if (messageIndex > 0) //At least, there is one UNREAD SMS
  { 
    gprsTest.readSMS(messageIndex, message, MESSAGE_LENGTH, phone, datetime);
    //In order not to full SIM Memory, is better to delete it
    digitalWrite(LED, HIGH); //some visual indication that I received a message
    delay(3000);
    digitalWrite(LED, LOW);
    gprsTest.deleteSMS(messageIndex);
    Serial.print("From number: ");
    Serial.println(phone);
    Serial.print("Datetime: ");
    Serial.println(datetime);
    Serial.print("Recieved Message: ");
    Serial.println(message);
  }
}

void sleep()
{
  LowPower.powerDown(SLEEP_FOREVER, ADC_OFF, BOD_OFF);
}
2

SoftwareSerial uses a pin change interrupt. That will wake the processor.

3

thank you for clarifying it :slight_smile:
How would one then avoid it reading the message if it's sleeping ?

4

Disable it somehow. I don't know what library you're using but there must be some way to shut it off. Or you can do it directly by calling the SoftwareSerial stopListening() method.

5

that did the job :smiley: yay thanks!