Send/Receive SMS with STM32F103C8 and SIM800C GSM Module

embedded

ByPramoth Thangavel 0

Interfacing SIM800C GSM Module with STM32F103C8 for Sending & Receiving SMS

GSM Modules are frequently used in IoT projects since it has the ability to send and receive data remotely. The GSM Modules doesn’t have much dependencies like the Wi-Fi modules. While other wireless modules such Wi-Fi or Zigbee have cons such as short range and cost, the GSM Module have the reliability, long range Since only one SIM card is required with valid plan. Though the GSM modules cannot send a vast amount of data generated by sensors, it is ideal for applications where small reasonable amount of data to be sent.

In this tutorial, such GSM module will be interfaced with STM32F103C8 ARM microcontroller to send and receive Text Messages (SMS) from the cellular mobile number configured in the program. To display the messages received and sent, one 16x2 LCD will be used with two Push Buttons which will initiate sending and receiving of text messages after pressing the relative button.

There are already several GSM projects are available interfaced with different kind of microcontrollers. You can go and check all the GSM Projects here, and try DIY by referencing previous tutorials on different GSM modules such as SIM900, SIM900A, SIM800, etc. Some of the projects with GSM modules are given below:

 

Components Required

  • STM32F103C8 Cortex-M3 Microcontroller
  • GSM Module (SIM800C is used in the tutorial)
  • 16x2 LCD Display
  • Push Buttons (2)
  • 10k Potentiometer
  • Breadboard
  • Connecting Wires

 

What is SIM800C GSM Module?

SIM800C GSM Module

 

SIM800C is a widely used GSM Module with a serial interface modem which runs in between 3.4V-4.4V Voltage level. SIM800C is a Quad-band GSM/GPRS Module which is used in embedded applications where the remote data transfer is required. SIM800C works on 850/900/1800/1900MHz. It can also receive & transmit Voice Call, SMS with low power consumption. The module is controlled by using AT commands. It supports one SIM card interface and has UART (TX & RX) pins along with one RS232 Serial Protocol that can be used to interface with different microcontrollers in embedded applications.

Powering SIM800C GSM Module

A DC Power adapter of 12V is used to power the SIM800C GSM module.

 

Inserting SIM in SIM800C GSM Module

A SIM card is inserted at the back of the SIM800C GSM module. Note that the SIM800C doesn’t support 4G, so do not insert a 4G SIM Card.

SIM800C GSM Module PCB

 

Interfacing GSM SIM800C with STM32F103C8

In order to interface SIM800C with STM32F103C8 microcontroller, the UART Serial port will be used which is a hardware serial interface of the STM32F103C8. The below image indicates the UART pins in STM32F103C8 which are A9 and A10.

STM32 Pin Details

 

The STM32F103C8 has three UART interface for connecting three external serial peripherals.

 

AT Commands

The AT-commands will be used to access the functions of GSM Module such as sending and receiving Voice Calls, Text Messages. Some of the AT commands are given below which will be important in this tutorial and will be used frequently.

AT

Replies with OK for Acknowledgement

AT+CPIN?

Check signal Quality

AT+COPS?

Find service provider name

ATDXXXXXXXXXX;

Call to the specific number, ends with semi-colon,replace X with mobile number

AT+CNUM

Find the number of SIM card (might not work for some SIM)

ATA

Answer the Incoming Call

ATH

Hang off the current Incoming call

AT+COLP

Show incoming call number

AT+VTS=(number)

Send DTMF number. You can use any number on your mobile keypad for (number)

AT+CMGR

AT+CMGR=1 reads message at first position

AT+CMGD=1

Delete message at first position

AT+CMGDA=”DEL ALL”

Delete All messages from SIM

AT+CMGL=”ALL”

Read all messaged from SIM

AT+CMGF=1

Set SMS configuration. “1” is for text only mode

AT+CMGS = “+91 XXXXXXXXXX”

>CircuitDigest Text<Ctrl+z>

Sends SMS to a particular number here XXXXXXXXXX. When you see “>” start entering the text. Press Ctrl+Z to send the text.

AT+CNMI=2,2,0,0,0

To receive Live SMS

AT+CGATT?

To check for internet connection on SIM card

AT+CIPSHUT

To close TCP connection, meaning to disconnect form internet

AT+CSTT = “APN”,”username”,”Pass”

Connect to GPRS with your APN and Pass key. Can be obtained from Network Provider.

AT+CIICR

Check if SIM card has data pack

AT+CIFSR

Get IP of the SIM network

AT+CIPSTART = “TCP”,”SERVER IP”,”PORT”

Used to set a TCP IP connection

AT+CIPSEND

This command is used to send data to server

 

Circuit Diagram

Connections for connecting GSM with STM32 is shown in the circuit diagram below.

Circuit Diagram for Interfacing SIM800C GSM Module with STM32F103C8

 

Circuit connections between STM32F103C8 & GSM SIM800C

STM32F103C8

GSM SIM800C

PA9 (TX)

RX

PA10 (RX)

TX

GND

GND

 

Circuit connections between STM32F103C8 & 16x2 LCD

LCD Pin No

LCD Pin Name

STM32 Pin Name

1

Ground (Gnd)

Ground (G)

2

VCC

5V

3

VEE

Pin from Centre of Potentiometer for contrast

4

Register Select (RS)

PB11

5

Read/Write (RW)

Ground (G)

6

Enable (EN)

PB10

7

Data Bit 0 (DB0)

No Connection (NC)

8

Data Bit 1 (DB1)

No Connection (NC)

9

Data Bit 2 (DB2)

No Connection (NC)

10

Data Bit 3 (DB3)

No Connection (NC)

11

Data Bit 4 (DB4)

PB0

12

Data Bit 5 (DB5)

PB1

13

Data Bit 6 (DB6)

PC13

14

Data Bit 7 (DB7)

PC14

15

LED Positive

5V

16

LED Negative

Ground (G)

 

Two Push buttons with Pull down resistor of 10k is connected to the pins PA0 & PA1 of STM32 microcontroller. The complete setup will look lik below:

Circuit Hardware for Interfacing SIM800C GSM Module with STM32F103C8

 

Programming STM32F103C8 Microcontroller for GSM interfacing

STM32F103C8 microcontroller can be programmed using ARDUINO IDE. In this tutorial, FTDI or ST-LINK programmer is not needed to program the STM32. For uploading code to STM32F103C8, simply plug microUSB port to STM32, and USB port to PC by using USB cable and start writing code in ARDUINO IDE. In case of any doubt follow our previous tutorials on how to program STM32 without using any external FTDI or ST-LINK Programmer.

 

The complete code and the working video can be found at the end of this tutorial.

 

Start with including necessary libraries for peripherals used in this tutorial. Also define the pin configuration of LCD.

#include <LiquidCrystal.h>                           
const int rs = PB11, en = PB10, d4 = PB0, d5 = PB1, d6 = PC13, d7 = PC14;      
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

 

Next, initialise LCD selecting the function lcd.begin(16,2);  where (16,2) tells the LCD type such as 16x2 block LCD display. Just print a message to debug that if LCD is giving Output and interfaced properly.

lcd.begin(16,2);           
 lcd.print("STM32 WITH GSM");   
 lcd.setCursor(0,1);
 lcd.print("SIM800C");

 

Define the Push button data direction as Input Mode and define the pin number used.

pinMode(PA0,INPUT);       
pinMode(PA1,INPUT);

 

Select the baud rate used for serial communication.

Serial1.begin(9600);

 

There are two functions in the code one to send message and other one to receive message. Detailed explanation is given below.

 

SendMessage

This function is used to send SMS to a number. AT command is sent to GSM module indicating the message text mode and on which number the message needs to be sent.

The below statement sets the GSM module in the Text Mode by sending AT command (AT+CMGF=1).

Serial1.println("AT+CMGF=1");  //Sets the GSM Module in Text Mode

 

After that AT command (AT+CMGS) indicating send an SMS with the mobile number is sent to GSM module via serial1 port.

Serial1.println("AT+CMGS=\"+91XXXXXXXXXX\"\r"); // Replace x with mobile number

 

The messages are sent using AT command after every one second and then CLRL+Z needs to be sent so an ASCII code of CTRL+Z is sent via Serial1 port to GSM module.

delay(1000);
 Serial1.println("Hi Circuit Digest from GSM Module"); // The SMS text you want to send
 Serial1.println((char)26);// ASCII code of CTRL+Z

 

Then the “SMS sent” is displayed in the 16X2 LCD displayed.

lcd.print("SMS SENT");

 

ReceiveMessage

In this function, the messages are received and printed in the LCD display screen.

Serial1.println("AT+CNMI=2,2,0,0,0");

 

So, to receive live messages the above AT command is used.

After receiving the SMS, it contains a string available at the serial1 port which also have the other data in it such as the time, date etc. So, after occurrence of the sixth double quote (“) the rest all is the SMS received. Hence the other information is omitted and the remaining information that is the received message is displayed in the 16x2 LCD display.

while(1)
  {
           if(Serial1.available())
            {
                do
                  {
                     while ( !Serial1.available() );    
                  } while ( '"' != Serial1.read() );
               do
                  {
                      while ( !Serial1.available() );    
                  } while ( '"' != Serial1.read() );
               do
                 {
                      while ( !Serial1.available() );    
                  } while ( '"' != Serial1.read() );
               do
                 {
                    while ( !Serial1.available() );    
                 } while ( '"' != Serial1.read() );
                do
                  {
                     while ( !Serial1.available() );              
                  } while ( '"' != Serial1.read() );
                do
                 {
                    while ( !Serial1.available() );  
                  } while ( '"' != Serial1.read() );
                  while ( !Serial1.available() );
                  receive = Serial1.read();     
                  while ( !Serial1.available() );
                  receive = Serial1.read();
    lcd.clear();
    while(1)
    {
        while ( !Serial1.available() ); 
        receive = Serial1.read();
        if ( receive == '\r' )   
              break;
     else
             lcd.write(receive);
     }
}
}

 

Demo to Send and Receive Text Message using STM32

1. To send Text Message, simply press the Left Push button. The SMS will be sent to the mobile number entered in the code.

Testing SIM800C GSM Module with STM32F103C8 for Sending & Receiving Messages

 

2. To receive Text Message, simply press the Right Push button and the SMS will be received and will be displayed in the LCD display screen.

Interfacing SIM800C GSM Module with STM32F103C8 for Sending & Receiving SMS

 

So that’s all about interfacing GSM module with STM32F103C8 ARM microcontroller. In case of any doubts or suggestions, please feel free to comment.

Recommended TI Whitepapers

Code

#include <LiquidCrystal.h>                           //Library for LCD display                    

const int rs = PB11, en = PB10, d4 = PB0, d5 = PB1, d6 = PC13, d7 = PC14;      //Pins that are connected between LCD and STM32

LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

int receive = 0;

void setup()

{
  
  lcd.begin(16,2);           //LCD set at 16x2 mode
  
  pinMode(PA0,INPUT);       //Push buttons as INPUT pins
  pinMode(PA1,INPUT);
  
  lcd.print("STM32 WITH GSM");   //Display Welcome message
  lcd.setCursor(0,1);
  lcd.print("SIM800C");
  
  Serial1.begin(9600);   // Setting the baud rate of GSM Module 

  delay(1000);

  

  lcd.clear();

}

void loop()

{
 
  lcd.clear();
  lcd.setCursor(0,0);
  lcd.print("S to Send sms");
  lcd.setCursor(0,1);
  lcd.print("R to Receive sms");
  delay(100);
  
  
  int a = digitalRead(PA0);          //Read status of the push buttons
  int b = digitalRead(PA1);
  
  if (a == 1)                         // Depeding upon which push button is pressed the respected function is called
       {
         SendMessage();
       } 
       
else if( b == 1)
      {
         RecieveMessage();
      }
       

 
 void SendMessage()      //Function to Send Message
{
  lcd.clear();
  
  lcd.print("Sending sms");
  delay(1000);
  Serial1.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode

  delay(1000);  

  Serial1.println("AT+CMGS=\"+91XXXXXXXXXX\"\r"); // Replace x with mobile number

  delay(1000);

  Serial1.println("Hi Circuit Digest from GSM Module"); // The SMS text you want to send

  delay(100);

  Serial1.println((char)26);// ASCII code of CTRL+Z

  delay(1000);
  lcd.clear();
  lcd.print("SMS SENT");
  delay(1000);
}

 void RecieveMessage()         //Function to Receive Message

{
 
  lcd.clear();
  lcd.print("Receiving sms");

  Serial1.println("AT+CNMI=2,2,0,0,0"); // AT Command to recieve a live SMS

  delay(1000);
  
  while(1)
  {
           if(Serial1.available())
            {
                do

                  {

                     while ( !Serial1.available() );    

                  } while ( '"' != Serial1.read() );

  

               do

                  {

                      while ( !Serial1.available() );    

                  } while ( '"' != Serial1.read() );

  

               do

                 {

                      while ( !Serial1.available() );    

                  } while ( '"' != Serial1.read() );

    

               do

                 {

                    while ( !Serial1.available() );    

                 } while ( '"' != Serial1.read() );

                do
        
                  {

                     while ( !Serial1.available() );  
                         

                  } while ( '"' != Serial1.read() );

                do

                 {
             
                    while ( !Serial1.available() );  
                       

                  } while ( '"' != Serial1.read() );

                  while ( !Serial1.available() );

                  receive = Serial1.read();     

                  while ( !Serial1.available() );

                  receive = Serial1.read();

    
    lcd.clear();

    while(1)

    {

        while ( !Serial1.available() ); 

        receive = Serial1.read();

        if ( receive == '\r' )
           
              break;
           
        else

             lcd.write(receive);
     }
}
}
}

Video

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