Interfacing between Nucleo(STM32F446RET) and A7672s (GSM Module)

[Nikitanagar]

I am working on interfacing an A7672S GSM module with an STM32F446RET microcontroller. I am using the STM32duino core, downloaded from the official STM32duino BoardManager Files. However, the GSM module is not responding to AT commands, and I don’t see any output on the Serial Monitor.

Hardware Setup:
Microcontroller: STM32F446RET
GSM Module: A7672S
Connections:
PA2 (TX) -> RX (GSM)
PA3 (RX) -> TX (GSM)
GND (STM32) -> GND (GSM)
Power supply for the GSM module is sufficient (verified).

Code:
Below is the code I am using:

HardwareSerial Serial2(PA3, PA2);

#define gsm Serial2

void setup()
{
  gsm.begin(115200);   // Setting the baud rate of GSM Module
  Serial.begin(115200);    // Setting the baud rate of Serial Monitor (Arduino)
  delay(100);
}


void loop()
{
  if (Serial.available() > 0)
    switch (Serial.read())
    {
      case 's':
        SendMessage();
        break;
      case 'r':
        RecieveMessage();
        break;
    }

  if (gsm.available() > 0)
    Serial.write(gsm.read());
}


void SendMessage()
{
  gsm.println("AT");
  gsm.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  gsm.println("AT+CMGS=\"+910000000000\"\r"); // Replace x with mobile number
  delay(1000);
  gsm.println("I am SMS from GSM Module");// The SMS text you want to send
  delay(100);
  gsm.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
}

void RecieveMessage()
{
  gsm.read();
  gsm.println("AT+CNMI=2,2,0,0,0"); // AT Command to receive a live SMS
  delay(1000);
}

Expected Behavior:
When I press 's' or 'r' in the Serial Monitor:
The GSM module should respond to AT commands.
SMS should be sent or received depending on the command.

Observed Behavior:
No response is observed in the Serial Monitor.
The GSM module appears unresponsive to AT commands.

Debugging Attempts:

• Verified the UART connections (TX ↔ RX).
• Tested the GSM module independently using a USB-to-serial adapter and verified it works.
• Ensured the GSM module has sufficient power supply.
• Checked the STM32 clock configuration in the Arduino IDE:
• Tools > U(S)ART Support: Enabled.

Questions:

1. Is the HardwareSerial configuration for Serial2 correct?
2. Could the baud rate mismatch be causing the issue? (The GSM module defaults to 115200.)
3. Is there any additional configuration required for the STM32F446RET to use HardwareSerial?
4. Are there any known issues with the STM32duino core for STM32F446 and GSM module communication?

Additional Information:

• Arduino IDE version: 2.3.4
• STM32duino core version: 2.9.0
• GSM module AT commands work correctly with other microcontrollers (tested with ESP32).

[fpistm]

Hi @Nikitanagar
Which board you used ? a custom board and then you used the generic target? Generic F446RETx?

I don't understand this:

Tools > CPU Speed: Set to 180 MHz.

This core does not provide such option. Are you sure you used this core?

[Nikitanagar]

yeah you are right, It was the mistake. I am using Nucleo32 which consist the STm32F446RET chip.

[fpistm]

It is a Nucleo 64 not 32.
So you have to Select the correct board in the menu:
Tools -> Boards: Nucleo-64
Then Tools -> Board part number: Nucleo F446RE

Your sketch will not build as the default Serial instance is mapped to Serial2 using PA3/PA2.

Arduino_Core_STM32/variants/STM32F4xx/F446R(C-E)T/variant_NUCLEO_F446RE.h

Lines 148 to 160 in ffb23d6

	// UART Definitions
	#ifndef SERIAL_UART_INSTANCE
	#define SERIAL_UART_INSTANCE 2 //Connected to ST-Link
	#endif
	// Default pin used for 'Serial' instance (ex: ST-Link)
	// Mandatory for Firmata
	#ifndef PIN_SERIAL_RX
	#define PIN_SERIAL_RX PA3
	#endif
	#ifndef PIN_SERIAL_TX
	#define PIN_SERIAL_TX PA2
	#endif

But your sketch use those pins for the GSM. So you have to instantiate an other one.

[fpistm]

Pay attention the pin D0 and D1 of the Arduino Uno connector are not connected to the pins PA2/PA3:

Those pins are connected to the STLink only .
You can use PA9 (D8) and PA10 (D2) for the GSM:

HardwareSerial gsm(PA10, PA9);

[Nikitanagar]

Yeah I configured the settings properly, then I did this. You are right, but it's not working. Don't know the proper issue, but yeah it's not working without mentioning the serialmon.

[fpistm]

With default option Serial works as expected. Hard to tell what is your issue.
Could you do a screenshot of the options?

[Nikitanagar]

yeah Here is the attached image:

[fpistm]

Ok.. disable the USB support then Serial will be on the USART...