Reset Handler in ARM Cortex-M: What It Is and How It Works

The reset handler in ARM Cortex-M is a special function that runs automatically when the processor starts or resets. It sets up the system by initializing memory and hardware before the main program runs.

How It Works

The reset handler acts like the starting point of a Cortex-M microcontroller after power-up or a reset event. Imagine it as the 'wake-up routine' that prepares everything before the main program begins. When the processor resets, it looks at a fixed memory location called the vector table to find the address of the reset handler.

Once found, the processor jumps to this reset handler code. This code typically initializes important parts like the stack pointer, copies initialized data from flash memory to RAM, clears uninitialized data areas, and sets up hardware clocks or peripherals. After these steps, it calls the main program function to start normal operation.

Example

This example shows a simple reset handler in C for an ARM Cortex-M microcontroller. It initializes memory and then calls the main function.

c

void Reset_Handler(void) {
    extern unsigned int _sidata; // start of initialized data in flash
    extern unsigned int _sdata;  // start of data section in RAM
    extern unsigned int _edata;  // end of data section in RAM
    extern unsigned int _sbss;   // start of bss section in RAM
    extern unsigned int _ebss;   // end of bss section in RAM

    unsigned int *src = &_sidata;
    unsigned int *dest = &_sdata;

    // Copy initialized data from flash to RAM
    while (dest < &_edata) {
        *dest++ = *src++;
    }

    // Clear the bss section (zero-initialize)
    dest = &_sbss;
    while (dest < &_ebss) {
        *dest++ = 0;
    }

    // Call the main program
    main();

    // If main returns, loop forever
    while (1) {}
}

int main(void) {
    // Your application code here
    return 0;
}

Output

No direct output; system initializes and runs main function.

When to Use

The reset handler is essential in every ARM Cortex-M based embedded system. It is used automatically at startup or after a reset to prepare the system for running your application code. You do not call it manually; the processor does this for you.

Use the reset handler to ensure your program's memory and hardware are correctly set up. For example, if you write firmware for a microcontroller controlling a robot or sensor, the reset handler makes sure everything is ready before your control code runs.

Key Points

• The reset handler runs first after power-up or reset.
• It initializes memory sections like data and bss.
• It sets up hardware and system state before main runs.
• The processor finds the reset handler address in the vector table.
• It is critical for reliable embedded system startup.

Key Takeaways

The reset handler is the first code run after an ARM Cortex-M reset.

It prepares memory and hardware before your main program starts.

The processor uses the vector table to find the reset handler address.

Proper reset handler setup is vital for stable embedded systems.

You typically write or customize the reset handler in embedded firmware.