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# 1.4.2.1 Bootloader Trigger Methods

Bootloader can be invoked in number of ways:

-. Bootloader will run automatically if there is no valid application firmware. - Firmware is considered valid if the first word at application start address is not 0xFFFFFFFF

    - Normally this word contains initial stack pointer value for MCUs, so it will never be **0xFFFFFFFF** unless device is erased.

- Bootloader application can implement the bootloader_Trigger() function which will be called during system initialization

  - A GPIO pin can be used as an external trigger to invoke bootloader at startup.

  - Bootloader can run on application (internal) request if the configured number of bytes from start of SRAM are equal to some trigger pattern (For MCUs).

Example Implementation of bootloader_Trigger()

    #define BTL_TRIGGER_PATTERN  0x5048434D

    static uint32_t *ramStart = (uint32_t *)BTL_TRIGGER_RAM_START;

    bool bootloader_Trigger(void)
    {
        /* Check for Bootloader Trigger Pattern in first 16 Bytes of RAM to enter
         * Bootloader.
         */
        if (BTL_TRIGGER_PATTERN == ramStart[0] && BTL_TRIGGER_PATTERN == ramStart[1] &&
            BTL_TRIGGER_PATTERN == ramStart[2] && BTL_TRIGGER_PATTERN == ramStart[3])
        {
            ramStart[0] = 0;
            return true;
        }

        /* Check for Switch press to enter Bootloader */
        if (SWITCH_Get() == 0)
        {
            return true;
        }

        return false;
    }

Application code to trigger bootloader (For OTA bootloader and bootloaders for MPU devices)

    bool bootloader_Trigger(void)
    {
        /* Check for Switch press to enter Bootloader */
        if (SWITCH_Get() == 0)
        {
            return true;
        }

        return false;
    }

Application code to trigger bootloader

    void invoke_bootloader(void)
    {
        uint32_t *sram = (uint32_t *)BTL_TRIGGER_RAM_START;

        sram[0] = 0x5048434D;
        sram[1] = 0x5048434D;
        sram[2] = 0x5048434D;
        sram[3] = 0x5048434D;

        NVIC_SystemReset();
    }