Adafruit NRF52 Boot Loader 启动流程

上一篇文章介绍了一下在NRF52840上运行zephyr application。在zephyr 运行之前，NRF52840会先运行boot loader，同时boot loader也提供了DFU的功能。接下来介绍一下整个boot loader的flow。有兴趣的可以去github下载源码研究。

GitHub - adafruit/Adafruit_nRF52_Bootloader: USB-enabled bootloaders for the nRF52 BLE SoC chips

DFU (Device Firmware Update)

DFU模式是用来对系统的firmware进行更新的模式。以“SuperMini”NFRF52840开发板为例，连续按两次reset可以进入DFU模式。开发板会模拟一个USB 驱动器，直接把zephyr.uf2拷贝到USB驱动器就能开始firmware update。

Boot Loader Flow

首先运行“thumb_crt0.s”汇编代码，thumb_crt0.s的作用是初始化硬件和运行c需要的栈空间，最后跳转到main函数入口。

main
    board_init  //initialize board hardware
    bootloader_init  
    check_dfu_mode  // Check if we need to enable DFU mode
        // Read reset pin status. If reset is pressed, reason_reset_pin = 1
        reason_reset_pin = (NRF_POWER->RESETREAS & POWER_RESETREAS_RESETPIN_Msk) ? true : false;
        // If (*dbl_reset_mem) == DFU_DBL_RESET_MAGIC, it means this is the 2nd reset
        dfu_start = ((*dbl_reset_mem) == DFU_DBL_RESET_MAGIC) && reason_reset_pin;

        if (reason_reset_pin)
            // Register our first reset for double reset detection
            (*dbl_reset_mem) = DFU_DBL_RESET_MAGIC;

            // Wait for 2nd reset
            NRFX_DELAY_MS(DFU_DBL_RESET_DELAY);
            // if RST is pressed during this delay (double reset), the below code will not be executed. 

        (*dbl_reset_mem) = 0; // Clear reset info
        
        if (dfu_start)
            // Initialize Tiny USB Driver which calls proc_read10_cmd or proc_write10_new_data to read/write DFU USB drive. 
            // When OS read block, "read_block" returns emulated USB drive info such as label, size. 
            // When OS write block (copy firmware to USB drive), USB driver will write firmware to flash.
            usb_init
                usb_desc_init
                uf2_init
                tusb_init 
    
    if (bootloader_app_is_valid() && !bootloader_dfu_sd_in_progress())
        bootloader_app_start
            bootloader_util_app_start  // Run Zephyr OS

    NVIC_SystemReset

当按下两次reset，boot loader进入DFU模式。用户拷贝.uf2格式的firmware到USB drive。 USB driver完成firmware的flash write。

如果只有一次reset，进入zephyr。

Adafruit NRF52 Boot Loader 启动流程

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