boot0启动的时候一直按 2 或者按 d，就能进入FEL烧录模式的原因

[memory]

void main(void)
{
        int dram_size;
        int status;
        phys_addr_t  uboot_base = 0, optee_base = 0, monitor_base = 0, \
                                rtos_base = 0, opensbi_base = 0;
        u16 pmu_type = 0, key_input = 0; /* TODO: set real value */

        sunxi_serial_init(BT0_head.prvt_head.uart_port, (void *)BT0_head.prvt_head.uart_ctrl, 6);
        printf("HELLO! BOOT0 is starting!\n");
        printf("BOOT0 commit : %s\n", BT0_head.hash);
        sunxi_set_printf_debug_mode(BT0_head.prvt_head.debug_mode);

        status = sunxi_board_init();
        if(status)
                goto _BOOT_ERROR;

        if (rtc_probe_fel_flag()) {
                rtc_clear_fel_flag();
                goto _BOOT_ERROR;
#ifdef CFG_SUNXI_PHY_KEY
        } else if (check_update_key(&key_input)) {
                goto _BOOT_ERROR;
#endif
        } else if (BT0_head.prvt_head.enable_jtag) {
                printf("enable_jtag\n");
                boot_set_gpio((normal_gpio_cfg *)BT0_head.prvt_head.jtag_gpio, 5, 1);
        }

#ifdef FPGA_PLATFORM
        dram_size = mctl_init((void *)BT0_head.prvt_head.dram_para);
#else
#ifdef CFG_DDR_SOFT_TRAIN
        if (BT0_head.prvt_head.dram_para[30] & (1 << 11))
                neon_enable();
#endif
        dram_size = init_DRAM(0, (void *)BT0_head.prvt_head.dram_para);
#endif
        if(!dram_size)
                goto _BOOT_ERROR;
        else {
                printf("dram size =%d\n", dram_size);
        }

        char uart_input_value = get_uart_input();

        if (uart_input_value == '2') {
                sunxi_set_printf_debug_mode(3);
                printf("detected user input 2\n");
                goto _BOOT_ERROR;
        } else if (uart_input_value == 'd') {
                sunxi_set_printf_debug_mode(8);
                printf("detected user input d\n");
        }

        mmu_enable(dram_size);
        malloc_init(CONFIG_HEAP_BASE, CONFIG_HEAP_SIZE);

lichee/brandy-2.0/spl/nboot/main/boot0_main.c