nRF5340 unable to enter MCUboot serial recovery mode via UART

[Ki0]

I am trying to enter MCUboot serial recovery mode on an nRF5340 through a UART command.

I mainly followed the Nordic tutorial below:

https://academy.nordicsemi.com/courses/nrf-connect-sdk-intermediate/lessons/lesson-9-bootloaders-and-dfu-fota/topic/exercise-1-dfu-over-uart/

My goal is:

• The application receives a specific UART command
• The application sets bootloader mode
• The device reboots
• MCUboot enters serial recovery mode
• Firmware update is performed through MCUmgr over UART

Below are my current configurations.

- prj.conf

CONFIG_FLASH=y 
CONFIG_FLASH_MAP=y 
CONFIG_NVS=y 
CONFIG_IMG_MANAGER=y 
CONFIG_STREAM_FLASH=y 
CONFIG_DFU_TARGET=y 
CONFIG_DFU_TARGET_MCUBOOT=y 

CONFIG_BOOTLOADER_MCUBOOT=y 

CONFIG_ZCBOR=y 
CONFIG_BASE64=y 
CONFIG_CRC=y 

CONFIG_MCUMGR=y 
CONFIG_MCUMGR_GRP_IMG=y 
CONFIG_MCUMGR_GRP_OS=y 
CONFIG_MCUMGR_GRP_STAT=y 
CONFIG_REBOOT=y 
CONFIG_RETENTION=y 
CONFIG_RETAINED_MEM=y 
CONFIG_RETENTION_BOOT_MODE=y

- sysbuild.conf

SB_CONFIG_BOOTLOADER_MCUBOOT=y

- sysbuild/mcuboot.conf

CONFIG_LOG=y

CONFIG_MCUBOOT_LOG_LEVEL_INF=y

CONFIG_SERIAL=n

CONFIG_CONSOLE=n

CONFIG_UART_CONSOLE=n

CONFIG_PRINTK=n

CONFIG_USE_SEGGER_RTT=n

CONFIG_MCUBOOT_SERIAL=y

CONFIG_MCUBOOT_INDICATION_LED=y

CONFIG_BOOT_SERIAL_UART=y
CONFIG_BOOT_SERIAL_BOOT_MODE=y

- sysbuild/mcuboot.overlay

/ { 
    chosen { 
        zephyr,uart-mcumgr = &uart0; 
    }; 
    
    aliases { 
        mcuboot-led0 = &led1; 
        mcuboot-button0 = &button0; 
    }; 
    
    leds { 
        compatible = "gpio-leds"; 
        led1: led_1 { 
            gpios = <&gpio0 12 GPIO_ACTIVE_HIGH>; 
        }; 
    }; 
    
    buttons { 
        compatible = "gpio-keys"; 
        button0: button_0 { 
            gpios = <&gpio0 31 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>; 
            label = "Power Switch"; 
        }; 
    }; //end buttons 
};

- app.overlay

/ { 
    chosen { 
        zephyr,console = &uart0; 
        zephyr,uart-mcumgr = &uart0; 
        zephyr,boot-mode = &boot_mode0; 
    }; 
};

- main.c

static void nrf53_reset_work_handler(struct k_work *work); 
K_WORK_DELAYABLE_DEFINE(nrf53_reset_work, nrf53_reset_work_handler);

static void nrf53_reset_work_handler(struct k_work *work) { 
    sys_reboot(SYS_REBOOT_WARM); 
} 

int main(void) { 
        ... 
        ret = bootmode_set(BOOT_MODE_TYPE_BOOTLOADER); 
        if (ret) { 
                printf("bootmode set failed: %d\r\n", ret); 
            } 
            
            if ((ret = bootmode_check(BOOT_MODE_TYPE_BOOTLOADER)) < 1) { 
                printf("bootmode check not set bootmode %d\r\n", ret); 
                break; 
            } 
            printf("bootmode start\r\n"); 
            k_work_reschedule(&nrf53_reset_work, K_NO_WAIT); 
    ... 
}

Current behavior

After rebooting, I connect using AuTerm over UART and open:

MCUmgr → Images → Go

When I press the Go button, the following data appears repeatedly on the UART terminal:

\06	AAsIAAABAAEBAKCYMQ==
\06	AAsAAAABAAEBAKCxzg==
\06	AAsIAAABAAEBAKCYMQ==
\06	AAsAAAABAAEBAKCxzg==

However, MCUmgr always times out.

Expected behavior

I expected MCUboot serial recovery mode to respond correctly and allow image upload through MCUmgr over UART.

Questions

1. Is my boot mode configuration correct for entering MCUboot serial recovery mode?
2. Do I need additional configurations related to:
   • CONFIG_MCUMGR_TRANSPORT_UART
   • CONFIG_UART_MCUMGR
   • zephyr,boot-mode
   • retained memory
   • UART ownership between app and MCUboot
3. Is there something incorrect in my MCUboot UART configuration?
4. The repeated Base64-like strings appear when pressing the MCUmgr button. Does this indicate that:
   • the PC is transmitting correctly,
   • but MCUboot is not responding?
5. Are there additional required settings for nRF5340 specifically when using MCUboot serial recovery over UART?

Any advice would be appreciated.