firmware-update-patterns

firmware-update-patterns

Knowledge Base

FOTA patterns for MCU firmware updates with reliability guarantees.

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Pattern 1: mcuboot Image Header Construction

/* Build tool (Python/imgtool) writes this header before the firmware binary */
/* MCU application starts at header_base + header_size (default 0x20 = 32 bytes) */

#define MCUBOOT_MAGIC  0x96f3b83dUL

typedef struct __attribute__((packed)) {
    uint32_t magic;        /* 0x96f3b83d */
    uint32_t load_addr;    /* 0: XIP from flash */
    uint16_t hdr_size;     /* 32 bytes */
    uint16_t protect_tlv;  /* 0 */
    uint32_t img_size;     /* Firmware body bytes */
    uint32_t flags;        /* IMAGE_F_* flags */
    uint8_t  img_ver[8];   /* major, minor, revision[2], build_num[4] */
    uint32_t _pad;
} mcuboot_header_t;        /* Exactly 32 bytes */

/* Verify on bootloader side: */
bool mcuboot_verify_header(const void *slot_base)
{
    const mcuboot_header_t *hdr = slot_base;
    return hdr->magic == MCUBOOT_MAGIC
        && hdr->hdr_size == 32U
        && hdr->img_size > 0U
        && hdr->img_size < MAX_IMAGE_SIZE;
}

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Pattern 2: FOTA Progress Tracking via MQTT

# Python host-side: publish firmware chunks, track progress
import paho.mqtt.client as mqtt
import struct, hashlib

CHUNK_SIZE = 1024
TOPIC_CMD  = "device/{id}/ota/cmd"
TOPIC_DATA = "device/{id}/ota/data"
TOPIC_ACK  = "device/{id}/ota/ack"

def send_firmware(client, device_id, fw_bytes):
    total = len(fw_bytes)
    sha256 = hashlib.sha256(fw_bytes).hexdigest()

    # Start command
    client.publish(TOPIC_CMD.format(id=device_id),
        f'{{"cmd":"start","size":{total},"sha256":"{sha256}"}}')

    # Send chunks
    for i, offset in enumerate(range(0, total, CHUNK_SIZE)):
        chunk = fw_bytes[offset:offset+CHUNK_SIZE]
        hdr = struct.pack(">IHH", offset, len(chunk), i)
        client.publish(TOPIC_DATA.format(id=device_id), hdr + chunk)

    # Finish
    client.publish(TOPIC_CMD.format(id=device_id), '{"cmd":"apply"}')