AN4486

OTA bootloader service description

DocID026293 Rev 2

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3.

After connection, the master will send “GATT DISC CHARAC BY UUID” commands in

order to read all OTA bootloader characteristic handles.

4.

The master will read the Btl Image characteristic through a “GATT READ

CHARACTERISTIC VALUE” command to be aware of free space on the target slave

device's Flash memory.

5.

Based on the information in the previous step, the master will select an appropriate

image to send over-the-air. The candidate image (residing somewhere on the master

as a *.bin file) must fit within the target's free Flash range and must be 512-byte

aligned as requested by STM32L interrupt vector constraints. Once the selection is

made, the master sends a “GATT WRITE CHARACTERISTIC VALUE” in order to

write the image base address and size into the Btl new image characteristic and read

it back through “GATT READ CHARACTERISTIC VALUE” to verify it.

6.

The master writes into the Btl expected image sequence number characteristic

descriptor to enable slave notifications for image block sequence numbers and errors.

Once the slave receives this command it also erases pages within the Flash memory

range written within the Btl new image characteristic by the master, and sends back a

notification.

7.

The image transfer begins. The master will send the image in block of 16 bytes

through a sequence of “GATT WRITE WITHOUT RESPONSE” commands (one for

each 16-byte block). Each time a new WRITE command lands on the target slave it

will write the new block of data within the Btl new image content characteristic. Each

16-byte block comes along with a 2-byte long sequence number and one-byte long

checksum field in order to check for sequencing and message integrity at destination.

Every time the slave's internal buffer is filled up by 16 byte blocks, it will download into

Flash memory. Once the slave has completed the management of the current 16-byte

block, it will send a notification message back to the master providing the block

number of the next expected block. It might notify Flash write errors and Flash verify

errors as well, in which case the bootloading session should stop under the

assumption that we are dealing with issues on the destination device Flash.

8.

If the number of bytes downloaded successfully on the destination device Flash is

equal to the initially provided image size information, the bootloader will write the base

address and end address of the new image at the last two locations of EEPROM

memory (at address 0x08080FF8) and transfer control to the location containing the

firmware of the new application.

2.2

BLE OTA service bootloader architecture

The BLE OTA service bootloader architecture includes the following components:



Reset manager



This should start after reset and pass control to the latest updated & valid BLE

application.



BLE application with BLE OTA service This is the running BLE application which

allows to receive new application image packets over-the-air and stores them in a

specific Flash section.



New BLE application



this is the application image downloaded through RF link



To take full advantage of the BLE OTA service approach, the new BLE

application can be built with the BLE service in order to provide itself with

OTA service bootloader capability.