最常见的固件更新协议

Question

I am supposed to pick (and may be implement) the firmware update protocol/software/procedure for the embedded device without USB and with limited program memory size. That device will work autonomously most of the time but once in a while a technician will be coming and updating the firmware.

What would be the most common choice for the update protocol if I wanted to use RS232 or CAN?

The requirements for the update are: complete after interrupted update (boot loader will be needed, I assume), small memory footprint, merge user settings with the newly introduced user data fields (in EEPROM), backup the previous version of the firmware with the possibility to roll the update back, safely update the boot loader itself.

It would be nice if the implementation of the boot loader and update client software existed already too (at least for Windows).

And just out of curiosity - are there any good alternatives to DFU for devices with USB?

Thanks in advance

Answer 1

I am not sure about "most common"; I am not sure anyone could answer that authoritatively or whether the answer is even useful. However I can tell you that I have implemented XMODEM-CRC/XMODEM-1K on a number of devices (ARM 7, ARM Cortem-M, PIC24, TI C55xx for example) in less than 4Kbytes. The bootloader sends an XMODEM start request on each port that is to support update, then for each port if a response is received within a short timeout (a few tens of milliseconds), then transfer continues. If no response is received the application is started normally.

complete after interrupted update (boot loader will be needed, I assume)

The approach I have taken is to not program the start address immediatly to flash on receipt but to copy it sideways and then program it last. The bootloader checks the start address on start-up and if it is 0xFFFFFFFF (ie not programmed) the transfer did not complete, and the bootloader restarts continuously polling for XMODEM start.

merge user settings with the newly introduced user data fields (in EEPROM),

In my case I have used Intel HEX files, but EEPROM memory is not commonly memory mapped. You could solve that by using a proprietary data format or set the address of the HEX data to an area that is invalid on the processor which the bootloader code will recognise as data to be sent to the EEPROM instead.

backup the previous version of the firmware with the possibility to roll the update back,

That is a function of the bootloader implementation rather than the protocol. It of course requires that you have space to store two copies of the application. The unused copy could possibly be zipped, but incorporating decompression in the bootloader will increase its size. A perhaps simpler and least costly approach would be to have the bootloader support output of the current application image via XMODEM allowing the back-up to be stored on the host. However by doing that you are potentially enabling a third party to access your code.

safely update the boot loader itself.

Again that is a function of your bootloader rather then the protocol. If the code runs from RAM (ie the bootloader is copied from ROM to RAM and executed, then it is straightforward. In this case it is safest if possible to load the entire bootloader data into RAM before programming flash memory in order to minimise the time the target has no bootloader and so that sucessful programming does not rely on the host connection being maintained throughout.

If however the bootloader runs from flash, replacing it from the bootloader itself is not possible. Instead you might load an application that the bootloader runs and which replaces the bootloader before loading (or reloading) the final application.

It would be nice if the implementation of the boot loader and update client software existed already too (at least for Windows).

Any terminal emulator software such as TeraTerm, Hyperterminal, PuTTY etc. will support XMODEM transfer. Implementing your own custom XMODEM sender is relatively straightforward with XMODEM source code widely available.

And just out of curiosity - are there any good alternatives to DFU for devices with USB?

What I have done is implement a CDC/ACM device class USB stack in the bootloader so that it appears to the host as a serial port, and then used the same XMODEM code as before to do the data transfer. This increases the size of the bootloader; in my case to about 12kbytes. It was implemented using a stack and CDC/ACM (virtual COM port) app-note provided by the chip vendor. Strictly speaking for this you will need a USB vendor-id (VID) registered to your company; you should not use just any old ID.