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A tiny CAN bootloader for AVR and MCP2515
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CANboot

About

CANboot is a tiny (440 bytes) CAN-enabled bootloader for AVRs (the current version is mildly specific to the ATmega88a).

It was originally written because our Hackerspace automation project needed a CAN bootloader, but existing bootloaders took up too much space (CANboot fits in a 256 word bootloader section, whereas others require at least 512 words). It can do flashing and verification, but there would probably be enough space to squeeze in EEPROM reading/writing, but that was not needed as of yet.

A daemon for answering to flash requests using the Linux SocketCAN-framework is included.

If you want to use the bootloader and it doesn’t work for you (or your μC) or you require additional features (like EEPROM support), feel free to contact me.

CANboot and bootflashd are BSD-licensed. If you use this software, I would be happy if you dropped me a line what you use it for.

Booting

CANboot checks whether it should just jump to the main program or request flashing by reading the value of the first digital input of the MCP2515.

Using Bootflashd

bootflashd requires two arguments: A SocketCAN interface to listen on and a directory containing firmware files. It will not daemonize itself. I recommend you use Runit or something similar.

When a boot request comes in, bootflashd will look for a file named like the CAN identifier (padded with 0s to 8 nibbles) that send the message, suffixed by .hex. If such a file exists, it is flashed to the μC. Either way, a reset is sent afterwards.

Firmware files

The firmware file format is a modified Intel HEX format. It may additionally contain comments (prefixed by #) and must contain a microcontroller type specification (like C atmega88) before the data starts. Currently, the only supported type is atmega88.

Other than that, only data records and the EOF record are supported.

Protocol

Addressing

Our CAN network has a custom addressing scheme, but it’s not hardwired into the bootloader. However, it only supports extended IDs (but changing that should not be all too difficult). All messages pertaining to a boot transaction use the same address (no matter whether the were sent from the device or the master), this address can be set during compilation like this:

make BOARD_ADDR=123ABCDE

Messages

Because it enabled some space-saving tricks with the MCP2515, the message format is slightly baroque, but you should never have do interact with it directly.

Firstly, when the bootloader starts, it sends out a CAN frame containing the 8 Bytes BOOTMAGC, requesting flashing. Other than that, the bootloader never sends out any frames without a prior request by the master.

Also, again to save size, the bootloader counts on the messages to be sensible. For example, a Set Z-pointer-Message that is followed by a page write must always set the Z Pointer to a page boundary.

Set Z-pointer (0x01)

01 ZH ZL

Sets the Z-pointer (the offset for all reading and writing operations) to (ZH << 8 | ZL).

Read flash (0x02)

02 length

Reads length (0 ≤ length ≤ 8) Bytes starting from the current Z-pointer position and advances it accordingly. An answer message containing only the results is sent.

Erase flash (0x03)

03

Erases the currently selected page.

Load buffer (0x04)

04 count d1 .. dN

Loads count words (a word is 2 Bytes!) into the current page buffer. N = 2 * count. The Z-pointer is advanced accordingly.

Write flash (0x05)

05

Stores the data in the page buffer to the flash. The Z-pointer is reset to the value it had during the last erase command.

Reset (0x06)

06

Jumps to the main program. Note that this is a simple jump. The watchdog reset method is intransparent to the main program on newer AVRs (like the ATmega88) since it remains active after triggering.

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