I’ve received my first production run of the fdcanusb CAN-FD USB adapters and they are up for sale at mjbots.com!
While this is necessary for interacting with the moteus controller, it is also a fine general purpose CAN-FD adapter. At the moment, the USB interface is a platform independent line based serial one (Windows, Linux, MacOS). It doesn’t yet interoperate with SocketCAN on linux, but hopefully that will be resolved in the not too distant future.
One final piece of porting that needed to happen for the moteus controller r4.x series was the bootloader. The r3.x series has a bootloader, which allowed re-flashing the device over the normal data link, but that was largely specific to the RS485 and mjlib/multiplex framing format. Thus, while not particularly challenging, I needed to update it for the FD-CAN interface used on the r4.x board.
The update itself was straightforward: https://github.com/mjbots/moteus/compare/406f01…1123a9
For now, on the assumption I will in the not too distant future deprecate the r3.x series, just duplicated the entire bootloader, replacing all the communication bits with FDCAN and stm32g4 appropriate pieces. As before, this bootloader is designed to only operate after the normal firmware has initialized the device, and also is required to be completely standalone. To make code size easier to manage, it makes no calls to any ST HAL library and manipulates everything it needs purely through the register definitions.
I introduced the fdcanusb previously, now I’ll describe some of the process and challenges in getting it to work.
My initial challenges were around the PCB design and manufacturing. To begin with, my very first revision was sent out for manufacturing with the same incorrect pinout as the moteus controller r4.0 and thus was only really usable as a paperweight. Second, the supply of STM32G474 chips seems to be spotty now, so for r2 I had to scavenge chips from the boards that had broken pinouts.
Last time I had an actually hard problem…, a mis-aligned stack which caused randomly misbehaving software. This time, I had a more prosaic problem. One that while not necessarily as interesting, was even more time consuming and frustrating.
The serial port wouldn’t work. I had copied the module I used for DMA based receive and transmit from the STM32F4 and it just wasn’t working. Nothing was written and nothing was received. I carefully inspected the software many times. I looked at the registers in the debugger and nothing seemed obviously amiss. I read the datasheet to look for subtle differences in the theory of operation between the STM32F4 and STM32G4 but came up empty.
In my continued quest to bring up the STM32G4 for the moteus controller r4.1 and other efforts, I get to discover and learn about the million and one ways in which things don’t work on my path to find the one or two ways in which they can work. I recently had an interesting enough problem that it is worth it to signal amplify google searches on the matter for the next unfortunate soul.
