After getting the power to work, the next step in bringing up the new quad’s raspberry pi interface board is getting the FDCAN ports to work. As described in my last roadmap, this board has multiple independent FDCAN buses. There are 2 STM32G4’s each with 2 FDCAN buses so that every leg gets a separate bus. There is a 5th auxiliary bus for any other peripherals driven from a third STM32G4. All 3 of the STM32G4’s communicate with the raspberry pi as SPI slaves.
Making this work was straightforward, if tedious. I designed a simple SPI based protocol that would allow transmission and receipt of FD-CAN frames at a high rate in a relatively efficient manner, then implemented that on the STM32s. On the raspberry pi side I initially used the linux kernel driver, but found that it didn’t give sufficient control over hold times during the transmission. Since the SPI slave is implemented in software, I needed to leave sufficient time after asserting the chip select and after transmitting the address bytes. The kernel driver gives no control over this at all, so I resorted to directly manipulating the BCM2837s peripheral registers and busy loop waiting in a real time thread.
After a decent supply of bugs were squashed, I got to a point where the host could send off 12 queries to all the servos with the four buses all being used simultaneously, then collating the responses back. I haven’t spent much time optimizing the cycle time, but the initial go around is at around 1.0ms for a query of all 12 devices which is about 1/3 of the 3.5ms I had in the previous single-bus RS485 version.
Here’s a scope trace of a full query cycle with 2 of the 4 CAN buses on the top, and the two chip selects on the bottom. Woohoo!
