CAN-FD Transceiver
All ECUs feature the same CAN-FD PHY (Physical layer) design, which is a simple CAN-FD transceiver IC, supported by bypass capacitors. The transceiver draws its power from the 5V line (VDD), but uses 3.3V for communication (Vio).
Most CAN transceiver ICs use identical packages and layout, and are therefore interchangeable. We built prototypes with either of the following components:
ATA6561-GAQW-N (BETA). It is qualified for automotive use (AEC-Q100), but its bitrate is limited to 5 Mbps.
MCP2558FD-H/SN (V1). It has a maximum bitrate of 8 Mbps, but is still pending AEC-Q100 qualification (?).
MCP2558FD-H/SN is our preferred choice, since it has a higher maximum bitrate.
The PHY Chip features a “STBY” (Standby) or “S” (Silent) pin to disable the transceiver:
When STBY/S is HIGH, the transceiver is disabled and the ECU is unable to communicate on the CAN-FD bus.
When STBY/S is LOW, the transceiver is enabled and the ECU can communicate on the CAN-FD bus
The STBY/S pin features an internal pull-up, meaning that the STBY/S pin must be actively set to LOW to enable communications. On a typical design, it is preferable to leave STBY/S pulled-up to make sure an unprogrammed ECU will not interfere with the CAN-FD bus. However, since RAMN uses the CAN-FD bus to program the ECUs from their (unprogrammed) factory state, the STBY/S pin is pulled-down externally, to allow the hardware built-in bootloader to communicate over CAN-FD.
Note
When an ECU is not used, It is preferable to program it to disable its CAN-FD transceiver.