I'm working on a modular CANBUS system for offroad racing. The idea right now is that the main Arduino (1284p at the moment) will be the only one connected to CANBUS, and other modules will have their own atmega's that will communicate with the master over I2C. They will not be physically on the same PCB, but will be physically within inches of each other. One module will have 8 MAX31865 IC's for interfacing with RTD's. The 31865 is not compatible with 5V microcontrollers, which right now we planned on running the Atmega's at 5V. Since the IC's are SPI, they can all share a MOSI, MISO, and CLK bus, so only two channels of a hex converter. For the Chip Select pin, would it be acceptable to use a resistor to decrease the voltage instead of using an additional 8 channels of hex converting?
What would the upsides/downsides be of using an atmega running at 3.3v for the module with the MAX31865's and just using a hex converter on SDA/SCL for the I2C communication to the master? What atmega I use for this is flexible.