Posted on April 17, 2015 at 19:53The FFA1R CAN register selects either FIFO 0 or FIFO 1 for a filter. There's no way to select both since it's a single bit. I don't understand how you can direct messages to both FIFOs? They will go to one or the other FIFO depending on filter match.
If you want a single message stream then both FIFO interrupts will have to write messages to the same queue set up by your program. Your CAN protocol listener pulls each message off a single queue, no priority for one FIFO over the other.
Be aware what you are doing is the opposite of how CAN is supposed to work. The two FIFOs are there for a reason. The CAN hardware resolves collisions by ensuring higher priority messages are sent first. High priority messages (emergencies, safety, synchronization, heartbeats) need to be processed as they arrive, not in order, since they are time critical. One FIFO is dedicated to high priority, the other to standard priority messages (data transfers). You can still merge messages into a single queue for the protocol listener but high priority arrivals are inserted at the beginning of the queue, standard arrivals at the end.
If you don't have a message queue you can still merge by polling:
set a flag for next FIFO to poll, start at 0
loop: poll FIFO selected by flag
if data ready, read message, toggle the FIFO polling flag, restart loop
if no data poll the FIFO not selected by flag
if second FIFO has data, get message, no change to polling flag
loop
That way you always poll the other FIFO after receiving a message so each one has equal priority. This isn't the recommended way for any net stack though, far better to have a message queue feeding a network stack task.
Jack Peacock
