TCAN4550: Difference between buffers (regular, FIFO, queue)

Hi Laura,

The TCAN4550 uses the MCAN IP developed by Bosch, so I will refer you to the MCAN Users Manual as a reference for a deeper description of the MCAN features.  (Link)

But I will try to briefly explain it as well.  You can configure up to 32 "Transmit Buffer Elements" that will store a message that should be transmitted on the CAN bus by setting the corresponding bit in the TX Buffer Add Request (TXBAR) register.  The allocation of these 32 buffers as a FIFO, Queue, or Dedicated Buffer comes down to your preference on how you want to transmit the messages and work with the CAN FD protocol message arbitration.

A Dedicated TX Buffer is just what it sounds like, a dedicated buffer.  Once you write a message into the buffer and set the corresponding TXBAR bit, it will try to be transmitted.   If you have multiple message buffers and messages to transmit, you will need to manage the buffers and keep track of which buffer elements have been transmitted, and which ones are still pending.

A FIFO or Queue can also be used which makes multiple message transmission easier to manage.  You can use either the FIFO or the Queue but not both at the same time.  You can however still have some dedicated TX Buffer elements in addition to either the FIFO or Queue buffers.

A FIFO is as described, a First In First Out approach and the device will transmit the messages in the order they were placed in the FIFO regardless of the message ID.

A Queue is similar to a FIFO, but the device will first compare the message ID value for all messages that are in the Queue and prioritize the transmission of the message with the lowest message ID based on the CAN FD Arbitration protocol.  This ensures that the highest priority message is always transmitted out of the queue first, unlike with the FIFO which transmits them in the order they were placed in the FIFO.

Regards,

Jonathan

Hello Laura,

Only one message can be written to a TX Buffer at a time.  If you want to use the same TX Buffer for multiple messages, you would have to write the first message to the TX Buffer and set the corresponding TX Buffer Add Request bit.  Then the device will set the corresponding bit in the TX Buffer Request Pending while the device tries to transmit the message.  Depending on the bus activity, and the message ID used for the Arbitration, the required to transmit a message may vary.  However, once the message has successfully been transmitted, the TX Buffer Request Pending bit will be de-asserted and the corresponding bit in the TX Buffer Request Transmission Occurred register will be set to indicate a successful transmission.  You can also enable device interrupts to relay this information to the MCU.

Once the TX Buffer is available again, it can be reused for an additional message and the process would repeat.

If you need to send multiple messages at a time, you would need to have multiple TX Buffers configured so that you can write all the messages to their own TX Buffer and then set the corresponding TX Buffer Address Request Bits for all messages you want to transmit.  The TCAN4550 will then try to transmit all messages using the same sequence of events as I've described for a single message.

You can configure up to 32 TX Buffer Elements.  If they are all dedicated buffers, you can monitor the TX Buffer Request Pending register to determine which buffer elements are available and which ones still have messages pending transmission.  You can then determine where to store your next message for transmission.

if you configure the TX buffers for either a FIFO or a Queue, you will need to monitor the TX FIFO/Queue Status register to determine which buffer elements are occupied and where to place your next message for transmission through the Put Index.

You can also have a combination of dedicated buffers and the FIFO/Queue if you would like (but still only up to the 32 max).  An example of why you might want to have a dedicated buffer is if there is a specific message that is frequently sent that does not change value.  By storing it in a dedicated TX Buffer that you didn't overwrite, you simply resend the message without having to write it back into the buffer element through SPI every time you wanted to send it again.  You would simply need to set the TX Buffer Add Request bit for that buffer again to start a new transmission.

Regards,

Jonathan