Datasheet

Table 39-6. Possible Configurations for Frame Transmission
CCCR Tx Buffer Element Frame Transmission
BRSE FDOE FDF BRS
ignored 0 ignored ignored Classic CAN
0 1 0 ignored Classic CAN
0 1 1 ignored FD without bit rate switching
1 1 0 ignored Classic CAN
1 1 1 0 FD without bit rate switching
1 1 1 1 FD with bit rate switching
Note: AUTOSAR requires at least three Tx Queue Buffers and support of transmit cancellation
The Tx Handler starts a Tx scan to check for the highest priority pending Tx request (Tx Buffer with
lowest Message ID) when the Tx Buffer Request Pending register TXBRP is updated, or when a
transmission has been started.
39.6.6.1 Transmit Pause
The transmit pause feature is intended for use in CAN systems where the CAN message identifiers are
(permanently) specified to specific values and cannot easily be changed. These message identifiers may
have a higher CAN arbitration priority than other defined messages, while in a specific application their
relative arbitration priority should be inverse. This may lead to a case where one ECU sends a burst of
CAN messages that cause another ECU’s CAN messages to be delayed because that other messages
have a lower CAN arbitration priority.
If e.g. CAN ECU-1 has the transmit pause feature enabled and is requested by its application software to
transmit four messages, it will, after the first successful message transmission, wait for two CAN bit times
of bus idle before it is allowed to start the next requested message. If there are other ECUs with pending
messages, those messages are started in the idle time, they would not need to arbitrate with the next
message of ECU-1. After having received a message, ECU-1 is allowed to start its next transmission as
soon as the received message releases the CAN bus.
The transmit pause feature is controlled by bit CCCR.TXP. If the bit is set, the CAN will, each time it has
successfully transmitted a message, pause for two CAN bit times before starting the next transmission.
This enables other CAN nodes in the network to transmit messages even if their messages have lower
prior identifiers. Default is transmit pause disabled (CCCR.TXP = ‘0’).
This feature looses up burst transmissions coming from a single node and it protects against "babbling
idiot" scenarios where the application program erroneously requests too many transmissions.
39.6.6.2 Dedicated Tx Buffers
Dedicated Tx Buffers are intended for message transmission under complete control of the CPU. Each
Dedicated Tx Buffer is configured with a specific Message ID. In case that multiple Tx Buffers are
configured with the same Message ID, the Tx Buffer with the lowest buffer number is transmitted first.
If the data section has been updated, a transmission is requested by an Add Request via TXBAR.ARn.
The requested messages arbitrate internally with messages from an optional Tx FIFO or Tx Queue and
externally with messages on the CAN bus, and are sent out according to their Message ID.
SAM D5x/E5x Family Data Sheet
CAN - Control Area Network
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 1222