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.
3.5.5.1 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, for example, 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 M_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.
3.5.5.2 Dedicated Tx Buffers
Dedicated Tx Buffers are intended for message transmission under complete control of
the Host 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.
Chapter 3 Modular CAN (M_CAN)
MPC5777C Reference Manual Addendum, Rev. 1, 12/2015
Freescale Semiconductor, Inc. 109
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