NXP Semiconductors MWCT101 S Series, Reference Manual

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49.5 Functional description
The FlexCAN module is a CAN protocol engine with a very flexible mailbox system for
transmitting and receiving CAN frames. The mailbox system is composed of a set of
message buffers (MB) that store configuration and control data, time stamp, message ID,
and data (see
Message buffer structure ). The memory corresponding to the first 38 MBs
can be configured to support a FIFO reception scheme with a powerful ID filtering
mechanism, capable of checking incoming frames against a table of IDs (up to 128
extended IDs or 256 standard IDs or 512 8-bit ID slices), with individual mask register
for up to 32 ID filter table elements.
For Classical CAN frames, simultaneous reception through FIFO and mailbox is
supported. For CAN FD frames, reception is supported through mailboxes only. For
mailbox reception, a matching algorithm makes it possible to store received frames only
into MBs that have the same ID programmed in the ID field. A masking scheme makes it
possible to match the ID programmed on the MB with a range of IDs on received CAN
frames. For transmission, an arbitration algorithm decides the prioritization of MBs to be
transmitted based on the message ID (optionally augmented by 3 local priority bits) or the
MB ordering.
Before proceeding with the functional description, an important concept must be
explained. A message buffer is said to be active at a given time if it can participate in
both the matching and arbitration processes. An Rx MB with a 0b0000 code is inactive
(see
Table 49-10 ). Similarly, a Tx MB with a 0b1000 or 0b1001 code is also inactive (see
Table 49-11 ).
The FlexCAN module is also able to receive and transmit messages in CAN FD format.
The message buffers are sized to adequately store the quantity of data bytes selected by
the FDCTRL[MBDSRn] fields. The quantity of FD MBs available for a given quantity of
data bytes is described in the FDCTRL register. See also
FlexCAN memory partition for
CAN FD .
49.5.1 Transmit process
To transmit a CAN frame, the CPU must prepare a message buffer for transmission by
executing the following procedure:
1. Check whether the respective interrupt bit is set and clear it.
2. If the MB is active (transmission pending), write the ABORT code (0b1001) to the
CODE field of the Control and Status word to request an abort of the transmission.
Chapter 49 FlexCAN
MWCT101xS Series Reference Manual, Rev. 3, 07/2019
NXP Semiconductors 1657