Preface
MPC5565 Microcontroller Reference Manual, Rev. 1.0
Freescale Semiconductor
21-29
21.4.5
CAN Protocol Related Features
21.4.5.1
Remote Frames
A remote frame is a special kind of frame. The user can program a MB to be a request remote frame by
writing the MB as transmit with the RTR bit set to 1. After the remote request frame is transmitted
successfully, the MB becomes a receive message buffer, with the same ID as before.
When a remote request frame is received by FlexCAN, its ID is compared to the IDs of the transmit
message buffers with the CODE field ‘1010’. If there is a matching ID, then this MB frame will be
transmitted. Note that if the matching MB has the RTR bit set, then FlexCAN2 will transmit a remote
frame as a response.
A received remote request frame is not stored in a receive buffer. It is only used to trigger a transmission
of a frame in response. The mask registers are not used in remote frame matching, and all ID bits (except
RTR) of the incoming received frame should match.
In the case that a remote request frame was received and matched a MB, this message buffer immediately
enters the internal arbitration process, but is considered as normal TX MB, with no higher priority. The
data length of this frame is independent of the DLC field in the remote frame that initiated its transmission.
21.4.5.2
Overload Frames
FlexCAN2 will transmit overload frames due to detection of following conditions on CAN bus:
•
Detection of a dominant bit in the first/second bit of intermission
•
Detection of a dominant bit at the 7th bit (last) of end of frame field (RX frames)
•
Detection of a dominant bit at the 8th bit (last) of error frame delimiter or overload frame delimiter
21.4.5.3
Time Stamp
The value of the free running timer is sampled at the beginning of the identifier field on the CAN bus, and
is stored at the end of ‘move in’ in the TIME STAMP field, providing network behavior with respect to
time.
The free running timer can be reset upon a specific frame reception, enabling network time
synchronization. Refer to TSYN description in
Section 21.3.3.2, “Control Register (CANx_CR)
.”
21.4.5.4
Protocol Timing
The clock source to the CAN protocol interface (CPI) can be either the system clock or a direct feed from
the oscillator pin EXTAL. The clock source is selected by the CLK_SRC bit in the CAN_CR. The clock
is fed to the prescaler to generate the serial clock (SCK).
The FlexCAN2 module supports a variety of means to setup bit timing parameters that are required by the
CAN protocol. The CAN
x
_CR has various fields used to control bit timing parameters: PRESDIV,
PROPSEG, PSEG1, PSEG2 and RJW. Refer to
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