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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
22-14
Freescale Semiconductor
22.3.3.3
Free Running Timer (CANx_TIMER)
CAN
x
_TIMER represents a 16-bit free running counter that can be read and written by the CPU. The timer
starts from 0x0000 after Reset, counts linearly to 0xFFFF, and wraps around.
The timer is clocked by the FlexCAN2 bit-clock (which defines the baud rate on the CAN bus). During a
message transmission/reception, it increments by one for each bit that is received or transmitted. When
there is no message on the bus, it counts using the previously programmed baud rate. During freeze mode,
the timer is not incremented.
The timer value is captured at the beginning of the identifier field of any frame on the CAN bus. This
captured value is written into the TIME STAMP entry in a message buffer after a successful reception or
transmission of a message.
Writing to the timer is an indirect operation. The data is first written to an auxiliary register and then an
internal request/acknowledge procedure across clock domains is executed. All this is transparent to the
user, except for the fact that the data will take some time to be actually written to the register. If desired,
software can poll the register to discover when the data was actually written.
27
LBUF
Lowest buffer transmitted first. This bit defines the ordering mechanism for message
buffer transmission.
0 Buffer with lowest ID is transmitted first
1 Lowest number buffer is transmitted first
28
LOM
Listen-only mode. Configures FlexCAN2 to operate in listen-only mode. In this mode, the
FlexCAN2 module receives messages without giving any acknowledge. It is not possible
to transmit any message in this mode.
0 FlexCAN2 module is in normal active operation, listen only mode is deactivated
1 FlexCAN2 module is in listen only mode operation
29–31
PROPSEG
[0:2]
Propagation segment. Defines the length of the propagation segment in the bit time. The
valid programmable values are 0–7.
1
One time quantum is equal to the S clock period.
Table 22-8. CANx_CR Field Descriptions (Continued)
Bits
Name
Description
Propagation Segment Time
(P 1)
Time Quanta
=
Time Quantum = one S clock period
Содержание MPC5553
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