MCF548x Reference Manual, Rev. 3
21-10 Freescale Semiconductor

21.3.2.3 FlexCAN Timer Register (TIMER)

This register represents a 16-bit free running counter that can be read and written to by the CPU. The timer

starts from 0x0000 after reset, counts linearly to 0xFFFF, and wraps around.

The timer is clocked by the FlexCAN 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.

6 BOFFREC Bus off recovery mode. This bit defines how FlexCAN recovers from bus off state. If this bit
is cleared, automatic recovering from bus off state occurs according to the CAN
Specification 2.0B. If the bit is set, automatic recovering from bus off is disabled and the
module remains in bus off state until the bit is cleared by the user. If the bit is cleared before
128 sequences of 11 recessive bits are detected on the CAN bus, then bus off recovery
happens as if the BOFFREC bit had never been set. If the bit is cleared after 128
sequences of 11 recessive bits occurred, then FlexCAN will re-synchronize to the bus by
waiting for 11 recessive bits before joining the bus. After negation, the BOFFREC bit can
be set again during bus off, but it will only be effective the next time the module enters bus
off. If BOFFREC was cleared when the module entered bus off, setting it during bus off will
not be effective for the current bus off recovery.
0 Automatic recovering from bus off-state enabled, according to CAN Spec 2.0 part B
1 Automatic recovering from bus off state disabled
5 TSYNC Timer synchronize mode. The TSYNC bit enables the mechanism that resets the
free-running timer each time a message is received in Message Buffer 0. This feature
provides the means to synchronize multiple FlexCAN stations with a special “SYNC”
message (global network time).
0 Timer synchronization disabled.
1 Timer synchronization enabled.
Note: There can be a bit clock skew of four to five counts between different FlexCAN
modules that are using this feature on the same network.
4 LBUF Lowest buffer transmitted first. This bit defines the ordering mechanism for message buffer
transmission.
0 Message buffer with lowest ID is transmitted first
1 Lowest numbered buffer is transmitted first
3 LOM Listen-only mode. This bit configures FlexCAN to operate in listen-only mode. In this mode,
transmission is disabled, all error counters are frozen and the module operates in a CAN
error passive mode [Ref. 1]. Only messages acknowledged by another CAN station will be
received. If FlexCAN detects a message that has not been acknowledged, it will flag a BIT0
error (without changing the REC), as if it was trying to acknowledge the message.
0 FlexCAN module is in normal active operation, listen-only mode is deactivated
1 FlexCAN module is in listen-only mode operation
2–0 PROPSEG Propagation segment. This 3-bit field defines the length of the propagation segment in the
bit time. The valid programmable values are 07.
Note: A time-quantum = 1 serial clock S clock period.

Table 21-3. CANCTRL Field Descriptions (Continued)

Bits Name Description
Propagation segment time (PROPSEG + 1) time-quanta=