MAC7200 datasheet(598/976 Pages) FREESCALE

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The Generic FlexCAN2 Module

MAC7200 Microcontroller Family Reference Manual, Rev. 2

520

Freescale Semiconductor

28.2.6.3.3

Free Running Timer (TIMER)

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

starts from $0000 after Reset, counts linearly to $FFFF, 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.

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.

TSYN

Timer Sync Mode. This bit enables a mechanism that resets the free-running timer each time a message

is received in Message Buffer 0. This feature provides means to synchronize multiple FlexCAN stations

with a special “SYNC” message (i.e., global network time).

0 Timer Sync feature disabled

1 Timer Sync feature enabled

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

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 Listen Only Mode is deactivated

1 FlexCAN module operates in Listen Only Mode

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 0–7.

Propagation Segment Time = (PROPSEG + 1) * Time-Quanta.

Time-Quantum = one Sclock period.

1. One time quantum is equal to the Sclock period.

Table 28-10. CTRL Field Descriptions (Continued)

Field

Description

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