Motorola MCF5281 用户手册

25-18

MOTOROLA

 

To exit low-power stop mode:

• Reset the FlexCAN either by asserting RSTI or by setting the SOFTRST bit 

CANMCR.

• Clear the STOP bit in CANMCR.
• The FlexCAN module can optionally exit low-power stop mode via the self-wake 

mechanism. If the SELFWAKE bit in CANMCR was set at the time the FlexCAN 
entered stop mode, then upon detection of a recessive to dominant transition on the 
CAN bus, the FlexCAN clears the STOP bit in CANMCR and its clocks begin 
running.

When in low-power stop mode, a recessive to dominant transition on the CAN bus causes
the WAKEINT bit in the error and status register (ESTAT) to be set. This event can generate
an interrupt if the WAKEMSK bit in CANMCR is set.

Consider the following notes regarding low-power stop mode:

• When the self-wake mechanism activates, the FlexCAN tries to receive the frame 

that woke it up. (It assumes that the dominant bit detected is a start-of-frame bit). It 
will not arbitrate for the CAN bus at this time.

• The CPU should disable all interrupts in the FlexCAN before entering low-power 

stop mode. Otherwise it may be interrupted while in STOP mode upon a non 
wake-up condition; If desired, the WAKEMASK bit should be set to enable the 
WAKEINT.

• If the STOP bit is set while the FlexCAN is in the bus off state, then the FlexCAN 

will enter low-power stop mode and stop counting recessive bit times. The count 
will continue when STOP is cleared.

• To place the FlexCAN in low-power stop mode with the self-wake mechanism 

engaged, write to CANMCR with both STOP and SELFWAKE set, then wait for the 
FlexCAN to set the STOPACK bit.

• To take the FlexCAN out of low-power stop mode when the self-wake mechanism 

is enabled, write to CANMCR with both STOP and SELFWAKE clear, then wait for 
the FlexCAN to clear the STOPACK bit.

• The SELFWAKE bit should not be set after the FlexCAN has already entered 

low-power stop mode.

• If both STOP and SELFWAKE are set and a recessive to dominant edge 

immediately occurs on the CAN bus, the FlexCAN may never set the STOPACK bit, 
and the STOP bit will be cleared.

• To prevent old frames from being sent when the FlexCAN awakes from low-power 

stop mode via the self-wake mechanism, disable all transmit sources, including 
transmit buffers configured for remote request responses, before placing the 
FlexCAN in low-power stop mode.