MPC555
/
MPC556
CAN 2.0B CONTROLLER MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
16-18
for the TouCAN to set the STOPACK bit.
• To take the TouCAN out of low-power stop mode when the self wake mechanism
is enabled, write to CANMCR with both STOP and SELFWAKE clear, and then
wait for the TouCAN to clear the STOPACK bit.
• The SELFWAKE bit should not be set after the TouCAN 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 TouCAN may never set the STOPACK
bit, and the STOP bit will be cleared.
• To prevent old frames from being sent when the TouCAN 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 Tou-
CAN in low-power stop mode.
• If the TouCAN is in debug mode when the STOP bit is set, the TouCAN assumes
that debug mode should be exited. As a result, it tries to synchronize with the CAN
bus, and only then does it await the conditions required for entry into low-power
stop mode.
• Unlike other modules, the TouCAN does not come out of reset in low-power stop
mode. The basic TouCAN initialization procedure should be executed before
placing the module in low-power stop mode. (Refer to
• If the TouCAN is in low-power stop mode with the self wake mechanism engaged
and is operating with a single IMB clock per time quantum, there can be extreme
cases in which TouCAN wake-up on recessive to dominant edge may not con-
form to the CAN protocol. TouCAN synchronization is shifted one time quantum
from the wake-up event. This shift lasts until the next recessive-to-dominant edge,
which resynchronizes the TouCAN to be in conformance with the CAN protocol.
The same holds true when the TouCAN is in auto power save mode and awakens
on a recessive to dominant edge.
16.5.3 Auto Power Save Mode
Auto power save mode enables normal operation with optimized power savings. Once
the auto power save (APS) bit in CANMCR is set, the TouCAN looks for a set of con-
ditions in which there is no need for the clocks to be running. If these conditions are
met, the TouCAN stops its clocks, thus saving power. The following conditions activate
auto power save mode.
• No Rx/Tx frame in progress
• No transfer of Rx/Tx frames to and from a serial message buffer, and no Tx frame
awaiting transmission in any message buffer
• No CPU access to the TouCAN module
• The TouCAN is not in debug mode, low-power stop mode, or the bus off state
While its clocks are stopped, if the TouCAN senses that any one of the aforementioned
conditions is no longer true, it restarts its clocks. The TouCAN then continues to mon-
itor these conditions and stops or restarts its clocks accordingly.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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