MC68336/376
CAN 2.0B CONTROLLER MODULE (TouCAN)
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
13-18
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
TouCAN in low-power stop mode.
If the TouCAN is in debug mode when the STOP bit is set, the TouCAN will
assume that debug mode should be exited. As a result, it will try to synchronize
with the CAN bus, and only then will 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
ization) should be executed before placing the module in low-power stop mode.
If the TouCAN is in low-power stop mode with the self-wake mechanism engaged
and is operating with a single system clock per time quantum, there can be ex-
treme cases in which TouCAN wake-up on recessive to dominant edge may not
conform to the CAN protocol. TouCAN synchronization will be shifted one time
quantum from the wake-up event. This shift lasts until the next recessive to dom-
inant 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.
13.6.3 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 will
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 CPU32 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/restarts its clocks accordingly.
13.7 Interrupts
The TouCAN is capable of generating one interrupt level on the IMB. This level is
programmed into the priority level bits in the interrupt configuration register (CANICR).
This value determines which interrupt signal is driven onto the bus when an interrupt
is requested.
When an interrupt is requested, the CPU32 initiates an IACK cycle. The TouCAN
decodes the IACK cycle and compares the CPU32 recognized level to the level that it