Chapter 23 Controller Area Network (CAN).pdf

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23.3.11 Local Power-Down Mode
The DCAN supports a local power-down mode, which can be controlled within the DCAN control registers.
23.3.11.1 Entering Local Power-Down Mode
The local power-down mode is requested by setting the PDR bit in CAN Control register.
The DCAN then finishes all transmit requests of the message objects. When all requests are done, DCAN
waits until a bus idle state is recognized. Then it will automatically set the Init bit in CAN control register to
prevent any further CAN transfers, and it will also set the PDA bit in CAN error and status register. With
setting the PDA bits, the DCAN module indicates that the local power-down mode has been entered.
During local power-down mode, the internal clocks of the DCAN module are turned off, but there is a
wakeup logic (see Section 23.3.11.2) that can be active, if enabled. Also, the actual contents of the control
registers can be read back.
NOTE: In local low-power mode, the application should not clear the Init bit while PDR is set. If there
are any messages in the message RAM which are configured as transmit messages and the
application resets the init bit, these messages may get sent.
23.3.11.2 Wakeup From Local Power Down
There are two ways to wake up the DCAN from local power-down mode:
• The application could wake up the DCAN module manually by clearing the PDR bit and then clearing
the Init bit in CAN Control register.
• Alternatively, a CAN bus activity detection circuit can be activated by setting the wakeup on bus activity
bit ( WUBA) in the CAN control register. If this circuit is active, on occurrence of a dominant CAN bus
level, the DCAN will automatically start the wakeup sequence. It will clear the PDR bit in the CAN
Control register and also clear the PDA bit in the error and status register. The WakeUpPnd bit in CAN
error and status register will be set. If status interrupts are enabled, also an interrupt will be generated.
Finally the Init bit in CAN control register will be cleared.
After the Init bit has been cleared, the module waits until it detects 11 consecutive recessive bits on the
CAN_RX pin and then goes bus-active again.
NOTE: The CAN transceiver circuit has to stay active in order to detect any CAN bus activity while
the DCAN is in local power down mode. The first CAN message, which initiates the bus
activity, cannot be received. This means that the first message received in power-down and
automatic wake-up mode, is lost.
Figure 23-11 shows a flow diagram about entering and leaving local power-down mode.
4052 Controller Area Network (CAN) SPRUH73E–October 2011–Revised May 2012
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