MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA
33742
33
Figure 20
illustrates how the wake-up signal is generated.
First the CAN signal is detected by a low consumption receiver
(WU receiver). Then the signal passes through a pulse width
filter, which discards the undesired pulses. The pulse must
have a width bigger than 0.5
μ
s and smaller than 500
μ
s to be
accepted. When a pulse is discarded, the pulse counter is reset
and no wake-up signal is generated. When a pulse is accepted,
the pulse counter is incremented and, after three pulses, the
internal wake-up signal is asserted.
Each one of the pulses must be spaced by no more than
500
μ
s. If not, the counter will be reset and no wake-up signal
will be generated. This is accomplished by the wake-up timeout
generator. The wake-up cycle is completed (and the wake-up
flag reset) when the CAN interface is brought to CAN Normal
mode.
Figure 20. Wake-Up Functional Block Diagram
CAN Wake-Up Report
The CAN wake-up reports depend upon the 33742 low
power mode.
If the 33742 is placed into Sleep mode (V
DD
and V2 off), the
CAN wake-up or any wake-up results in V
DD
regulator turn on,
leading to MCU supply turn on and reset release. If the 33742
is in Stop mode (V2 off and V
DD
active), the CAN wake-up or
any wake-up is signalled by a pulse on the INT output. In
addition the CAN-WU bit is set in the CAN register.
If the 33742 is in Normal or Standby mode and the CAN
interface is in Sleep mode with wake-up enable, the CAN wake-
up is reported by the bit CANWU in the CAN register.
In the event the 33742 is in Normal mode and CAN Sleep
mode with wake-up enable, it is recommended that the user
check for the CAN WU bit prior to setting the 33742 in Sleep or
Stop mode in case bus traffic has occurred while the CAN
interface was in Sleep mode.
After CAN wake-up, a flag is set in the CAN register. Bit
CAN-WU reports the CAN wake-up event while the 33742 was
in Sleep or Stop mode. This bit is set until the CAN is in placed
by SPI command into TXRX mode and the CAN register read.
+
Internal Wake-Up
Signal
Latch
RST
Timeout
RST
Counter
Pulse OK
Narrow
Pulse
Pulse Width
Filter
Timeout
Generator
Standby
WU Receiver
CANH
CANL
F
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Go to: www.freescale.com
n
.