STMicroelectronics SPC572L series Reference Manual page 1061

RM0400
44.3.7.7 Power down (Sleep mode)
The M_CAN can be set into power down mode controlled by input signal clock stop request
or via CC Control Register CCCR[CSR]. As long as the clock stop request signal is active,
bit CCCR[CSR] is read as one. When all pending transmission requests have completed,
the M_CAN waits until bus idle state is detected. Then the M_CAN sets then CCCR[INIT] to
one to prevent any further CAN transfers. Now the M_CAN acknowledges that it is ready for
power down by setting clock stop acknowledge output signal to one and CCCR[CSA] to
one. In this state, before the clocks are switched off, further register accesses can be made.
A write access to CCCR[INIT] will have no effect. Now the module clock inputs (CAN clock
and host clock) may be switched off. To leave power down mode, the application has to turn
on the module clocks before resetting signal clock stop request signal resp. CC Control
Register flag CCCR[CSR]. The M_CAN will acknowledge this by resetting clock stop
acknowledge output signal and resetting CCCR[CSA]. Afterwards, the application can
restart CAN communication by resetting bit CCCR[INIT].
44.3.7.8 Test modes
To enable write access to register TEST (see
CCCR[TEST] has to be set to one. This allows the configuration of the test modes and test
functions.
Four output functions are available for the CAN transmit pin m_can_tx by programming
TEST.TX. Additionally to its default function – the serial data output – it can drive the CAN
Sample Point signal to monitor the M_CAN's bit timing and it can drive constant dominant or
recessive values. The actual value at M_CAN Rx pin can be read from TEST[RX]. Both
functions can be used to check the CAN bus physical layer.
Due to the synchronization mechanism between CAN clock and Host clock domain, there
may be a delay of several Host clock periods between writing to TEST[TX] until the new
configuration is visible at output Tx pin. This applies also when reading input Rx pin via
TEST[RX].
Note: Test modes should be used for production tests or self test only. The software control for Tx
pin interferes with all CAN protocol functions. It is not recommended to use test modes for
application.
44.3.7.8.1 External Loop Back mode
The M_CAN can be set in External Loop Back mode by programming TEST.LBCK to one. In
Loop Back mode, the M_CAN treats its own transmitted messages as received messages
and stores them (if they pass acceptance filtering) into Rx FIFOs.
connection of Tx and Rx signals to the M_CAN in External Loop Back mode.
This mode is provided for hardware self-test. To be independent from external stimulation,
the M_CAN ignores acknowledge errors (recessive bit sampled in the acknowledge slot of a
data/remote frame) in Loop Back mode. In this mode the M_CAN performs an internal
feedback from its Tx output to its Rx input. The actual value of the CAN Rx input pin is
disregarded by the M_CAN. The transmitted messages can be monitored at the M_CAN Tx
pin.
Section 44.3.5.2.4: Test
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CAN Subsystem
Register), bit
Figure 522 shows the
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