setting 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 input clocks: 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 CC Control Register flag CCCR[CSR]. The M_CAN will acknowledge this by
resetting output signal clock stop acknowledge and resetting CCCR[CSA]. Afterwards,
the application can restart CAN communication by resetting bit CCCR[INIT].
3.5.1.9 Test Modes
To enable write access to register TEST, bit 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 pin M_CAN_Rx 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 pin M_CAN_Tx. This applies also when reading input
pin M_CAN_Rx via TEST[RX].
Note
Test modes should be used for production tests or self test only.
The software control for pin M_CAN_Tx interferes with all
CAN protocol functions. It is not recommended to use test
modes for application.
3.5.1.9.1 External Loopback Mode
The M_CAN can be set in External Loopback Mode by programming TEST[LBCK] to
one. In Loopback Mode, the M_CAN treats its own transmitted messages as received
messages and stores them (if they pass acceptance filtering) into an Rx Buffer or an Rx
FIFO. The following figure shows the connection of signals M_CAN_Tx and
M_CAN_Rx to the M_CAN in External Loopback 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 Loopback Mode. In this mode the M_CAN
Chapter 3 Modular CAN (M_CAN)
MPC5777C Reference Manual Addendum, Rev. 1, 12/2015
Freescale Semiconductor, Inc. 95
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