UM0306 Controller area network (bxCAN)
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14.4.6 Loop back mode
The bxCAN can be set in Loop Back Mode by setting the LBKM bit in the CAN_BTR
register. In Loop Back Mode, the bxCAN treats its own transmitted messages as received
messages and stores them (if they pass acceptance filtering) in a Receive mailbox.
Figure 122. bxCAN in loop back mode
This mode is provided for self-test functions. To be independent of external events, the CAN
Core ignores acknowledge errors (no dominant bit sampled in the acknowledge slot of a
data / remote frame) in Loop Back Mode. In this mode, the bxCAN performs an internal
feedback from its Tx output to its Rx input. The actual value of the CANRX input pin is
disregarded by the bxCAN. The transmitted messages can be monitored on the CANTX pin.
14.4.7 Loop back combined with silent mode
It is also possible to combine Loop Back mode and Silent mode by setting the LBKM and
SILM bits in the CAN_BTR register. This mode can be used for a “Hot Selftest”, meaning the
bxCAN can be tested like in Loop Back mode but without affecting a running CAN system
connected to the CANTX and CANRX pins. In this mode, the CANRX pin is disconnected
from the bxCAN and the CANTX pin is held recessive.
Figure 123. bxCAN in combined mode
14.5 Functional description
14.5.1 Transmission handling
In order to transmit a message, the application must select one empty transmit mailbox, set
up the identifier, the data length code (DLC) and the data before requesting the transmission
by setting the corresponding TXRQ bit in the CAN_TIxR register. Once the mailbox has left
bxCAN
CANTX CANRX
Tx Rx
bxCAN
CANTX CANRX
Tx Rx
=1
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