Controller area network (bxCAN)
RM0090
1074/1731
DocID018909 Rev 11
remain in recessive state. Silent mode can be used to analyze the traffic on a CAN bus
without affecting it by the transmission of dominant bits (Acknowledge Bits, Error Frames).
Figure 337. bxCAN in silent mode
32.5.2
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 338. 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.
32.5.3
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.
bxCAN
CANTX CANRX
Tx
Rx
=1
bxCAN
CANTX CANRX
Tx
Rx