
DocID024597 Rev 5
RM0351
Controller area network (bxCAN)
46.5.1 Silent
mode
The bxCAN can be put in Silent mode by setting the SILM bit in the CAN_BTR register.
In Silent mode, the bxCAN is able to receive valid data frames and valid remote frames, but
it sends only recessive bits on the CAN bus and it cannot start a transmission. If the bxCAN
has to send a dominant bit (ACK bit, overload flag, active error flag), the bit is rerouted
internally so that the CAN Core monitors this dominant bit, although the CAN bus may
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 504. bxCAN in silent mode
46.5.2
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 505. 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.
46.5.3
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
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