Loop Back Mode; Loop Back Combined With Silent Mode; Figure 226. Bxcan In Silent Mode; Figure 227. Bxcan In Loop Back Mode - ST STM32F40 Series Reference Manual

RM0090
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 226. bxCAN in silent mode

24.5.2

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 227. 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.
24.5.3

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
Tx Rx
=1
CANTX CANRX
bxCAN
Tx
CANTX CANRX
Doc ID 018909 Rev 4
Controller area network (bxCAN)
Rx
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