CONTROLLER AREA NETWORK (bxCAN)
CONTROLLER AREA NETWORK (Cont’d)
Figure 151. . CAN Error State Diagram
When TEC or REC > 127
ERROR ACTIVE
ERROR PASSIVE
When TEC and REC < 128,
When 128 * 11 recessive bits occur:
When TEC > 255
BUS OFF
10.10.5.6 Error Management
Bus-Off Recovery
The error management as described in the CAN
protocol is handled entirely by hardware using a
Transmit Error Counter (TECR register) and a Re-
ceive Error Counter (RECR register), which get in-
cremented or decremented according to the error
condition. For detailed information about TEC and
REC management, please refer to the CAN stand-
ard.
The Bus-Off state is reached when TECR is great-
er then 255, this state is indicated by BOFF bit in
CESR register. In Bus-Off state, the bxCAN is no
longer able to transmit and receive messages.
Depending on the ABOM bit in the CMCR register
bxCAN will recover from Bus-Off (become error
active again) either automatically or on software
request. But in both cases the bxCAN has to wait
at least for the recovery sequence specified in the
CAN standard (128 x 11 consecutive recessive
bits monitored on CANRX).
Both of them may be read by software to deter-
mine the stability of the network. Furthermore, the
CAN hardware provides detailed information on
the current error status in CESR register. By
means of CEIER register and ERRIE bit in CIER
register, the software can configure the interrupt
generation on error detection in a very flexible
way.
If ABOM is set, the bxCAN will start the recovering
sequence automatically after it has entered Bus-
Off state.
If ABOM is cleared, the software must initiate the
recovering sequence by requesting bxCAN to en-
ter and to leave initialization mode.
Note: In initialization mode, bxCAN does not mon-
itor the CANRX signal, therefore it cannot com-
plete the recovery sequence. To recover, bxCAN
must be in normal mode.
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