MSM9225B User’s Manual
Chapter 2 Register Descriptions
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(3) Acknowledgment error flag: ACK
This bit becomes “1” when an acknowledgment error occurs.
At reset or after release of the bus off state, this bit becomes “0”.
(4) CRC error flag: CRC
This bit becomes “1” when a CRC error occurs.
At reset or after release of the bus off state, this bit becomes “0”.
(5) Form error flag: FEF
This bit becomes “1” when a form error occurs.
At reset or after release of the bus off state, this bit becomes “0”.
(6) Bus off flag: BOF
This bit becomes “1” when a bus off error occurs.
It is set to “0” when “0” is written from the microcontroller. Also, when the COMPAT bit of the CAN
control register 2 (CANC2: 8Fhex) is “1”, BOF is automatically set to “0” if the bus off state is released
(that is, if 11 consecutive “recessive” bits are received).
A bus off flag (BOFF: bit 6) is provided in the CAN status register (CANS: 9Fhex). This flag is assigned
to bit 8 of the transmit error counter. Therefore, when the transmit error counter is set to “0_0000_0000”
because of bus off release, BOFF is also set to “0”. For this reason, if the microcontroller does not access
the CANS register during the period from the time the bus off state is entered to the time it is released, the
microcontroller cannot detect whether the CAN controller has entered a bus off state.
In the case of BOF, the CANS2 bus off flag, it holds “1” until “0” is written to it from the microcontroller
(when COMPAT = “0”) or the bus off release operation does not start before the initialization bit INIT of
the CAN control register (CANC: 0Ehex) is manipulated. Therefore, the microcontroller can detect
whether the CAN controller is (or was) in the bus off state.
At reset, BOF is set to “0”.
2.4.16 Bus Off Release Counter (BOCO: BFhex)
BOCO is a read-only register for checking the bus off release.
This counter is incremented by 1 every time 11 consecutive “recessive” bits are detected. If the counter has
been incremented up to a maximum of 128, the bus off state will be released.
If any change is found in the value of this register after the bus off state is entered, it can be confirmed that the
CAN bus is not fixed as “dominant” due to some effect.
At reset or after the bus off state is released, BOCO is set to “0”.
MSB BOC7 BOC6 BOC5 BOC4 BOC3 BOC2 BOC1 BOC0
LSB
BOCO (BFhex), R/W: R
Initial
value:
00000000
Figure 2-41 Bus Off Release Counter (BOCO)