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bus arbitration procedure and continues with the frame transmission when the bus was
found in idle state. While the data transmission is running, the Bitstream Processor
continuously monitors the I/O line. If (outside the CAN bus arbitration phase or the
acknowledge slot) a mismatch is detected between the voltage level on the I/O line and
the logic state of the bit currently sent out by the transmit shift register, a CAN error
interrupt request is generated, and the error code is indicated by the Node x Status
Register bit field NSRx.LEC.
The data consistency of an incoming frame is verified by checking the associated CRC
field. When an error has been detected, a CAN error interrupt request is generated and
the associated error code is presented in the Node x Status Register NSRx.
Furthermore, an Error Frame is generated and transmitted on the CAN bus. After
decomposing a faultless frame into identifier and data portion, the received information
is transferred to the message buffer executing remote and Data Frame handling,
interrupt generation and status processing.
20.3.5.3 Error Handling Unit
The Error Handling Unit of a CAN node x is responsible for the fault confinement of the
CAN device. Its two counters, the Receive Error Counter REC and the Transmit Error
Counter TEC (bit fields of the Node x Error Counter Register NECNTx, see
are incremented and decremented by commands from the Bitstream Processor. If the
Bitstream Processor itself detects an error while a transmit operation is running, the
Transmit Error Counter is incremented by 8. An increment of 1 is used when the error
condition was reported by an external CAN node via an Error Frame generation. For
error analysis, the transfer direction of the disturbed message and the node that
recognizes the transfer error are indicated for the respective CAN node x in register
NECNTx. Depending on the values of the error counters, the CAN node is set into error-
active, error-passive, or bus-off state.
The CAN node is in error-active state if both error counters are below the error-passive
limit of 128. The CAN node is in error-passive state, if at least one of the error counters
is equal to or greater than 128.
The bus-off state is activated if the Transmit Error Counter is equal to or greater than the
bus-off limit of 256. This state is reported for CAN node x by the Node x Status Register
flag NSRx.BOFF. The device remains in this state, until the "bus-off" recovery sequence
is finished. Additionally, Node x Status Register flag NSRx.EWRN is set when at least
one of the error counters is equal to or greater than the error warning limit defined by the
Node x Error Count Register bit field NECNTx.EWRNLVL. Bit NSRx.EWRN is reset if
both error counters fall below the error warning limit again (see
User's Manual
MultiCAN, V2.24
Controller Area Network Controller (MultiCAN)
20-24
TC1728
Page
20-87)
Page
20-75).
V1.0, 2011-12
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