XC886/888CLM
Controller Area Network (MultiCAN) Controller
User’s Manual
15-10
V1.3, 2010-02
MultiCAN, V1.0
Processor starts the CAN bus arbitration procedure and continues with the frame
transmission when the bus was found in idle state. While the data transmission is
running, the Bit Stream Processor monitors continuously 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 ‘Last Error’ interrupt request is generated and the error code is indicated
by the 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, the ‘Last Error’ interrupt request is generated
and the error code is indicated by the bit field NSRx.LEC. 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.
15.1.3.3
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 NECNTx.REC and the
Transmit Error Counter NECNTx.TEC are incremented and decremented by commands
from the Bit Stream Processor. If the Bit Stream 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, recognizing the transfer error, are indicated for the respective
CAN node x in register NECNTx. According to the values of the error counters, the CAN
node is set into the states
“error active”, “error passive”, and “bus-off”.
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 or greater than 128.
The “bus-off” state is activated if the Transmit Error Counter is equal or greater than the
“bus-off” limit of 256. This state is reported by flag NSRx.BOFF. The device remains in
this state, until the “bus-off” recovery sequence is finished. Additionally, bit NSRx.EWRN
is set when at least one of the error counters is equal or greater than the error warning
limit defined by bit field NECNTx.EWRNLVL. Bit NSRx.EWRN is reset if both error
counters fall below the error warning limit again.
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