AN2283 Freescale Semiconductor / Motorola, AN2283 Datasheet - Page 25

• Manufacturer Part Number: AN2283
• Description: Scalable Controller Area Network (MSCAN)
• Manufacturer: Freescale Semiconductor / Motorola

Error Detection
Bit Error
Stuff Error
CRC Error
Form Error
Acknowledgement
Error
MOTOROLA
NOTE:
Motorola Scalable Controller Area Network (MSCAN) Interrupts
The MSCAN is able to detect five types of errors. They are:
A node sending a bit on the bus also monitors the bus. That is, each node’s
transmit state machine compares the bit levels detected on the bus with the bit
levels being transmitted.
A node’s transmitter detects a bit error if the bit value monitored is different from
the bit value transmitted. An exception to this is when sending a recessive [1]
bit and receiving a dominant [0] bit during the arbitration field or the ACK slot.
Also, a node transmitting a PASSIVE Error Flag and receiving a dominant [0]
bit doesn’t interpret this as a Bit Error.
Whenever five consecutive bits of equal value are transmitted, an extra bit of
complementary value is automatically inserted into the bit stream. This
provides edges for clock resynchronization. The receivers automatically de-
stuff the extra bit of complementary value. So, when a receiver detects a 6
consecutive equal level value (six dominant [0] bits or six recessive [1] bits)
during a message field that should be encoded by bit stuffing a stuff error is
detected.
A CRC (Cyclic Redundancy Check) error is detected by the node’s receiver if
the CRC calculated by the receiver is different from the CRC received in the
message.
The node’s receiver detects a Form Error if a fixed form bit field contains one
or more illegal bits. For example, a fixed form bit field would be the arbitration
field which can be 12 (standard mode) / 32 (extended mode) bits in length; or
the control field which has two reserved bits (r1 and r0) that need to be sent out
as two dominants [0] in a row; or the CRC field which has the CRC Delimiter
and needs to be a recessive [1] bit.
A node’s transmitter detects an Acknowledge Error if it doesn’t receive a
dominant [0] bit during the ACK Slot. A dominant [0] bit in the ACK Slot
indicates an acknowledgment occurred. The ACK Slot is in the Acknowledge
field of the data frame.
consists of and shows what is inside the Acknowledge field.
1. Bit Error — detected by the MSCAN’s transmit state machine
2. Stuff Error — detected by the MSCAN’s receiver state machine
3. CRC Error — detected by the MSCAN’s receiver state machine
4. Form Error — detected by the MSCAN’s receiver state machine
5. Acknowledgement Error — detected by the MSCAN’s transmitter state
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Figure 19
provides a reminder of what the data frame
Error Overview
AN2283/D
th
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