1996 Jun 27 61
Philips Semiconductors Product specification
8-bit microcontroller with on-chip CAN P8xC592
13.6.8.3 CRC Error
To ensure the validity of a transmitted message all
receivers perform a CRC check. Therefore, in addition to
the (destuffed) information digits (Start-Of-Frame up to
Data Field), every message includes some control digits
(CRC Sequence; generated by the transmitting
CAN-controller of the respective message) used for error
detection.
The code used by all CAN-controllers is a (shortened)
BCH code, extended by a parity check and has the
following attributes:
• 127 bits as maximum length of the code.
• 112 bits as maximum number of information digits
(max. 83 bits are used by the CAN-controller).
• Length of the CRC Sequence amounts to 15 bits.
• Hamming distance d = 6.
As a result, ‘(d−1)’ random errors are detectable (some
exceptions exist).
The CRC Sequence is determined (calculated) by the
following procedure:
1. The destuffed bit stream consisting of Start-Of-Frame
up to the Data Field (if present) is interpreted as
polynomial with coefficients 0 or 1.
2. This polynomial is divided (modulo-2) by the following
generator polynomial, which includes a parity check:
(x + 1) = 1100010110011001 B.
3. The remainder of this polynomial division is the
CRC Sequence.
Burst errors are detected up to a length of 15
[degree of f(x)]. Multiple errors (number of disturbed bits at
least d = 6) are not detected with a residual error
probability of by CRC check only.
13.6.8.4 Form Error
Form Errors result from violations of the fixed form of the
following bit fields:
• CRC Delimiter
• Acknowledge Delimiter
• End-Of-Frame
• Error Delimiter
• Overload Delimiter.
During the transmission of these bit fields an error
condition is recognized if a dominant bit level instead of a
recessive one is detected.
fx() x
14
x
9
x
8
x
6
x
5
x
4
x
2
x1++++++++()=
2
15–
310
5–
×()
13.6.8.5 Acknowledgement Error
This is detected by a transmitter whenever it does not
monitor a dominant bit during the Acknowledge Slot.
13.6.8.6 Error detection by an Error Flag from
another CAN-controller
The detection of an error is signalled by transmitting an
Error Flag. An Active Error Flag causes a Stuff Error, a Bit
Error or a Form Error at all other CAN-controllers.
13.6.8.7 Error Detection Capabilities
Errors which occur at all CAN-controllers (global errors)
are 100% detected. For local errors, i.e. for errors
occurring at some CAN-controllers only, the shortened
BCH code, extended by a parity check, has the following
error detection capabilities:
• Up to five single Bit Errors are 100% detected, even if
they are distributed randomly within the code.
• All single Bit Errors are detected if their total number
(within the code) is odd.
• The residual error probability of the CRC check amounts
to (3 × 10
−5
). As an error may be detected not only by
CRC check but also by other detection processes
described above the residual error probability is several
magnitudes less than (3 × 10
−5
).
13.6.9 E
RROR CONFINEMENT DEFINITIONS
13.6.9.1 Bus-OFF
A CAN-controller which has too many unsuccessful
transmissions, relative to the number of successful
transmissions, will enter the Bus-OFF state. It remains in
this state, neither receiving nor transmitting messages
until the Reset Request bit is set LOW (absent) and both
Error Counters set to 0 (see Section 13.6.10).
13.6.9.2 Acknowledge
A CAN-controller which has received a valid message
correctly, indicates this to the transmitter by transmitting a
dominant bit level on the bus during the Acknowledge Slot,
independent of accepting or rejecting the message.
13.6.9.3 Error-Active
An error-active CAN-controller in its normal operating state
is able to receive and to transmit normally and also to
transmit an Active Error Flag (see Section 13.6.10).
