COP87L88EB Datasheet, PDF (32/72 Pages) National Semiconductor (TI) – 8-Bit CMOS OTP Microcontrollers with 16k or 32k Memory, CAN Interface, 8-Bit A/D, and USART

Frame Formats (Continued)
ERROR MANAGEMENT AND DETECTION
There are multiple mechanisms in the CAN protocol, to de-
tect errors and to inhibit erroneous modules from disabling
all bus activities.
FIGURE 26. Order of Bit Transmission within a CAN Frame
DS100044-31
The following errors can be detected:
• Bit Error
A CAN device that is sending also monitors the bus. If the
monitored bit value is different from the bit value that is sent,
a bit error is detected. The reception of a “dominant” bit in-
stead of a “recessive” bit during the transmission of a pas-
sive error flag, during the stuffed bit stream of the arbitration
field or during the acknowledge slot, is not interpreted as a
bit error.
• Stuff error
A stuff error is detected, if the bit level after 6 consecutive bit
times has not changed in a message field that has to be
coded according to the bit stuffing method.
• Form Error
A form error is detected, if a fixed frame bit (e.g., CRC delim-
iter, ACK delimiter) does not have the specified value. For a
receiver a “dominant” bit during the last bit of End of Frame
does NOT constitute a form error.
• Bit CRC Error
A CRC error is detected if the remainder of the CRC calcula-
tion of a received CRC polynomial is non-zero.
• Acknowledgment Error
An acknowledgment error is detected whenever a transmit-
ting node does not get an acknowledgment from any other
node (i.e., when the transmitter does not receive a “domi-
nant” bit during the ACK frame).
The device can be in one of three states with respect to error
handling:
• Error active
An error active unit can participate in bus communication
and sends an active (“dominant”) error flag.
• Error passive
An error passive unit can participate in bus communication.
However, if the unit detects an error it is not allowed to send
an active error flag. The unit sends only a passive (“reces-
sive”) error flag. A device is error passive when the transmit
error counter is greater than 127 or when the receive error
counter is greater than 127. A device becoming error passive
sends an active error flag. An error passive device becomes
error active again when both transmit and receive error
counter are less than 128.
• Bus off
A unit that is “bus off” has the output drivers disabled, i.e., it
does not participate in any bus activity. A device is bus off
when the transmit error counter is greater than 255. A bus off
device will become error active again in one of two ways de-
pending on which mode is selected by the user through the
Fault Confinement Mode select bit (FMOD) in the CAN Bus
Control Register (CBUS). Setting the FMOD bit to “0” (de-
fault after power on reset) will select the Standard Fault Con-
finement mode. In this mode the device goes from “bus off”
to “error active” after monitoring 128*11 recessive bits (in-
cluding bus idle) on the bus. This mode has been imple-
mented for compatibility reasons with existing solutions. Set-
ting the FMOD bit to “1” will select the Enhanced Fault
Confinement mode. In this mode the device goes from “bus
off” to “error active” after monitoring 128 “good” messages,
as indicated by the reception of 11 consecutive “recessive”
bits including the End of Frame. The enhanced mode offers
the advantage that a “bus off” device (i.e., a device with a se-
rious fault) is not allowed to destroy any messages on the
bus until other devices can transmit at least 128 messages.
This is not guaranteed in the standard mode, where a defec-
tive device could seriously impact bus communication. When
the device goes from “bus off” to “error active”, both error
counters will have the value “0”.
In each CAN module there are two error counters to perform
a sophisticated error management. The receive error
counter (REC) is 7 bits wide and switches the device to the
error passive state if it overflows. The transmit error counter
(TEC) is 8 bits wide. If it is greater than 127, the device is
switched to the error passive state. As soon as the TEC
overflows, the device is switched bus-off, i.e., it does not par-
ticipate in any bus activity.
www.national.com
32