COP888EB Datasheet, PDF (35/75 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 8k Memory, CAN Interface, 8-Bit A/D, and USART

Frame Formats (Continued)
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.
The counters are modified by the device’s hardware accord-
ing to the following rules:
TABLE 8. Receive Error Counter Handling
Condition
A receiver detects a Bit Error
during sending an active error flag.
A receiver detects a “dominant” bit
as the first bit after sending an
error flag.
Receive Error
Counter
Increment by 8
Increment by 8
Condition
After detecting the 14th consecutive
“dominant” bit following an active
error flag or overload flag or after
detecting the 8th consecutive
“dominant” bit following a passive
error flag. After each sequence of
additional 8 consecutive “dominant”
bits.
Any other error condition (stuff,
frame, CRC, ACK).
A valid reception or transmission.
Receive Error
Counter
Increment by 8
Increment by 1
Decrement by 1 if
Counter is not 0
TABLE 9. Transmit Error Counter Handling
Condition
Transmit Error
Counter
A transmitter detects a Bit Error
during sending an active error
flag.
Increment by 8
After detecting the 14th
consecutive “dominant” bit
following an active error flag or
overload flag or after detecting
the 8th consecutive “dominant”
bit following a passive error
flag. After each sequence of
additional 8 consecutive
“dominant” bits.
Increment by 8
Any other error condition (stuff, Increment by 8
frame, CRC, ACK).
A valid reception or
transmission.
Decrement by
1 if Counter is not 0
Special error handling for the TEC counter is performed in
the following situations:
• A stuff error occurs during arbitration, when a transmitted
“recessive” stuff bit is received as a “dorminant” bit. This
does not lead to an incrementation of the TEC.
• An ACK-error occurs in an error passive device and no
“dominant” bits are detected while sending the passive
error flag. This does not lead to an incrementation of the
TEC.
• If only one device is on the bus and this device transmits
a message, it will get no acknowledgment. This will be
detected as an error and message will be repeated.
When the device goes “error passive” and detects an ac-
knowledge error, the TEC counter is not incremented.
Therefore the device will not go from “error passive” to
the “bus off” state due to such a condition.
Figure 27 shows the connection of different bus states ac-
cording to the error counters.
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