COP888EB datasheet(35/75 Pages) NSC

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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

Receive Error

Counter

A receiver detects a Bit Error

during sending an active error flag.

Increment by 8

A receiver detects a “dominant” bit

as the first bit after sending an

error flag.

Increment by 8

Condition

Receive Error

Counter

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,

frame, CRC, ACK).

Increment by 1

A valid reception or transmission.

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,

frame, CRC, ACK).

Increment by 8

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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