National CP3BT26 manual Error Active, Error Warning, Error Passive, Bus Off, Error Counters

CP3BT26

Error Active

An error active unit can participate in bus communication and may send an active (“dominant”) error flag.

Error Warning

The Error Warning state is a sub-state of Error Active to in- dicate a heavily disturbed bus. The CAN module behaves as in Error Active mode. The device is reset into the Error Active mode if the value of both counters is less than 96.

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 or the receive error counter is greater than

127.A device becoming error passive will send an active er- ror flag. An error passive device becomes error active again when both transmit and receive error counter are less than

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 after monitoring 128 × 11 “recessive” bits (including bus idle) on the bus. When the device goes from “bus off“ to “error active“, both error counters will have a value of 0.

19.2.5Error Counters

There are multiple mechanisms in the CAN protocol to de- tect errors and inhibit erroneous modules from disabling all bus activities. Each CAN module includes two error counters to perform error management. The receive error counter (REC) and the transmit error counter (TEC) are 8- bits wide, located in the 16-bit wide CANEC register. The counters are modified by the CAN module according to the rules listed in Table 45. This table provides an overview of the CAN error conditions and the behavior of the CAN mod- ule; for a detailed description of the error management and fault confinement rules, refer to the CAN Specification 2.0B.

If the MSB (bit 7) of the REC is set, the node is error passive and the REC will not increment any further.

The Error counters can be read by application software as described under CAN Error Counter Register (CANEC) on page 139.

	Table 45 Error Counter Handling
	Condition			Action
	Receive Error Counter Conditions
	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
	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.			Increment by 8
	After each sequence of additional 8 consecutive “dominant” bits.
	Any other error condition (stuff, frame, CRC, ACK)			Increment by 1
	A valid reception or transmission			Decrement by 1 unless
				counter is already 0
	Transmit Error Counter Conditions
	A transmitter detects a bit error while 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.			Increment by 8
	After each sequence of additional 8 consecutive ‘dominant’ bits.
	Any other error condition (stuff, frame, CRC, ACK)			Increment by 8
	A valid reception or transmission			Decrement by 1 unless
				counter is already 0
	Special error handling for the TEC counter is performed in		„ If only one device is on the bus and this device transmits
	the following situations:		a message, it will get no acknowledgment. This will be
	„ A stuff error occurs during arbitration, when a transmitted		detected as an error and the message will be repeated.
			When the device goes “error passive” and detects an ac-
	“recessive” stuff bit is received as a “dominant” bit. This
			knowledge error, the TEC counter is not incremented.
	does not lead to an increment of the TEC.
			Therefore the device will not go from ”error passive” to
	„ An ACK-error occurs in an error passive device and no
			the “bus off” state due to such a condition.
	“dominant” bits are detected while sending the passive
	error flag. This does not lead to an increment of the TEC.
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