Analog Integrated Circuit Device Data

Freescale Semiconductor

19

33902

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

CAN BUS DIAGNOSTIC

The aim is to implement a diagnostic of bus short-circuit to

GND, VBAT, and the internal application circuit board 5.0 V.

Several comparators are implemented on the CANH and

CANL lines. These comparators monitor the bus level in the

recessive and dominant states. The information is then

managed by the logic circuitry to properly determine the

failure and report it.

Figure 14. CAN Bus Simplified Structure Truth Table for Failure Detection

Table 7 indicates the state of the comparators in case of a bus failure, and depending upon the driver state.

Detection Principle

In the recessive state, if one of the two bus lines are

shorted to GND, VDD, or VBAT, the voltage at the other line

follows the shorted line, due to the bus termination

resistance. For example: if CANL is shorted to GND, the

CANL voltage is zero, the CANH voltage measured by the Hg

comparator is also close to zero.

In the recessive state, the failure detection to GND or

VBAT is possible. However, it is not possible with the above

implementation to distinguish which of the CANL or CANH

lines are shorted to GND or VBAT. A complete diagnostic is

possible once the driver is turned on, and in the dominant

state.

Number of Samples for Proper Failure Detection

The failure detector requires at least one cycle of the

recessive and dominant states to properly recognize the bus

failure. The error will be fully detected after 5 cycles of the

recessive-dominant states. As long as the failure detection

circuitry has not detected the same error for 5 recessive-

dominant cycles, the error is not reported.

Bus clamping detection

If the bus is detected to be in dominant for a time longer

in Normal mode.

Such conditions could occur if the CANH line is shorted to

a high voltage. In this case, current will flow from the high

voltage short-circuit through the bus termination resistors

(60

Ω) and then in the Split terminal (if used), and through the

device CANH and CANL input resistors, which are terminated

to an internal 2.5 V biasing or to GND (Sleep mode).

Depending upon the high voltage short-circuit, the number

node state (sleep or active), the voltage developed across the

bus termination can be sufficient to create a positive

dominant voltage between CANH and CANL. The RXD pin

will be low. This would prevent the start of any CAN

Table 7. Failure Detection Truth Table

Failure description

Driver recessive state

Driver dominant state

Lg (threshold 1.75 V)

Hg (threshold 1.75 V)

Lg (threshold 1.75 V)

Hg (threshold 1.75 V)

No failure

1

1

0

1

CANL to GND

0

0

0

1

CANH to GND

0

0

0

0

No failure

0

0

0

0

CANL to VBAT

1

1

1

1

CANH to VBAT

1

1

0

1

No failure

0

0

0

0

CANL to 5.0 V

1

1

1

1

CANH to 5.0 V

1

1

0

1

Hg

CANH

CANL

Lg

Hb

Lb

Logic

TX

Diag

VDD (5.0 V)

GND (0.0 V)

Recessive level (2.5 V)

VBAT (12-14 V)

VRVB (VSUP-2.0 V)

VRG (1.75 V)

CANL dominant level (1.4 V)

CANH dominant level (3.6 V)

L5

H5

VR5 (VDD-.43 V)