33742 datasheet(39/65 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33742

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

DETECTION PRINCIPLE

In the recessive state, if one of the two bus lines is shorted

to GND, VDD, or VSUP, then voltage at the other line follows

the shorted line due to bus termination resistance and the

high impedance of the driver. For example, if CANL is shorted

to GND, CANL voltage is zero, and CANH voltage, as

measured by the Hg comparator, is also close to zero.

In the recessive state the failure detection to GND or

VSUP is possible. However, it is impossible to distinguish

which bus line, CANL or CANH, is shorted to GND or VSUP.

In the dominant state, the complete diagnostic is possible

once the driver is turned on.

CAN BUS FAILURE REPORTING

CANL bus line failures (for example, CANL short to GND)

is reported in the SPI register TIM1/2. CANH bus line (for

example, CANH short to VSUP) is reported in the LPC

In addition CAN-F and CAN-UF bits in the CAN register

indicate that a CAN bus failure has been detected.

NON-IDENTIFIED AND FULLY IDENTIFIED BUS

FAILURES

As indicated in Table 11, page 38, when the bus is in a

recessive state it is possible to detect an error condition;

however, is it not possible to fully identify the specific error.

This is called “non-identified” or “under-acquisition” bus

failure. If there is no communication (i.e., bus idle), it is still

possible to warn the MCU that the SBC has started to detect

a bus failure.

In the CAN register, bits D2 and D1 (CAN-F and CAN-UF,

respectively) are used to signal bus failure. Bit D2 reports a

bus failure and bit D1 indicates if the failure is identified or not

(bit D1 is set to logic [1} if the error is not identified).

When the detection mechanism is fully operating any bus

error will be detected and reported in the TIM1/2 and LPC

registers and bit D1 will be reset to logic [0].

NUMBER OF SAMPLES FOR PROPER FAILURE

DETECTION

The failure detector requires at least one cycle of

recessive and dominant state to properly recognize the bus

failure. The error will be fully detected after five cycles of

recessive-dominant states. As long as the failure detection

circuitry has not detected the same error for five recessive-

dominant cycles, the bit “non-identified failure” (CAN-UF) will

be set.

RXD PERMANENT RECESSIVE FAILURE

The purpose of this detection mechanism is to diagnose

an external hardware failure at the RXD output pin and to

ensure that a permanent failure at the RXD pin does not

disturb network communication.In the event RXD is shorted

to a permanent high level signal (i.e., 5.0 V), the CAN

protocol module within the MCU cannot receive any incoming

message. Additionally, the CAN protocol module cannot

distinguish the bus idle state and could start communication

at any time. To prevent this, an RXD failure detection, as

illustrated in Figure 25 and explained below, is necessary.

Figure 25. RXD Path and RXD Permanent Recessive Detection Principle

CANH

CANL

Diff

VDD

RXD Sense

RXD

TXD

Ω

Logic

Diag

CANL

Diff Output

RXD Output

RXD Short to V1

Prop Delay

RXD Flag

RXD Flag Latched

2.0 V

Sampling

Sampling Sampling

Note

RXD Flag is neither the RXPR bit in the LPC register nor the CAN-F bit in the INTR register.

CANH

Driver

33742 Datasheet(PDF) 39 Page - Freescale Semiconductor, Inc

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