TJA1054T データシートの表示（PDF） - Philips Electronics

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TJA1054T

Fault-tolerant CAN transceiver

Philips Electronics 

Philips Semiconductors

Fault-tolerant CAN transceiver

Preliminary specification

TJA1054

FUNCTIONAL DESCRIPTION

The TJA1054 is the interface between the CAN protocol

controller and the physical wires of the CAN bus

(see Fig.7). It is primarily intended for low speed

applications, up to 125 kBaud, in passenger cars.

The device provides differential transmit capability to the

CAN bus and differential receive capability to the CAN

controller.

To reduce RFI, the rise and fall slope are limited. This

allows the use of an unshielded twisted pair or a parallel

pair of wires for the bus lines. Moreover, it supports

transmission capability on either bus line if one of the wires

is corrupted. The failure detection logic automatically

selects a suitable transmission mode.

In normal operating mode (no wiring failures) the

differential receiver is output on pin RXD (see Fig.1).

The differential receiver inputs are connected to

pins CANH and CANL through integrated filters.

The filtered input signals are also used for the single-wire

receivers. The receivers connected to pins CANH

and CANL have threshold voltages that ensure a

maximum noise margin in single-wire mode.

A timer has been integrated at pin TXD. This timer

prevents the TJA1054 from driving the bus lines to a

permanent dominant state.

Failure detector

The failure detector is fully active in the normal operating

mode. After the detection of a single bus failure the

detector switches to the appropriate mode (see Table 1).

Table 1 Bus failures

FAILURE

DESCRIPTION

1

CANH wire interrupted

2

CANL wire interrupted

3

CANH short-circuited to battery

3a CANH short-circuited to VCC

4

CANL short-circuited to ground

5

CANH short-circuited to ground

6

CANL short-circuited to battery

6a CANL short-circuited to VCC

7

CANL mutually short-circuited to CANH

The differential receiver threshold voltage is set at

−3.2 V typically (VCC = 5 V). This ensures correct

reception with a noise margin as high as possible in the

normal operating mode and in the event of failures 1, 2,

4 and 6a. These failures, or recovery from them, do not

destroy ongoing transmissions.

Failures 3 and 6 are detected by comparators connected

to the CANH and CANL bus lines, respectively. If the

comparator threshold is exceeded for a certain period of

time, the reception is switched to the single-wire mode.

This time is needed to avoid false triggering by external RF

fields. Recovery from these failures is detected

automatically after a certain time-out (filtering) and no

transmission is lost. In the event of failure 3 the CANH

driver and pin RTH are switched off. In the event of

failure 6 the CANL driver and pin RTL are switched off.

The pull-up current on pin RTL and the pull-down current

on pin RTH will not be switched off.

Failures 3a, 4 and 7 initially result in a permanent

dominant level on pin RXD. After a time-out, the CANL

driver and pin RTL are switched off (failures 4 and 7) or

the CANH driver and pin RTH are switched off (failure 3a).

Only a weak pull-up on pin RTL or a weak pull-down on

pin RTH remains. Reception continues by switching to the

single-wire mode via pins CANH or CANL. When

failures 3a, 4 or 7 are removed, the recessive bus levels

are restored. If the differential voltage remains below the

recessive threshold level for a certain period of time,

reception and transmission switch back to the differential

mode.

If any of the wiring failure occurs, the output signal on

pin ERR will become LOW. On error recovery, the output

signal on pin ERR will become HIGH again.

During all single-wire transmissions, the EMC

performance (both immunity and emission) is worse than

in the differential mode. The integrated receiver filters

suppress any HF noise induced into the bus wires.

The cut-off frequency of these filters is a compromise

between propagation delay and HF suppression. In the

single-wire mode, LF noise cannot be distinguished from

the required signal.

1999 Feb 11

6

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