TJA1054

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Product data sheet

Rev. 4 — 3 August 2010

5 of 25

NXP Semiconductors

TJA1054

Fault-tolerant CAN transceiver

7.

Functional description

The TJA1054 is the interface between the CAN protocol controller and the physical wires

of the CAN bus (see Figure 7). It is primarily intended for low-speed applications, up to

125 kBd, in passenger cars. The device provides differential transmit capability to the

CAN bus and differential receive capability to the CAN controller.

To reduce EME, the rise and fall slopes are limited. This allows the use of an unshielded

twisted pair or a parallel pair of wires for the bus lines. Moreover, the device 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 Figure 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 function (TxD dominant time-out function) has been integrated to prevent the bus

lines from being driven into a permanent dominant state (thus blocking the entire network

communication) due to a situation in which pin TXD is permanently forced to a LOW level,

caused by a hardware and/or software application failure.

If the duration of the LOW level on pin TXD exceeds a certain time, the transmitter will be

disabled. The timer will be reset by a HIGH level on pin TXD.

7.1 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 4). The

differential receiver threshold voltage is set at

−3.2 V typical (V

correct reception with a noise margin as high as possible in the normal operating mode

and in the event of failures 1, 2, 5 and 6a. These failures, or recovery from them, do not

destroy ongoing transmissions. The output drivers remain active, the termination does not

change and the receiver remains in differential mode (see Table 4).

Failures 3, 3a and 6 are detected by comparators connected to the CANH and CANL bus

lines. Failures 3 and 3a are detected in a two-step approach. If the CANH bus line

exceeds a certain voltage level, the differential comparator signals a continuous dominant

condition. Because of inter operability reasons with the predecessor products

PCA82C252 and TJA1053, after a first time-out the transceiver switches to single-wire

operation through CANH. If the CANH bus line is still exceeding the CANH detection

voltage for a second time-out, the TJA1054 switches to CANL operation; the CANH driver

is switched off and the RTH bias changes to the pull-down current source. The time-outs

(delays) are needed to avoid false triggering by external RF fields.