Philips Semiconductors
Product specification
Fault-tolerant CAN transceiver
FUNCTIONAL DESCRIPTION
The TJA1054A is the interface between the CAN protocol
controller and the physical wires of the CAN bus (see
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 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 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 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.
TJA1054A
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.
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).
The differential receiver threshold voltage is set at
−3.2
V
typical (V
CC
= 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, 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 1).
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 TJA1054A 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.
Table 1
Bus failures
DESCRIPTION
CANH wire interrupted
CANL wire interrupted
CANH short-circuited to battery
CANH short-circuited to V
CC
CANL short-circuited to ground
CANH short-circuited to ground
CANL short-circuited to battery
CANL short-circuited to V
CC
CANL and CANH mutually
short-circuited
TERMINATION TERMINATION CANH
CANL
CANH (RTH)
CANL (RTL) DRIVER DRIVER
on
on
weak; note 1
weak; note 1
on
on
on
on
on
on
on
on
on
weak; note 2
on
weak; note 2
on
weak; note 2
on
on
off
off
on
on
on
on
on
on
on
on
on
off
on
off
on
off
RECEIVER
MODE
differential
differential
CANL
CANL
CANH
differential
CANH
differential
CANH
FAILURE
1
2
3
3a
4
5
6
6a
7
Notes
1. A weak termination implies a pull-down current source behaviour of 75
µA
typical.
2. A weak termination implies a pull-up current source behaviour of 75
µA
typical.
2004 Mar 23
6
PHILIPS [ NXP SEMICONDUCTORS ]
