Philips Semiconductors
Product specification
High speed CAN transceiver
TJA1040
FUNCTIONAL DESCRIPTION
Operating modes
to the centre tap of the split termination (see Fig.4). In case
of a recessive bus voltage <0.5VCC due to the presence of
an unsupplied transceiver in the network with a significant
leakage current from the bus lines to ground, the split
circuit will stabilize this recessive voltage to 0.5VCC. So a
start of transmission does not cause a step in the
common-mode signal which would lead to poor
The TJA1040 provides two modes of operation which are
selectable via pin STB. See Table 1 for a description of the
modes of operation.
Table 1 Operating modes
ElectroMagnetic Emission (EME) behaviour.
PIN RXD
PIN
Wake-up
MODE
STB
LOW
HIGH
In the standby mode the bus lines are monitored via a
low-power differential comparator. Once the low-power
differential comparator has detected a dominant bus level
for more than tBUS, pin RXD will become LOW.
normal
LOW
bus dominant
bus recessive
standby
HIGH wake-up request no wake-up
detected request detected
Over-temperature detection
NORMAL MODE
The output drivers are protected against over-temperature
conditions. If the virtual junction temperature exceeds the
shutdown junction temperature Tj(sd), the output drivers will
be disabled until the virtual junction temperature becomes
lower than Tj(sd) and TXD becomes recessive again.
By including the TXD condition, the occurrence of output
driver oscillation due to temperature drifts is avoided.
In this mode the transceiver is able to transmit and receive
data via the bus lines CANH and CANL. See Fig.1 for the
block diagram. The differential receiver converts the
analog data on the bus lines into digital data which is
output to pin RXD via the multiplexer (MUX). The slope of
the output signals on the bus lines is fixed and optimized
in a way that lowest ElectroMagnetic Emission (EME) is
guaranteed.
TXD dominant time-out function
STANDBY MODE
A ‘TXD dominant time-out’ timer circuit prevents the bus
lines from being driven to a permanent dominant state
(blocking all network communication) if pin TXD is forced
permanently LOW by a hardware and/or software
application failure. The timer is triggered by a negative
edge on pin TXD.
In this mode the transmitter and receiver are switched off,
and the low-power differential receiver will monitor the bus
lines. A HIGH level on pin STB activates this low-power
receiver and the wake-up filter, and after tBUS the state of
the CAN bus is reflected on pin RXD.
If the duration of the LOW level on pin TXD exceeds the
internal timer value (tdom), the transmitter is disabled,
driving the bus lines into a recessive state. The timer is
reset by a positive edge on pin TXD. The TXD dominant
time-out time tdom defines the minimum possible bit rate of
40 kBaud.
The supply current on VCC is reduced to a minimum in
such a way that ElectroMagnetic Immunity (EMI) is
guaranteed and a wake-up event on the bus lines will be
recognized.
In this mode the bus lines are terminated to ground to
reduce the supply current (ICC) to a minimum. A diode is
added in series with the high-side driver of RXD to prevent
a reverse current from RXD to VCC in the unpowered state.
In normal mode this diode is bypassed. This diode is not
bypassed in standby mode to reduce current consumption.
Fail-safe features
Pin TXD provides a pull-up towards VCC in order to force a
recessive level in case pin TXD is unsupplied.
Pin STB provides a pull-up towards VCC in order to force
the transceiver into standby mode in case pin STB is
unsupplied.
Split circuit
Pin SPLIT provides a DC stabilized voltage of 0.5VCC. It is
turned on only in normal mode. In standby mode pin SPLIT
is floating. The VSPLIT circuit can be used to stabilize the
recessive common-mode voltage by connecting pin SPLIT
In the event that the VCC is lost, pins TXD, STB and RXD
will become floating to prevent reverse supplying
conditions via these pins.
2003 Oct 14
4
NXP [ NXP ]
