Texas Instruments | Improved CAN network security with TI’s SN65HVD1050 transceiver | Application notes | Texas Instruments Improved CAN network security with TI’s SN65HVD1050 transceiver Application notes

Texas Instruments Improved CAN network security with TI’s SN65HVD1050 transceiver Application notes
Interface
Texas Instruments Incorporated
Improved CAN network security with
TI’s SN65HVD1050 transceiver
By Steve Corrigan (Email: s-corrigan1@ti.com)
New Product Definition
A CAN transceiver must reliably transmit data in extremely
harsh operating environments that place an extraordinary
electrical burden on the device. Since the transceiver is
typically the only interface between expensive node
electronics and the CAN bus, many operational security
features of the Texas Instruments (TI) SN65HVD1050
take on additional importance in CAN applications. These
features include electromagnetic (EM) immunity, low EM
emissions, noise rejection, electrostatic discharge (ESD)
protection, fault tolerance, and protection during hot
plugging or power cycling.
Measurement of the common-mode output signal is
believed to provide all the information necessary to predict
performance of system-level emissions. Both time and
frequency representations of the output common-mode
signal can be analyzed for the purpose of evaluating
emissions behavior.
Figure 1. Typical CAN bus VOC(PP) waveform
As the EM spectrum becomes more fully utilized, EM fields
radiated by a wide range of devices are increasing the
probability of interference with other electronic equipment.
Due in part to the wireless revolution in electronics, EM
interference is increasingly becoming a widespread concern.
Every electronic device has its own unique EM characteristics. The inductance and capacitance of any circuit
may develop a common-mode resonance at discrete
frequencies that either amplify or attenuate emissions.
The HVD1050 CAN transceiver is designed and tested
for EM compatibility without malfunction or degradation of
performance in rugged EM environments. “Compatibility”
in this definition means both low emissions and high
immunity to external EM fields.
Amplitude: 200 mV/div
EM immunity
Time: 500 ns/div
Low EM emissions: Balanced signaling and
common-mode output
An important requirement of products intended for networking applications is that they behave in a way that does
not interfere with the operation of other nearby components
or systems. The desired behavior is referred to as “low
radiated emissions” and is typically tested according to
various quantitative requirements specified at the system
or electronic module level.
EM noise is generated by imbalanced high-frequency
voltage or current switching. In a CAN transceiver, systemlevel emission performance must be translated into transceiver characteristics. Specifically, driver output signals
are typically mismatched on CANH and CANL, and the
resultant EM fields fail to differentially cancel each other
as equal and opposite. This output mismatch (displayed in
Figures 1 and 2) is referred to in TI datasheets as the
peak-to-peak common-mode output voltage, VOC(PP), and
may be considered to be a figure of merit for balanced
differential signaling.
Figure 2. VOC(PP) test setup
VOC
CANH
30 Ω
CANL
30 Ω
+
VOC
–
VOC(PP)
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Amplitude: 1 V/div
Figure 3. Common-mode noise coupled onto
four twisted-pair bus lines
Time: 50 ns/div
High immunity: Common-mode noise rejection
Common-mode noise rejection is an inherent feature of true
differential receivers, just as it is with other differentialinput circuits such as operational amplifiers. Differential
signal pairs are physically close together and are generally
exposed to the same noise sources—noise common to each
wire. This ensures that exposure to EM fields is nearly
equal and common to each line, canceling the differential
influence from magnetic field coupling by reversing the
polarity in adjacent loops of twist in twisted-pair wiring.
Unwanted noise of various magnitudes easily links the
antenna-like bus lines of CAN applications. Pulsing motor
controllers, switch-mode power supplies, and fluorescent
lighting are typical noise sources that couple onto bus
lines (displayed in Figure 3).
A CAN transceiver not specifically designed to reject
this coupled noise will respond as if it were data on a bus
and send corrupted and meaningless data to a controller.
TI’s HVD1050 CAN transceiver is specifically designed
and tested for its ability to reject this noise over an
extremely wide (–12-V to +12-V) common-mode operating
range. The high degree to which the differential receiver
rejects coupled noise is evidence of the careful electrical
and mechanical design of the receiver components that
ensures that input matching is as close to ideal as possible.
Voltage transients and integrated-circuit protection
ESD can occur in any of four ways: A charged body can
touch an IC; a charged IC can touch a grounded surface; a
charged machine can touch an IC; or an electrostatic field
can induce a voltage across a dielectric that is sufficient to
break it down. It becomes readily apparent that a high
ESD rating indicates not only a robust transceiver but a
robust circuit design as well.
Comparable CAN transceivers on the market provide
only 4-kV ESD protection, while the HVD1050 CAN transceiver has an ESD rating of 8 kV when tested in accordance
with the Human Body Model (HBM) of JEDEC Standard
22 A114-B. With this ESD rating, the HVD1050 is much
better suited to harsh electrical environments than the
earlier transceiver versions of other vendors.
To ensure the HVD1050’s robustness, it is also tested to
±200 V in accordance with ISO 7637, test pulses 1, 2, 3a,
3b, 5, 6, and 7.
Fault tolerance
Bus hangs and dominant time-out
CAN bus operators occasionally report that all bus communication comes to a halt when a faulty node places a
continuous dominant bit on the bus. This stuck-dominant
condition occurs either from a faulty controller or from
random slivers of wire, a solder ball, or metal shaving
shorts across a transceiver’s input (TXD) pin and the
adjoining ground (GND) pin.
A dominant-time-out circuit in the HVD1050 prevents
the driver from blocking network communication with a
hardware or software failure. The time-out circuit is triggered by a falling edge on TXD. If no rising edge occurs
before the time-out constant of the circuit expires, the
driver is output-disabled, releasing the bus from the stuckdominant condition. Once the fault is corrected, the circuit
is reset by the next rising edge on TXD.
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Crushed-cable and short-circuit protection
Bus-cable polarity reversals, accidentally crushed cable,
and unintentional shorts of the bus wires to power supplies or ground are common in many CAN applications. To
provide protection from these real-world operating events
that are common on 24-V industrial buses, the HVD1050
provides short-circuit protection from –27 V up to +40 V.
This short-circuit protection extends over any length of
time and guarantees that the device will continue normal
operation once the fault is removed.
Additional circuit protection is provided with the thermal shutdown circuitry. In the event of a bus short with a
runaway current condition, the HVD1050 automatically
shuts the device down when thermal conditions exist that
could damage internal circuitry.
Hot plugging, power cycling, and glitch-free outputs
Adding additional components to a network most often
requires shutting down the entire network to prevent
costly system errors. Therefore the ability to plug directly
into an operating system becomes a valued asset in many
CAN applications.
Plugging an unpowered module directly into a powered
system is referred to as “hot plugging” and requires that
the transceiver output remain stable during the unpowered
to power-up transition without disturbing ongoing network
communications.
Many CAN transceivers on the market today have a very
low output impedance when unpowered. This causes the
device to sink any signal present on the bus and effectively
shuts down all data transmission.
For this purpose, the HVD1050’s bus pins are biased
internally to a high-impedance recessive state. This provides for a power-up into a known recessive condition
without disturbing ongoing bus communication. It also
maintains the integrity of the bus when power or ground
is added to or removed from the circuit.
Together these features of the HVD1050 serve to greatly
enhance the operational security of any CAN application.
Related Web sites
interface.ti.com
www.ti.com/sc/device/SN65HVD1050
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