A controller area network (CAN) is ideally suited to the many high-level industrial protocols embracing CAN and ISO-11898:2003 as their physical layer. Its cost, performance, and upgradeability provide for tremendous flexibility in system design. This application report presents an introduction to the CAN fundamentals, operating principles, and the implementation of a basic CAN bus with TI's CAN transceivers and DSPs. The electrical layer requirements of a CAN bus are discussed along with the importance of the different features of a TI CAN transceiver.
CAN Transceiver Features:
- 3.3-V Supply Voltage
Most CAN transceivers require a 5-V power supply to reach the signal levels required by the ISO 11898 standard. However, by superior attention to high-efficiency circuit design, the TI 3.3-V CAN transceiver family operates with a 3.3-V power supply and is fully interoperable with 5-V CAN transceivers on the same bus. This allows designers to reduce total node power by 50% or more
- ESD Protection
Static charge is an unbalanced electrical charge at rest, typically created by the physical contact of different materials. One surface gains electrons, whereas the other surface loses electrons. This results in an unbalanced electrical condition known as a static charge. When a static charge moves from one surface to another, it is referred to as an electrostatic discharge (ESD). It can occur only when the voltage differential between the two surfaces is sufficiently high to break down the dielectric strength of the medium separating the two surfaces. ESD can occur in any one of four ways: a charged body can touch an integrated circuit (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 sufficient to break it down. The main threat of ESD damage occurs during the assembly and manufacturing of circuits. After assembly and installation, the main protection required for the bus pins is surge protection.
- Common-Mode Voltage Operating Range
Common-mode voltage is the difference in potential between grounds of sending and receiving nodes on a bus. This is often the case in the networked equipment typically found in a CAN application. Possible effects of this problem are intermittent reboots, lock-ups, bad data transfer, or physical damage to a transceiver. Network interface cards, parallel ports, serial ports, and especially transceivers are prime targets for some form of failure if not designed to accommodate high levels of ground shift and power supply imbalance between typical CAN nodes
- Common-Mode Noise Rejection
Common-mode noise of varied magnitudes exist within the networks associated with CAN applications. Noise from pulsing motor controllers, switch-mode power supplies, or from fluorescent lighting load are the typical sources of noises that couple onto bus lines These would otherwise be straight lines.
- Controlled Driver Output Transition Times
Controlling the driver output slew rate dampens the rise time of a dominant bit to improve signal quality and provides for longer stub lengths and a better bit-error rate. For a discussion on how slew-rate control provides for longer stub lengths, see application report SLLA270.
- Low-Current Bus Monitor, Standby and Sleep Modes
Many applications are looking to lower-power opportunities as more electronics are added to designs. The standby mode in many TI transceivers is generally referred to as the “listen only” mode, because in standby, the driver circuitry is switched off while the receiver continues to monitor bus activity. In the occurrence of a dominant bit on the bus, the receiver passes this information along to its DSP/CAN controller which in turn activates the circuits that are in standby. This is achieved by placing a logic-low level on the Rs pin (pin 8) of the device.
- Bus Pin Short-Circuit Protection
V-Split The CAN Bus www.ti.com The ISO 11898 Standard recommends that a transceiver survive bus wire short-circuits to each other, to the power supply, and to ground. This ensures that transceivers are not damaged by bus cable polarity reversals, cable crush, and accidental shorts to high power supplies. The short-circuit protection in TI devices protects for an unlimited time. Once a problem is removed, the devices perform as designed whereas the CAN transceivers offered from competing vendors are permanently damaged and require replacement.
- Thermal Shutdown Protection
Another desirable safety feature for a CAN transceiver is the thermal shutdown circuitry of TI CAN transceivers. This feature protects a device against the destructive currents and resulting heat that can occur in a short-circuit condition. Once thermal shutdown is activated, the device remains shut down until the circuitry is allowed to cool. Once cooled down to normal operating temperature, the device automatically returns to active operation without damage.
- Bus Input Impedance
A high bus input impedance increases the number of nodes that can be added to a bus above the ISO 11898 Standard’s 30 nodes. The high impedance restricts the amount of current that a receiver sinks or sources onto a bus over common-mode voltage conditions. This ensures that a driver transmitting a message into such a condition is not required to sink or source a damaging amount of current from the sum of the receiver leakage currents on a bus.
V-split is a fortified Vcc/2 Vref pin with the same ESD protection rating, short-circuit protection, and common-mode operating range as the bus pins. It is used to stabilize bus voltage at Vcc/2 and prevent it from drifting to a high common-mode voltage during periods of inactivity.