Nuts and Volts - December 2016

Datasheets and documents for CAN based microcontrollers might make them seem complicated to use. Thankfully, MCU manufacturers provide code libraries and examples that can get you off to a good start. You need not understand every aspect of the CAN bus and CAN controllers to communicate between devices. This article will help you learn enough to use basic bus communications in projects. Later articles will supply more details of CAN signals and protocols.

People who use RS-232 type communications understand how positive and negative voltages represent logic 1s or 0s. The CAN bus, on the other hand, uses the voltage difference between two wires — labeled CANH and CANL — to represent logic levels. The SN65HVD251 bus driver, for example, includes a differential driver and a differential receiver (Figure 1).

When you apply a logic 1 to the driver input (D) on an SN65HVD251 IC, the CANH and CANL outputs will have the same voltage level; about 2.5V for a 5V supply. We call this a recessive state on the bus. A differential receiver — including the one in the SN65HVD251 — would detect no voltage difference between CANH and CANL, so its receiver output (R) would give us a logic 1. Apply a logic 0 to the D input and the CANH output rises to about 4V, while the CANL output drops to about 1.4V to create a 2.6V difference between the two signals as shown in Figure 2. We call this the dominant state, and all differential receivers would produce a logic 0 at their R output.

Even if all other drivers produce recessive signals, a logic 0 input to

The Controller Area Network (CAN) began as a communication path for electronic equipment in vehicles. Unlike the RS-232 or RS-485 standards, the CAN standard (ISO 11898) provides electrical specifications and a format for information exchanges. Many newer microcontrollers include one or two CAN controllers that properly format data, set device addresses, provide hardware error detection, and identify communication errors if any. As a bonus, CAN based devices may operate in a multi-master network that uses more than one controller to supervise operations.

Take a CAN Bus for a Spin

By Jon Titus KZ1G

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FIGURE 1. An SN65HVD251 provides a bus driver (top) and a bus receiver (bottom). The receiver output always indicates the state of the CAN bus. Ground the Rs input on this device, and do not make a connection to the reference-voltage output on pin 5 (not shown). A CAN bus does not need a common ground between devices (pin numbers for an eight-pin DIP).