Datasheet
Copyright © 2013 ARM Ltd. All rights reserved
CAN: Controller Area Network Lab using ST STM32 Cortex-M processors. www.keil.com
5
Physical Layer: the wires and the voltages…
There are three physical layers used in CAN: Hi-Speed, Fault Tolerant and Single Wire. Hi-Speed is the most common and
is the only one we will use in this article. Fault Tolerant offers more robustness as its name implies and is used more often in
European autos. Single Wire is used by General Motors and a few others as a low speed body network.
Hi-Speed in cars has a speed of 500 Kbps, trucks are 250 Kbps. CANopen runs up to 1 Mbps. Fault Tolerant is usually 125
Kbps and GM Single Wire is normally 33.33 Kbps. TIP: 1 Mbps in a large system is difficult to handle. 500Kbps is easier
to use and maintain and will present fewer design problems. In general, the longer the physical wires and more nodes, the
frequency should be lowered to maintain stability and reduce bus errors. You do not need to use these exact frequencies in
your own nodes that will not be connected to other nodes and therefore do not require bus speed compatibility.
To change from one layer to the other requires only the transceiver chip need be exchanged and probably the speed changed.
These three flavors of CAN cannot be physically connected to each other as the voltage levels are different. You need to use
a router or gateway to join different CAN networks together. Any CAN controller
will properly service all three flavors of CAN with the appropriate transceiver.
The Hi-speed CAN physical layer is merely a twisted pair of wires with a 120 ohm
termination resistor at each end and twisted wire drops to the individual CAN nodes.
You can connect your node’s transceiver chip directly to the bus. It is possible to
implement isolation techniques using appropriate devices.
CAN Hi voltage with respect to ground changes between 2.5 to 4 volts nominal.
CAN Lo changes from 2.5 to 1 volt. Therefore the difference between the two is
either 0 volts (is logical “1”) or 2 volts (is logical “0”).
0 volts is known as the “recessive” state and 2 volts is the “dominant” state.
These two signals, CAN Hi and CAN Lo, are 180 degrees out of phase as indicated in this diagram. Bus idle is when the
CAN Hi and CAN Lo voltage difference is near zero (Recessive) for at least 11 successive bit times.
A 2 node CAN cable assembly:
Two wires and two 120 Ω resistors.
A ground connection is preferred. It
facilitates certain bus defects such as
o
p
en or shorted lines
A CAN frame:
The top two traces are
CAN_Hi and CAN_Lo
respectively. Note they are
180 degrees out of phase.
These are the differential
signals.
The bottom trace is the
algebraic sum of the top two.
This is from the Keil CAN
example program using
transceiver chips.