AN1105 datasheet(4/100 Pages) STMICROELECTRONICS

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ST7 pCAN PERIPHERAL DRIVER

1 CAN COMMUNICATION PROTOCOL

In the early 80s, electronic applications appeared in the automotive world. The need gradually

arose to establish real-time communications between various types of on-board equipment.

In 1986, the CAN bus protocol, designed by Robert Bosch GmbH, was presented to the public

for the first time. In 1989, the first silicon implementing the protocol was issued and in 1991,

the first car equipped with a CAN network was produced.

Today, the protocol is an international norm (ISO 11898). Its relative simplicity, the wide avail-

ability of products and services in terms of chips, software libraries, starter kits, courses and

training makes the development of applications relatively fast and cheap.

The CAN protocol provides the user with very reliable communication, with a powerful error

detection/confinement mechanism. It allows baud rates up to 1Mbaud (for networks of up to

40m) which makes it able to support real time applications. The bus can of course be config-

ured with lower speeds for larger networks (50kbaud for networks up to 1km long for ex-

ample).

There were more than 150 million CAN nodes installed at the beginning of 1999.

1.1 GENERAL CHARACTERISTICS

1.1.1 Protocol properties

Asynchronous serial bus (See “Bit timing” on page 9)

– No clock signal is transmitted, allowing a two-wires (high speed CAN see ISO 11898) or

even one-wire communication (low speed CAN, see ISO 11519)

– Each node resynchronizes itself on every falling edge occurring on the bus.

CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)

– Several nodes can request the bus simultaneously (CSMA).

– When such a situation occurs, there is no loss of data and the message with the highest

priority is immediately sent (CA).

Multimaster capability (See “Arbitration Phase” on page 10)

– Every CAN node in the network is able to transmit data.

– The arbitration mechanism is decentralized.

Object oriented communication

– Each message on the bus carries its own identifier which is an 11-bit (or 29-bit) number.

This field describes the content of the message. It is not an address.

– Consequently, a transmitter doesn’t address data to a peer but simply broadcasts its data.

Each node in the network picks off data from the bus when it recognizes an identifier that

concerns it.

AN1105 Datasheet(PDF) 4 Page - STMicroelectronics