 2001 Microchip Technology Inc.

DS00754A-page 1

M

AN754

INTRODUCTION

The Controller Area Network (CAN) protocol is an

asynchronous serial bus with Non-Return to Zero

(NRZ) bit coding designed for fast, robust communica-

tions in harsh environments, such as automotive and

industrial applications. The CAN protocol allows the

user to program the bit rate, the sample point of the bit,

and the number of times the bit is sampled. With these

features, the network can be optimized for a given

application.

There are relationships between bit timing parameters,

the physical bus propagation delays, and the oscillator

tolerances throughout the system. This application

note investigates these relationships as they pertain to

Microchip’s CAN module and assists in optimizing the

bit timing for given physical system attributes.

THE CAN BIT TIME

The CAN bit time is made up of non-overlapping seg-

ments. Each of these segments are made up of integer

units called Time Quanta (TQ) and are explained later

in this application note. The Nominal Bit Rate (NBR) is

defined in the CAN specification as the number of bits

per second transmitted by an ideal transmitter with no

resynchronization and can be described with the

equation:

Nominal Bit Time

overlapping segments (Figure 1), therefore, the NBT is

the summation of the following segments:

Associated with the NBT are the Sample Point, Syn-

chronization Jump Width (SJW), and Information Pro-

cessing Time (IPT), which are explained later.

SYNCHRONIZATION SEGMENT

The Synchronization Segment (SyncSeg) is the first

segment in the NBT and is used to synchronize the

nodes on the bus. Bit edges are expected to occur

within the SyncSeg. This segment is fixed at 1TQ.

PROPAGATION SEGMENT

The Propagation Segment (PropSeg) exists to com-

pensate for physical delays between nodes. The prop-

agation delay is defined as twice the sum of the signal’s

propagation time on the bus line, including the delays

associated with the bus driver. The PropSeg is pro-

grammable from 1 - 8TQ.

PHASE SEGMENT 1 AND PHASE SEGMENT 2

The two phase segments, PS1 and PS2 are used to

compensate for edge phase errors on the bus. PS1 can

be lengthened or PS2 can be shortened by resyncroni-

zation. PS1 is programmable from 1 - 8TQ and PS2 is

programmable from 2 - 8TQ.

FIGURE 1:

CAN BIT TIME SEGMENTS

Author:

Pat Richards

Microchip Technology Inc.

NBR

1

-------

==

++

+

=

Sample

Point

SyncSeg

PropSeg

PhaseSeg1 (PS1)

PhaseSeg2 (PS2)

Understanding Microchip’s CAN Module Bit Timing