AN228 / A CAN Physical Layer Discussion

http://ww1.microchip.com/downloads/en/AppNotes/00228a.pdf

 2002 Microchip Technology Inc. Preliminary DS00228A-page 1

M AN228

INTRODUCTION

Many network protocols are described using the seven

layer Open System Interconnection (OSI) model, as

shown in Figure 1. The Controller Area Network (CAN)

protocol defines the Data Link Layer and part of the

Physical Layer in the OSI model. The remaining physical

layer (and all of the higher layers) are not defined by

the CAN specification. These other layers can either be

defined by the system designer, or they can be implemented

using existing non-proprietary Higher Layer

Protocols (HLPs) and physical layers.

The Data Link Layer is defined by the CAN specification.

The Logical Link Control (LLC) manages the overload

control and notification, message filtering and

recovery management functions. The Medium Access

Control (MAC) performs the data encapsulation/decapsulation,

error detection and control, bit stuffing/destuffing

and the serialization and deserialization

functions.

The Physical Medium Attachment (PMA) and Medium

Dependent Interface (MDI) are the two parts of the

physical layer which are not defined by CAN. The

Physical Signaling (PS) portion of the physical layer is

defined by the CAN specification. The system designer

can choose any driver/receiver and transport medium

as long as the PS requirements are met.

The International Standards Organization (ISO) has

defined a standard which incorporates the CAN specification

as well as the physical layer. The standard,

ISO-11898, was originally created for high-speed invehicle

communications using CAN. ISO-11898 specifies

the physical layer to ensure compatibility between

CAN transceivers.

A CAN controller typically implements the entire CAN

specification in hardware, as shown in Figure 1. The

PMA is not defined by CAN, however, it is defined by

ISO-11898. This document discusses the MCP2551

CAN transceiver and how it fits in with the ISO-11898

specification.

FIGURE 1: CAN AND THE OSI MODEL

Author: Pat Richards

Microchip Technology Inc.

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical Medium Attachment

Physical Signaling

Medium Dependent Interface

- Bit encoding/decoding

- Bit timing/synchronization

- Driver/receiver characteristics

- Connectors/wires

Logical Link Control (LLC)

Medium Access Control (MAC)

- Data encapsulation/decapsulation

- Acceptance filtering

- Overload notification

- Recovery management

- Frame coding (stuffing/de-stuffing)

- Error detection/signaling

- Serialization/deserialization

Defined by

ISO11898

7- Layer OSI

CAN Controller

Transceiver

MCP2551

A CAN Physical Layer DiscussionAN228

DS00228A-page 2 Preliminary  2002 Microchip Technology Inc.

ISO11898-2 OVERVIEW

ISO11898 is the international standard for high-speed

CAN communications in road vehicles. ISO-11898-2

specifies the PMA and MDA sublayers of the Physical

Layer. See Figure 3 for a representation of a common

CAN node/bus as described by ISO-11898.

Bus Levels

CAN specifies two logical states: recessive and dominant.

ISO-11898 defines a differential voltage to represent

recessive and dominant states (or bits), as shown

in Figure 2.

In the recessive state (i.e., logic ‘1’ on the MCP2551

TXD input), the differential voltage on CANH and CANL

is less than the minimum threshold (<0.5V receiver

input or <1.5V transmitter output)(See Figure 4).

In the dominant state (i.e., logic ‘0’ on the MCP2551

TXD input), the differential voltage on CANH and CANL

is greater than the minimum threshold. A dominant bit

overdrives a recessive bit on the bus to achieve

nondestructive bitwise arbitration.

FIGURE 2: DIFFERENTIAL BUS

Connectors and Wires

ISO-11898-2 does not specify the mechanical wires

and connectors. However, the specification does

require that the wires and connectors meet the electrical

specification.

The specification also requires 120Ω (nominal) terminating

resistors at each end of the bus. Figure 3 shows

an example of a CAN bus based on ISO-11898.

FIGURE 3: CAN BUS

Voltage Level (V)

Time (t)

VDIFF

Dominant

Recessive Recessive

CANH

CANL

120Ω 120Ω

MCU

CAN Controller

Transceiver

Node Node 2002 Microchip Technology Inc. Preliminary DS00228A-page 3

AN228

FIGURE 4: ISO11898 NOMINAL BUS LEVELS

Robustness

The ISO11898-2 specification requires that a compliant

or compatible transceiver must meet a number of electrical

specifications. Some of these specifications are

intended to ensure the transceiver can survive harsh

electrical conditions, thereby protecting the

communications of the CAN node. The transceiver

must survive short circuits on the CAN bus inputs from

-3V to +32V and transient voltages from -150V to

+100V. Table 1 shows the major ISO11898-2 electrical

requirements, as well as MCP2551 specifications.

TABLE 1: COMPARING THE MCP2551 TO ISO11898-2

2.5

3.5

1.5

0.9

5.0

0.5

-1.0

-0.5

0.05

1.5

3.0

V

V V

Recessive

Differential

Input

Range

Dominant

Differential

Input

Range

Dominant

Differential

Output

Range

Recessive

Differential

Output

Range

CANH

CANL

Parameter

ISO-11898-4 MCP2551

Unit Comments

min max min max

DC Voltage on CANH and CANL -3 +32 -40 +40 V Exceeds ISO-11898

Transient voltage on CANH and CANL -150 +100 -250 +250 V Exceeds ISO-11898

Common Mode Bus Voltage -2.0 +7.0 -12 +12 V Exceeds ISO-11898

Recessive Output Bus Voltage +2.0 +3.0 +2.0 +3.0 V Meets ISO-11898

Recessive Differential Output Voltage -500 +50 -500 +50 mV Meets ISO-11898

Differential Internal Resistance 10 100 20 100 kΩ Meets ISO-11898

Common Mode Input Resistance 5.0 50 5.0 50 kΩ Meets ISO-11898

Differential Dominant Output Voltage +1.5 +3.0 +1.5 +3.0 V Meets ISO-11898

Dominant Output Voltage (CANH) +2.75 +4.50 +2.75 +4.50 V Meets ISO-11898

Dominant Output Voltage (CANL) +0.50 +2.25 +0.50 +2.25 V Meets ISO-11898

Permanent Dominant Detection (Driver) Not Required 1.25 — ms

Power-On Reset and Brown-Out Detection Not Required Yes --AN228

DS00228A-page 4 Preliminary  2002 Microchip Technology Inc.

Bus Lengths

ISO11898 specifies that a transceiver must be able to

drive a 40m bus at 1 Mb/s. A longer bus length can be

achieved by slowing the data rate. The biggest limitation

to bus length is the transceiver’s propagation

delay.

PROPAGATION DELAY

The CAN protocol has defined a recessive (logic ‘1’)

and dominant (logic ‘0’) state to implement a nondestructive

bit-wise arbitration scheme. It is this arbitration

methodology that is affected most by propagation

delays. Each node involved with arbitration must be

able to sample each bit level within the same bit time.

For example, if two nodes at opposite ends of the bus

start to transmit their messages at the same time, they

must arbitrate for control of the bus. This arbitration is

only effective if both nodes are able to sample during

the same bit time. Figure 5 shows a one-way propagation

delay between two nodes. Extreme propagation

delays (beyond the sample point) will result in invalid

arbitration. This implies that bus lengths are limited at

given CAN data rates.

A CAN system’s propagation delay is calculated as

being a signal’s round-trip time on the physical bus

(tbus), the output driver delay (tdrv) and the input comparator

delay (tcmp). Assuming all nodes in the system

have similar component delays, the propagation delay

is explained mathematically:

EQUATION 1:

FIGURE 5: ONE-WAY PROPAGATION DELAY

tprop 2 tbus tcmp tdrv = ⋅ ( ) + +

SyncSeg

Sample Point

SyncSeg

Transmitted Bit from “Node A”

“Node A” bit received by “Node B”

Propagation Delay

Time (t)

PropSeg PhaseSeg1 (PS1) PhaseSeg2 (PS2)

PropSeg PhaseSeg1 (PS1) PhaseSeg2 (PS2) 2002 Microchip Technology Inc. Preliminary DS00228A-page 5

AN228

MCP2551 CAN TRANSCEIVER

The MCP2551 is a CAN Transceiver that implements

the ISO-11898-2 physical layer specification. It supports

a 1 Mb/s data rate and is suitable for 12 V and 24

V systems. The MCP2551 provides short-circuit

protection up to ±40V and transient protection up to

±250V.

In addition to being ISO-11898-2-compatible, the

MCP2551 provides power-on reset and brown-out protection,

as well as permanent dominant detection to

ensure an unpowered or faulty node will not disturb the

bus. The device implements configurable slope control

on the bus pins to help reduce RFI emissions. Figure

6 shows the block diagram of the MCP2551.

General MCP2551 Operation

TRANSMIT

The CAN protocol controller outputs a serial data

stream to the logic TXD input of the MCP2551. The corresponding

recessive or dominant state is output on the

CANH and CANL pins.

RECEIVE

The MCP2551 receives dominant or recessive states

on the same CANH and CANL pins as the transmit

occurs. These states are output as logic levels on the

RXD pin for the CAN protocol controller to receive CAN

frames.

RECESSIVE STATE

A logic ‘1’ on the TXD input turns off the drivers to the

CANH and CANL pins and the pins “float” to a nominal

2.5V via biasing resistors.

DOMINANT STATE

A logic ‘0’ on the TXD input turns on the CANH and

CANL pin drivers. CANH drives ~1V higher than the

nominal 2.5V recessive state to ~3.5V. CANL drives

~1V less than the nominal 2.5V recessive state to

~1.5V.

FIGURE 6: MCP2551 BLOCK DIAGRAM

VDD

VSS

CANH

CANL

TXD

RS

RXD

VREF

VDD

Slope

Control

Power-On

Reset

Reference

Voltage

Receiver

GND

0.5 VDD

TXD

Dominant

Detect

Thermal

Shutdown

Driver

ControlAN228

DS00228A-page 6 Preliminary  2002 Microchip Technology Inc.

Modes of Operation

There are three modes of operation that are externally

controlled via the RS pin:

1. High-Speed

2. Slope Control

3. Standby

HIGH-SPEED

The high-speed mode is selected by connecting the RS

pin to VSS. In this mode, the output drivers have fast

rise and fall times that support the higher bus rates up

to 1 Mb/s and/or maximum bus lengths by providing the

minimum transceiver loop delays.

SLOPE CONTROL

If reduced EMI is required, the MCP2551 can be placed

in slope control mode by connecting a resistor (REXT)

from the RS pin to ground. In slope control mode, the

single-ended slew rate (CANH or CANL) is basically

proportional to the current out of the RS pin. The current

must be in the range of 10 µA < -IRS < 200 µA, which

corresponds to a voltage on the pin of 0.4 VDD < VRS <

0.6 VDD respectively (or 0.5 VDD typical).

The decreased slew rate implies a slower CAN data

rate at a given bus length, or a reduced bus length at a

given CAN data rate.

STANDBY

Standby (or sleep) mode is entered by connecting the

RS pin to VDD. In sleep mode, the transmitter is

switched off and the receiver operates in a reduced

power mode. While the receive pin (RXD) is still

functional, it will operate at a slower rate.

Standby mode can be used to place the device in low

power mode and to turn off the transmitter in case the

CAN controller malfunctions and sends unexpected

data to the bus.

Permanent Dominant Detection on

Transmitter

The MCP2551 will turn off the transmitter to CANH and

CANL if an extended dominant state is detected on the

transmitter. This ability prevents a faulty node (CAN

controller or MCP2551) from permanently corrupting

the CAN bus.

The drivers are disabled if TXD is low for more than

~1.25 ms (minimum) (See Figure 7).

The drivers will remain disabled as long as TXD

remains low. A rising edge on TXD will reset the timer

logic and enable the drivers.

FIGURE 7: TXD PERMANENT DOMINANT DETECTION

Dominant

Recessive

tDOM

Transmitter

Disabled

Transmitter

Enabled

Recessive

Dominant

TXD

CANH

CANL 2002 Microchip Technology Inc. Preliminary DS00228A-page 7

AN228

Power-On Reset and Brown-Out

The MCP2551 incorporates both Power-On Reset

(POR) and Brown-Out Detection (BOD) (see Figure 8).

POWER-ON RESET (POR)

When the MCP2551 is powered on, the CANH and

CANL pins remain in the high impedance state until

VDD reaches the POR high voltage (VPORH).

Additionally, if the TXD pin is low at power-up, the

CANH and CANL pins will remain in high impedance

until TXD goes high. After which, the drivers will

function normally.

BROWN-OUT DETECTION (BOD)

BOD occurs when VDD goes below the power-on reset

low voltage (VPORL). At this point, the CANH and CANL

pins enter a high impedance state and will remain there

until VPORH is reached.

FIGURE 8: POWER-ON RESET AND BROWN-OUT DETECTION

3.0

3.5

4.0

V

t

TXD

CANH

CANL

High

Impedance

High

Impedance

VPORH

VPORL

VDD

POR

BODAN228

DS00228A-page 8 Preliminary  2002 Microchip Technology Inc.

Ground Offsets

Since it is not required to provide a common ground

between nodes, it is possible to have ground offsets

between nodes. That is, each node may observe different

single-ended bus voltages (common mode bus

voltages) while maintaining the same differential voltage.

While the MCP2551 is specified to handle ground

offsets from -12V to +12V, the ISO-11898 specification

only requires -2V to +7V. Figure 9 and Figure 10

demonstrate how ground offsets appear between

nodes.

Figure 9 shows the transmitting node with a positive

ground offset with respect to the receiving node. The

MCP2551 receiver can operate with CANH = +12V.

The maximum CAN dominant output voltage

(VO(CANH)) from the transmitting node is 4.5V. Subtracting

this maximum yields an actual ground offset (with

respect to the receiving node) of 7.5V for the transmitting

node. In the recessive state, each node attempts to

pull the CANH and CANL pins to their biasing levels

(2.5V typical). However, the resulting common mode

voltage in the recessive state becomes 6.25V for the

receiving node and -1.25V for the transmitting node.

Figure 10 shows the transmitting node with a negative

ground offset with respect to the receiving node. The

MCP2551 receiver can operate with CANL = -12V. The

minimum CAN dominant output voltage (VO(CANL))

from the transmitting node is 0.5V. Subtracting this minimum

yields an actual ground offset, with respect to the

receiving node, of -12.5V. The common mode voltage

for the recessive state is -6.25V for the receiving node

and 6.25V for the transmitting node.

Since all nodes act as a transmitter for a portion of each

message (i.e., each receiver must acknowledge (ACK)

valid messages during the ACK slot), the largest

ground offset allowed between nodes is 7.5V, as shown

in Figure 9.

Operating a CAN system with large ground offsets can

lead to increased electromagnetic emissions. Steps

must be taken to eliminate ground offsets if the system

is sensitive to emissions.

FIGURE 9: RECEIVING (NODE GROUND) BELOW TRANSMITTING (NODE GROUND)

Common Mode

Bus Voltage

(Single Ended)

Transmitting Node Ground

Receiving Node Ground

0

6

12

CANH

CANL

VDIFF(max)

3V

VO(CANH)(max)

4.5V

6.25V

-1.25V

7.5V

12V 2002 Microchip Technology Inc. Preliminary DS00228A-page 9

AN228

FIGURE 10: RECEIVING (NODE GROUND) ABOVE TRANSMITTING (NODE GROUND)

BUS TERMINATION

Bus termination is used to minimize signal reflection on

the bus. ISO-11898 requires that the CAN bus have a

nominal characteristic line impedance of 120Ω. Therefore,

the typical terminating resistor value for each end

of the bus is 120Ω. There are a few different termination

methods used to help increase EMC performance (see

Figure 11).

1. Standard Termination

2. Split Termination

3. Biased Split Termination

Standard Termination

As the name implies, this termination uses a single

120Ω resistor at each end of the bus. This method is

acceptable in many CAN systems.

Split Termination

Split termination is a concept that is growing in popularity

because emission reduction can be achieved very

easily. Split termination is a modified standard termination

in which the single 120Ω resistor on each end of

the bus is split into two 60Ω resistors, with a bypass

capacitor tied between the resistors and to ground. The

two resistors should match as close as possible.

Common Mode

Bus Voltage

(Single-Ended)

Transmitting Node Ground

Receiving Node Ground

-13

-6

0

CANH

CANL

VDIFF(max)

6.25V

-6.25V

12.5V -12V

VO(CANL)(max) 3V

0.5V

Note: EMC performance is not determined solely

by the transceiver and termination method,

but rather by careful consideration of all

components and topology of the system.AN228

DS00228A-page 10 Preliminary  2002 Microchip Technology Inc.

Biased Split Termination

This termination method is used to maintain the common

mode recessive voltage at a constant value,

thereby increasing EMC performance. This circuit is

the same as the split termination with the addition of a

voltage divider circuit to achieve a voltage of VDD/2

between the two 60 Ω resistors (see Figure 11).

FIGURE 11: TERMINATION

CONCEPTS

REFERENCES

MCP2551 Data Sheet, “High Speed CAN Transceiver”,

DS21667, Microchip Technology, Inc.

AN754, “Understanding Microchip’s CAN Module Bit

Timing”, DS00754, Microchip Technology, Inc.

ISO-11898-2, “Road Vehicles - Interchange of Digital

Information - Part 2: High Speed Medium Access Unit

and Medium Dependant Interface”, International

Organization for Standardization.

CAN System Engineering, “From Theory to Practical

Applications”, Wolfhard Lawrenz, Springer.

Note: The biasing resistors in Figure 11, as well

as the split termination resistors, should

match as close as possible.

Standard

Termination

Split

Termination

Biased

Termination

Split

120 Ω

60 Ω

60 Ω

60 Ω

R1 60 Ω

R2

C

C 2002 Microchip Technology Inc. DS00228A - page 11

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The Company’s quality system processes and

procedures are QS-9000 compliant for its

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products.DS00228A-page 12  2002 Microchip Technology Inc.

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