NXP AN10364 Technical data

AN10364

Opto-electrical isolation of the I2C-bus (operating the bus
between points with different local ground potential)

Rev. 1 — 12 November 2010 Application note

Document information

Info Content
Keywords P82B96, PCA9600, PCA9601, RS-485, I2C-bus, Fast-mode Plus, CAN

bus, opto-isolation, safety isolation, PoE

2

Abstract This report discusses factors that need consideration when an I

used to communicate between two points that do not share a common
logic ground potential. I
and it is expected that connected devices share that reference.

In practice, when an I2C-bus is extended to link devices that are separated
by distances exceeding about 1 meter there can be many factors that
cause differences in the local ground potential of those devices and that in
turn affects the noise margins of the system and can lead to device ratings
being exceeded.

In other cases where an I2C-bus could provide an attractive data link there
is no possibility for a shared ground reference. Examples include the need
to provide safety isolation between the I
power, telephone lines, or patient monitoring medical equipment. In other
cases, such as Power over Ethernet, standards require isolation of the
grounds.

Solutions are provided to deal with ground differences ranging from less
than 1 V up to kilovolt levels by using Opto or transformer isolators in the
signal path.

Key to all solutions is the availability of I2C buffers with the capability to
split the bidirectional I
logic streams that can be then be handled using conventional methods.

Solutions for links from 3 kHz to 1 MHz speeds are described.

2

C logic signals are referenced to a ground level

2

C control signals and AC mains

2

C signals into pairs of conventional uni-directional

C-bus is

NXP Semiconductors

AN10364

Opto-electrical isolation of the I2C-bus

Revision history

Rev Date Description

v.1 20101112 application note; initial release

Contact information

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AN10364 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.

Application note Rev. 1 — 12 November 2010 2 of 16

NXP Semiconductors

1. Introduction

The I2C-bus provides an attractive maintenance and control communication interface
between parts of a system since it uses only two signal wires yet has powerful addressing
and a reasonably fast, up to 1 MHz, bidirectional data handling capability.

2

I

C-bus logic signals are referenced to a common (ground) potential. Any potential
difference between the grounds of different devices on an I
the logic signal level between the sending and receiving devices that is equivalent to
noise-induced signal level disturbances on the bus wiring. It increases the possibility of
communication errors or exceeding the voltage ratings of the connected devices. Most

2

I

C-bus ICs have I/Os ratings that prohibit applying negative voltages greater than

0.3 V to 0.5 V. When one IC has a ground potential 1 V lower than another, then when
that IC sends a LOW of, say, +0.4 V with respect to its ground the receiving input will be
driven to 0.6 V relative to its ground and exceed its rating. Even when I
share a nominally common ground the possibility that large currents, sharing that ground,
can cause significant differences in local potential should be considered.

AN10364

Opto-electrical isolation of the I2C-bus

2

C-bus introduces a change in

2

C components

Many systems that can benefit from the advan tages of I

2

C-bus control are not allowed to
share any common reference signal (grou nd ) potential. To prevent any chance of
electrocution, AC mains control and medical patient monitoring equipment require a
safety isolation barrier, to kilovolt levels, between the system components. Regulations
require isolation from telephone lines and Standards require isolation from Ethernet
wiring, especially when power is also supplied by the Ethernet wiring (PoE). Techniques
enabling reliable I

2

C communication while addressing these different require m en ts all
require first splitting the normally bidirectional SDA/SCL signals into two uni-directional
components and then applying conventional techniques to provide the necessary
tolerance to difference in local ground potentials, or to provide total galvanic isolation.

The conventional techniques include:

• Increasing the logic signal amplitudes and limiting negative voltages using clamp

diodes.

• Conversion to differential mode signals usin g CAN or RS-485 drivers or dedicated I

2

parts

• Providing true galvanic isolation by including opto or magnetic/transformer signal

couplers

Buffers that split the I
simply re-combined by re-connecting them include P82B96, PCA9600 and PCA9601.
They can all provide the drive signals to directly interface with the isolating ICs/devices
while offering a range of I
Plus (Fm+).

2

C-bus signal into uni-directional components and allow them to be

2

C-bus interfacing speeds and drive levels including Fast-mode

C

Figure 1

shows the separation of one I2C-bus signal (SDA or SCL) into two separate Send
and Receive components and clamping negative voltages with Schottky diodes as in the
‘4-signal’ arrangement discussed in AN10658 (Ref. 1

). Operating as shown with a 5 V

bus, it can operate between points having a ground potential difference of more than 2 V.

AN10364 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.

Application note Rev. 1 — 12 November 2010 3 of 16

NXP Semiconductors

1

/

2

P82B96

SCL

002aaf769

V

CC1

2.7 kΩ

R1

150 Ω

Tx

RxSx

ground 1

V

CC

1

/2 BAT54A

1 kΩ

long wiring

5 V 5 V

R1
150 Ω

1 kΩ

1

/2 BAT54A

1

/

2

P82B96

Rx

Tx

Sx

V

CC

V

CC2

2.7 kΩ

SCL

ground 2

002aaf770

5 V

2 kΩ

1

/

2

P82B96

Tx

(SDA)

Rx

(SDA)

3.3 V
to 5 V

I2C-bus

SDA

PCA82C250

RxD

TxD

ground 3

5 V

2 kΩ

1

/

2

P82B96

Tx

(SDA)

Rx

(SDA)

3.3 V
to 5 V

I2C-bus

SDA

PCA82C250

RxD

TxD

ground 2

line termination

CANH

CANL

PCA82C250

1

/

2

P82B96

RxD

TxD

I2C-bus

SDA

Tx
(SDA)

Rx
(SDA)

2 kΩ

5 V

ground 1

3.3 V

to 5 V

AN10364

Opto-electrical isolation of the I2C-bus

Fig 1. T olerating moderate ground potential differences using separated uni-directional

signals

2. Using a differential signal technique to carry the SDA and SCL signals

Figure 2 and Figure 3 show how I2C-bus signals can be carried as differential signals over

either CAN bus hardware, that supports multi- drop bidirectional signals carried on a single
twisted pair, or RS-485 hardware that uses separate transmit and receive paths and
therefore requires using four twisted pairs to carry the two I
arrangements provide the same common-mode immunity as offered by those systems,
normally around 7V.

2

C signals. Both these

For Figure 2

duplicate this arrangement to distribute the SCL signal. Because CAN

supports the bidirectional SCL/SDA signals only two twisted pairs are required.
For Figure 3

these differential drivers are uni-direction al so fo ur twisted pairs are required

(e.g., Cat5 cable).

AN10364 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.

Application note Rev. 1 — 12 November 2010 4 of 16

Fig 2. Transmitting one I2C signal as a differential signal using CAN bus ha rdware

NXP Semiconductors

P82B96

002aaf771

2 kΩ

Ty

Ry

Sx

ground 1

V

CC

5 V

Tx

Rx

Sy

SCL

SDA

I2C

CKT

3.3 V to 5 V

2 kΩ

1

/2 26LS31

1

/2 26LS32

long wiring

e.g., Cat5 cable

1

/2 26LS32

5 V

1

/2 26LS31

74LVC
2G07

3.3 V to 5 V

SCL

SDA

I2C

CKT

ground 2

1

/

2

P82B96

I2C-bus

SDA

002aab987

+5 V

R1

+V

CC

R2

R3

Rx
(SDA)

Tx
(SDA)

R5

R4

+V

CC1

I2C-bus
SDA

Opto-electrical isolation of the I2C-bus

Fig 3. Transmitting the I2C signals as differential signals using RS-485 hardware

3. Opto-coupling supports very large ground differences

AN10364

Figure 4 shows the simplest arrangement from application note AN460 (Ref. 2) using

P82B96 and using the lowest cost 4N36 opto-couplers to opto-isolate the I

2

C-bus signal.
These couplers allow saturation of the photo-transistor in the opto-coupler, resulting in
very long turn-off delays caused by charge storage effects in their output transistor. For
the component values given in AN460 the total switching delays will be around 50 s and
that will limit the bus speed of this simple circuit to around 5 kHz clock.

Fig 4. Simple 5 kHz opto-electrical isolation circuit

AN10364 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.

Application note Rev. 1 — 12 November 2010 5 of 16