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App Notes > INTERFACE
CIRCUITS   SWITCHES
AND MULTIPLEXERS

APP 955: Apr 12, 2002

Keywords:
I2C, analog switch, clock data SDA SCL port expansion, analogue switch

Related
Parts: MAX4562
MAX4572

Download, PDF Format
(144K)

Analog Switch Expands I2C
Interface

Design
idea outlining how to use an I2C-controlled analog switch to
triple the number of devices connected to an I2C bus.

Perhaps
the most effective way to gain board space and increase component density is
to minimize wiring on the board. A widely used architecture that allows such
miniaturization is the I2C bus. Comprising only a bidirectional
data line, SDA, and a clock line, SCL, this bus requires no chip selects or
other additional connections.

Microcontrollers from
Philips, Microchip, and other manufacturers include dedicated I2C
interfaces, but you can also implement the interface in software. To complete
this task, you associate a 7-bit address with each master or slave
transceiver and factory- or pin-program the device with two to four address
options.

An increasing number of
slaves now include the I2C interface, but some of their 128
address locations are reserved for special functions, so not all locations
are available to a designer. Yet, two or more devices could have the same
address in some applications.

In Figure 1, analog switch
IC1, which is I2C-controlled, connects auxiliary
branches that contain devices with the same address to the main I2C
bus. IC2 and IC3, for example, have the same address
but are located on different auxiliary buses.

Figure 1. This I2C-controlled
analog switch expands by three the number of devices connected to the bus.

The arrangement in Figure
1 prevents the master from addressing multiple slaves at the same time. If
that situation occurs, the data becomes corrupted during a master-read
protocol, and all slaves may not receive data during a master-write protocol.
The analog switch accepts bidirectional signals as required for the SDA line.
The switch has low on-state resistance, adds almost no leakage on the lines,
and provides four selectable slave addresses. You simultaneously control the
switches by using the simple SendByte protocol (address plus 8-bit command).

You can switch the three
auxiliary buses on the fly. Power-up sets the switches to soft mode, an off
state with 12-msec switching time. Then, a command byte of 0b11000000 sets
the switches to hard mode (400-nsec switching time). Subsequent commands
select the desired auxiliary bus. Command 0b1000011, for example, selects
auxiliary bus 1.

The main I2C
bus includes necessary pullup resistors, and the auxiliary buses include
weaker pullups that ensure a high state when you deselect the bus. The
circuit in Figure 1 allows you to add three times more devices on the bus.
For a wider selection, you can replace the MAX4562 with a MAX4572, whose 14
switches allow you to add as many as seven auxiliary buses.

A similar version of this
article appeared in the November 22, 2001 issue of EDN magazine.

DI360, February 2002

More
Information

APP 955: Apr 12, 2002

MAX4562:

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Data Sheet (232k)

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MAX4572:

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Data Sheet (328k)

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Samples

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(144K)

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