Analog Switch Expands I2C Interface

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= App Notes >= ; IN= TERFACE CIRCUITS SW= ITCHES AND MULTIPLEXERS	APP 955: Apr 12, 2002 =
= Keywords: I2C, analog switch, clock data SDA SCL port expansion, analogue switch<= o:p>
= Related Parts: MAX4562 MAX4572<= /a>	Download, PDF For= mat (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 s= uch 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 I^{2C
interfaces, but you can also implement the interface in software. To comp=
lete
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 addre=
ss
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 I^{2C
bus. IC₂ and IC₃, for example, have the same address
but are located on different auxiliary buses.}

Figure 1. This I<= sup>2C-controlled analog switch expands by three the number of devices connected to the bus= .

The arrangement in F= igure 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 proto= col. The analog switch accepts bidirectional signals as required for the SDA l= ine. The switch has low on-state resistance, adds almost no leakage on the lin= es, and provides four selectable slave addresses. You simultaneously control = the switches by using the simple SendByte protocol (address plus 8-bit comman= d).

You can switch the t= hree auxiliary buses on the fly. Power-up sets the switches to soft mode, an o= ff 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 I^{2C
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 o= f this article appeared in the November 22, 2001 issue of EDN magazine.

DI360, February 2002

More Information

APP 955: Apr 12, 2002 =

MAX4562:	QuickView=	-- Full (= PDF) Data Sheet (232k)	-- Free Samples
MAX4572:	QuickView=	-- Full (= PDF) Data Sheet (328k)	-- Free Samples

3D"Download, Download, PDF For= mat (144K)

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