Designing and building your own board

Solving the Addressing Problem

The way to solve this addressing problem is to use an I2C bus multiplexer (mux), such as the TCA9545A from Texas Instruments [2]. This allows you to multiplex together up to four I2C buses, all with their own unique address space. No more address problems! A side benefit is that you can set each bus to use either 3.3V or 5.0V levels, thus allowing the use of many more I2C sensors.

Because the TCA9545A (and most other modern ICs) comes in an SMD package, it is difficult to build a breadboard using this chip. If you are building a project, you want to find a breakout board that allows you to wire up your sensors easily. This is the reason behind building the product I talk about in this article.

The Process

Manufacturing and selling the I2C mux breakout board involves five major steps:

  1. Select the parts (the TCA9545A).
  2. Draw the schematic of the board.
  3. Draw the layout of the board.
  4. Have the board manufactured (and tested).
  5. Sell your board.

Several free and public domain schematic capture and PCB layout packages are available, including Fritzing, ExpressPCB, PCB123, Eagle, and others. I chose Eagle for three reasons: It generates standard Gerber files that manufactures can use, it has a large library of parts available from many sources, and it has an upgrade path available to build bigger boards with more than two layers. Many good tutorials are available for using Eagle. One of the best is from Sparkfun [3].

Although the I2C mux breakout board may look complicated, it is really very simple. The example application diagrams in the TCA9545A specification tell you just what to do to make the chip work.

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