Designing and building your own board

The Schematic

The I2C mux breakout board consists of the TCA9454A IC, a decoupling capacitor, five sets of pin header outputs (one for the master I2C and one for each of the four I2C mux buses), and pull-up resistors for the SDA/SCL of each I2C bus and for each of the active low I2C bus interrupts. Why pull-up resistors? By specification, the I2C bus lines (SCL and SDA) are active low input/outputs (they pull the line toward ground to activate) and need to be pulled high when not active. The interrupt lines are also active low.

The Board Layout

The first thing to do to lay out the board is place the parts (Figure 1). Most likely you want to place the parts to minimize the connection length. This is usually an iterative process, moving back and forth from the autorouter (seeing what lines are not being connected) to the parts placement to achieve a good layout. You will almost always have to route some lines by hand, but the Eagle autorouter can help you see what problems you might have. In this board, I had a problem with routing the SCL/SDA lines from the master I2C bus to the chip, so I routed those lines by hand and let the autorouter take care of the rest.

Figure 1: The four-channel I2C mux breakout board schematic.

After the final parts placement (Figure 2), I routed four lines by hand and then used the autorouter to complete the job. The Eagle autorouter is pretty good, but it often fails to make a few connections that are obvious to the human operator. You will quickly get good at this process as you learn the tool.

Figure 2: Final parts placement on the breakout board.

SwitchDoc Labs Tip: Not surprisingly, the best layouts tend to have the shortest connections between parts. A symmetric and visually pleasing layout is often the best layout. Autoplacement of parts is a very difficult algorithm to write for a computer, which is why Eagle does not have a native parts function placement. I have tried several Eagle extensions for autoplacement and have not been able to get any really reasonable results.

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