I2C Multiplexers

What is a multiplexer?

In simple terms in the context of I²C communication, a multiplexer allows for one I²C bus to appear to be 4 or 8 or more separate buses. In the simple case of a single multiplexor on an I²C bus, it allows the processor on the motherboard to switch between the 4 or 8 channels and see only devices on each channel at once. This multiplies the number of I²C devices that can be connected to a command station.

This enables:

• Using multiple devices with the same, hard coded I²C address (including displays)

• Being able to use more devices than normal due to limited address ranges

• Split an I²C bus electrically to overcome capacitance/resistance issues which can dramatically improve reliability

At present up to 8 multiplexors can be supported on a single command station, resulting in up to 64 distinct I²C buses and hundreds of devices. In practise you will be limited by the amount of RAM available on your command station.

Which multiplexers are supported?

Both TCA9547 8-channel, and TCA9548A 4-channel multiplexers have been tested and are supported at the moment. Equivalent and compatible multiplexers should work, but we have not completed testing of the others at this time.

How do I use a multiplexer?

Shown below is an Arduino Mega with a TCA9548A 4-channel multiplexor attached.

As you can see, we have two identically addressed I²C peripherals attached to independent channels SC2 and SC3 so that they don’t clash. The EX‑CommandStation software is able to distinguish between the two with new extended addressing formats. As a helpful aid, the command station will discover the multiplexor and then probe each channel to see what, if any, I²C devices are attached.

<* License GPLv3 fsf.org (c) dcc-ex.com *>
<* I2C Device found at 0x3D, OLED Display? *>
<* I2C Device found at 0x70, I2C Mux? *>
<* I2C Device found at {I2CMux_0,SubBus_0,0x3C}, OLED Display? *>
<* I2C Device found at {I2CMux_0,SubBus_0,0x60}, ?? *>
<* I2C Device found at {I2CMux_0,SubBus_1,0x40}, PWM? *>
<* I2C Device found at {I2CMux_0,SubBus_1,0x41}, PWM? *>
<* I2C Device found at {I2CMux_0,SubBus_3,0x20}, GPIO Expander? *>
<* I2C Device found at {I2CMux_0,SubBus_4,0x3C}, OLED Display? *>
<* I2C Device found at {I2CMux_0,SubBus_5,0x3C}, OLED Display? *>
<* 128x64 OLED display configured on I2C:0x3d *>
<* LCD0:DCC-EX v4.2.34 *>
<* LCD1:Lic GPLv3 *>

In this example, we see OLED display 0 is attached directly to the main I²C bus on address 0x3D, and that an 8-channel multiplexor (Mux) is also seen at address 0x70. Hanging off this multiplexor we have a variety of devices off SubBus_0, SubBus_1, SubBus_3, SubBus_4 and SubBus_5. You will notice that on SubBus_4 and SubBus_5 we have two identical OLED displays both using address 0x3C. But because they are isolated from one another by the multiplexor, the command station is able to write to them entirely independently.

In order to declare these OLED displays, we need a way to tell the code where to find them. So for the example above, we need to add lines to “myHal.cpp”:

// Create a 128x32 OLED display device as display number 1
// (line 0 is written by EX-RAIL 'SCREEN(1, 0, "text")').

HALDisplay<OLED>::create(1, {SubBus_4, 0x3c}, 128, 64);

// Create a 128x32 OLED display device as display number 2
// (line 0 is written by EX-RAIL 'SCREEN(2, 0, "text")').

HALDisplay<OLED>::create(2, {SubBus_5, 0x3c}, 128, 64);

As you can see from the comments in this code, both of these displays can now be written to from EX‑RAIL using the SCREEN() directive. See the EX‑RAIL command reference for notes on using it.

Note that when there are multiple multiplexors on the I²C bus, we will need to also specify which mulitplexor the display is connected to. So that declaration looks like this in “myHal.cpp”:

// Create a 128x32 OLED display device as display number 1
// (line 0 is written by EX-RAIL 'SCREEN(1, 0, "text")').

HALDisplay<OLED>::create(1, {I2CMux_0, SubBus_4, 0x3c}, 128, 64);

// Create a 128x32 OLED display device as display number 2
// (line 0 is written by EX-RAIL 'SCREEN(2, 0, "text")').

HALDisplay<OLED>::create(2, {I2CMux_0, SubBus_5, 0x3c}, 128, 64);

As you can see, things can get quite complex, but this also means it is very flexible.

Note that any currently supported I²C device can be connected to any port of any multiplexor, provided they are declared in “myHal.cpp” with the correct addressing for multiplexor and sub-bus. It is probably easiest to connect all the I²C devices to your multiplexors and let EX‑CommandStation probe for the them and any devices off each sub-bus, so you can use the console output to guide you in creating the declarations for “myHal.cpp”.