TCA9548A I²C Multiplexer

Contents

• What is an I²C Multiplexer?
• TCA9548A Multiplexer Description
• Pin Configuration and Functions
• I²C Address Selection
• Using the TCA9548A Multiplexer
• TCA9548A MicroPython Driver for the micro:bit
• More Interfacing Tools

What is an I²C Multiplexer?

The I²C serial protocol has much going for it. While it is relatively slow compared to some other communication protocols it is inexpensive, robust, easy to use and has a written international standard. As a consequence of being widely used I²C has a well developed software support base.

The standard allows many I²C slaves to share a single master controlled bus. The slaves are identified by and respond to a unique 7-bit (sometimes 10-bit) address. Herein lies one of the few issues with I²C. Each device on the bus must have a unique address.

Some slave devices provide mechanisms to choose from a pool of addresses. However many don't have this option as the address is fixed at the point of manufacture.

Often it is desirable to have more than one of the same type of e.g. sensor on the same bus. The solution is to use an I²C multiplexer. These allow each slave with a clashing address to be selectively "switched" on or off from the serial bus.

Two popular I²C multiplexer chips are the PCA9548A and TCA9548A chips. The older NXP Semiconductor's PCA9548A is almost functionally the same as the newer Texas Instruments Incorporated's TCA9548A chip. The main difference is the TCA9548A can be powered by a slightly larger voltage range than the PCA9548A.

Both chips can be considered interchangeable in the context of the discussion on this webpage.

TCA9548A Multiplexer Description

The TCA9548A is described in the product datasheet as a device that has eight bidirectional translating switches that can be controlled through the I²C bus. Ok, what does that mean?

Firstly, the TCA9548A is a slave that is controlled through the microcontroller's I²C bus. Additionally it has eight 'client' I²C channels. Each channel can host one or more I²C slaves and can be opened or closed with respect to the microcontroller.

The microcontroller can control the state (opened or closed) of each of these channels very simply by writing to a control register on the TCA9548A. This means that by selectively opening and closing channels the microcontroller is able to communicate with up to eight I²C slaves that all have the same address.

This ability to manage multiple I²C devices, all with the same address and on the same bus, is known as multiplexing . The following diagram from the Texas Instruments' product datasheet explains it well.

The TCA9548A chip is available in two different packages; TSSOP^[1] (7.8mm x 4.4mm x 1.2mm) and VQFN^[1] (4.0mm x 4.0mm x 1mm). Both have 24 pins/tabs. The TSSOP form is the one usually found on breakout boards.

Fig 1 - TCA9548A (from the datasheet): [L] TSSOP package shape and pinout, [R] VQFN-package form.

Pin Configuration and Functions

Breakout boards generally expose all these pins.

I²C Address Selection

The A0, A1  and A2 pins enable the user to select from eight possible I²C addresses. These pins, in any combination, are connected to GND through pulldown resistors or VCC through pullup resistors. The following table shows the combinations that allows an address range of 0x70 through 0x77.

Using the TCA9548A Multiplexer

The TCA9548A has a bidirectional I²C standardised interface controlled by a master (usually a microcontroller) and can operate in Standard Mode (0 - 100kHz) and Fast Mode (0 - 400kHz).

There is only a single register (known as the Control Register) available for the user to read or write to through the I²C bus. This makes this device incredibly easy to use.

The Control Register is 8-bits with each bit controlling one channel. The following diagram (Texas Instruments, TCA9548A Datasheet, p18) illustrates the concept.

If the bit value = 1, then that channel is open. Conversely, if the bit value = 0, then that channel is closed.

As an example if the contents of the Control Register  were 0b00000110 then Channel 1 and Channel 2 would be open with the other channels all closed.

As already mentioned, the TCA9548A has only one user accessible register. A register address is not used to access this Control Register. Updating the register in order to open or close channels is simply done by writing a byte on the I²C bus, addressed to the TCA9548A.

Similarly to read the status (opened or closed) of all the channels it is only necessary to read one byte from the I²C bus.

TCA9548A MicroPython Driver for the micro:bit

A simple to use TCA9548A MicroPython driver is available here. It is free to use, extend, etc but comes with no warranty!

The driver is implemented as a class. The class constructor allows the user to optionally provide:

• The I²C address (default is 0x70) of the TCA9548A.
• A digital pin on the micro:bit that is connected to the RESET pin of the TCA9548A.

The driver has methods for the following:

• Determine whether a channel is open
• Open one or more channels
• Close one or more channels
• Perform a soft rest of the TCA9548A (if the RESET pin is wired to a digital pin on the micro:bit)

Note: This driver is also fully compatible with the PCA9548A multiplexer chip.