It's easy to use the PCA9546 with Python or CircuitPython, and the Adafruit_CircuitPython_TCA9548A module. This module allows you to easily write Python code that allows you to multiplex up to 4 STEMMA boards with the PCA9546 I2C multiplexer. You can use this multiplexer with any CircuitPython microcontroller board or with a computer that has GPIO and Python thanks to Adafruit_Blinka, our CircuitPython-for-Python compatibility library.
If you're curious why you'd need an I2C multiplexer, be sure to check out this guide that goes in depth on working with multiple copies of the same I2C device, which most likely have the same I2C address.
Why the Adafruit_CircuitPython_TCA9548A Module?
The PCA9546 is the four output version of the PCA9548 and the PCA9548 is the 'fraternal twin sister' of the TCA9548. The PCA954x chips are easier to get during the great chip shortage of 2022-23. They work exactly the same, they just can't go down to 1.8V power which is OK because QT boards are 3V or 5V only anyways. You can still use any example code or library for the TCA9546, but a PCA9546A class has been added to the library that only allows for reading up to four attached devices.
CircuitPython Microcontroller Wiring
First, wire up a PCA9546 to your board exactly as shown below. Here's an example of wiring a Feather RP2040 to the PCA9546 with I2C using one of the handy STEMMA QT connectors. Then, plug two TSL2591 STEMMA boards into the PCA9548 via STEMMA plug 0 and STEMMA plug 1:
- Feather 3V to multiplexer VIN (red wire)
- Feather GND to multiplexer GND (black wire)
- Feather SCL to multiplexer SCL (yellow wire)
- Feather SDA to multiplexer SDA (blue wire)
- TSL2591 1 to multiplexer STEMMA port 0
- TSL2591 2 to multiplexer STEMMA port 1
Python Computer Wiring
Since there's dozens of Linux computers/boards you can use, below shows wiring for Raspberry Pi. For other platforms, please visit the guide for CircuitPython on Linux to see whether your platform is supported.
Here's the Raspberry Pi wired to the I2C multiplexer using I2C and a STEMMA QT connector:
- Pi 3V to multiplexer VIN (red wire)
- Pi GND to multiplexer GND (black wire)
- Pi SCL to multiplexer SCL (yellow wire)
- Pi SDA to multiplexer SDA (blue wire)
- TSL2591 1 to multiplexer STEMMA port 0
- TSL2591 2 to multiplexer STEMMA port 1
Python Installation of TCA9548A Library
You'll need to install the Adafruit_Blinka library that provides the CircuitPython support in Python. This may also require enabling I2C on your platform and verifying you are running Python 3. Since each platform is a little different, and Linux changes often, please visit the CircuitPython on Linux guide to get your computer ready!
Once that's done, from your command line run the following command:
sudo pip3 install adafruit-circuitpython-tca9548a
If your default Python is version 3, you may need to run pip instead. Make sure you aren't trying to use CircuitPython on Python 2.x, it isn't supported!
CircuitPython Usage
To use with CircuitPython, you need to first install the TCA9548A library, and its dependencies, into the lib folder on your CIRCUITPY drive. Then you need to update code.py with the example script.
Thankfully, we can do this in one go. In the example below, click the Download Project Bundle button below to download the necessary libraries and the code.py file in a zip file. Extract the contents of the zip file, and copy the entire lib folder and the code.py file to your CIRCUITPY drive.
Your CIRCUITPY/lib folder should contain the following folders and file:
- adafruit_bus_device/
- adafruit_tca9548a.mpy
- adafruit_tsl2591.mpy
Python Usage
Once you have the library pip3 installed on your computer, copy or download the following example to your computer, and run the following, replacing code.py with whatever you named the file:
python3 code.py
# SPDX-FileCopyrightText: 2021 Carter Nelson for Adafruit Industries
# SPDX-License-Identifier: MIT
# This example shows using TCA9548A to perform a simple scan for connected devices
import board
import adafruit_tca9548a
# Create I2C bus as normal
i2c = board.I2C() # uses board.SCL and board.SDA
# i2c = board.STEMMA_I2C() # For using the built-in STEMMA QT connector on a microcontroller
# Create the PCA9546A object and give it the I2C bus
mux = adafruit_tca9548a.PCA9546A(i2c)
for channel in range(4):
if mux[channel].try_lock():
print("Channel {}:".format(channel), end="")
addresses = mux[channel].scan()
print([hex(address) for address in addresses if address != 0x70])
mux[channel].unlock()
If running CircuitPython: Once everything is saved to the CIRCUITPY drive, connect to the serial console to see the data printed out!
If running Python: The console output will appear wherever you are running Python.
In this simple test for the PCA9546, an I2C scan is performed for all four of its ports. If any devices are connected, then the I2C address will be printed to the REPL next to the channel number. If no device is connected, then the port will print with empty brackets ([]).
# SPDX-FileCopyrightText: 2021 ladyada for Adafruit Industries
# SPDX-License-Identifier: MIT
# This example shows using two TSL2491 light sensors attached to PCA9546A channels 0 and 1.
# Use with other I2C sensors would be similar.
import time
import board
import adafruit_tsl2591
import adafruit_tca9548a
# Create I2C bus as normal
i2c = board.I2C() # uses board.SCL and board.SDA
# i2c = board.STEMMA_I2C() # For using the built-in STEMMA QT connector on a microcontroller
# Create the PCA9546A object and give it the I2C bus
mux = adafruit_tca9548a.PCA9546A(i2c)
# For each sensor, create it using the PCA9546A channel instead of the I2C object
tsl1 = adafruit_tsl2591.TSL2591(mux[0])
tsl2 = adafruit_tsl2591.TSL2591(mux[1])
# After initial setup, can just use sensors as normal.
while True:
print(tsl1.lux, tsl2.lux)
time.sleep(0.1)
In the multi-sensor example, the PCA9546 is used as an I2C multiplexer with two TSL2591 light sensors. When the connected sensors are instantiated over I2C, the I2C pins declared are the ports from 0 and 1 on the PCA9546.
In the example, the first TSL2591 light sensor, instantiated as tsl1, is plugged into STEMMA QT port 0 (tca[0]) and the second TSL2591 light sensor, instantiated as tsl2, is plugged into STEMMA QT port 1 (tca[1]).
# Create the PCA9546A object and give it the I2C bus mux = adafruit_tca9548a.PCA9546A(i2c) # For each sensor, create it using the PCA9546A channel instead of the I2C object tsl1 = adafruit_tsl2591.TSL2591(mux[0]) tsl2 = adafruit_tsl2591.TSL2591(mux[1])
In the loop, the readings from the two light sensors are printed to the REPL every 0.1 seconds.
while True:
print(tsl1.lux, tsl2.lux)
time.sleep(0.1)
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