Adapt PCA9548 multiplexer overlay to GPIO I2C bus for Raspberry Pi 4 and the BUSTER release of Raspian.
Raspberry Pi 4B v1.2 Buster 2/23/2020
Raspberry Pi 3b+ v1.3 Stretch Debian version 9.11
This is most useful for devices that are SMbus compliant (<=100khz clock) or have fixed addresses that prevent using more than one of the same device on the same bus. This is also a good work-around on the Raspberry Pi for the clock-stretching bug that some devices (AM2315 for instance) misbehave with the clock artifacts from the hardware ARM I2C bus. Control the clock speed by modifying the delay ( i2c_gpio_delay_us ). This software bus provides some additional SMbus support.
For details on the SMbus functions: [https://www.kernel.org/doc/Documentation/i2c/smbus-protocol]
pi@raspberry:~ $ sudo i2cdetect -F 1
Functionalities implemented by /dev/i2c-1:
I2C yes
SMBus Quick Command yes
SMBus Send Byte yes
SMBus Receive Byte yes
SMBus Write Byte yes
SMBus Read Byte yes
SMBus Write Word yes
SMBus Read Word yes
SMBus Process Call yes
SMBus Block Write yes
SMBus Block Read no
SMBus Block Process Call no
SMBus PEC yes
I2C Block Write yes
I2C Block Read yes
pi@raspberry:~ $ sudo i2cdetect -F 3
Functionalities implemented by /dev/i2c-3:
I2C yes
SMBus Quick Command yes
SMBus Send Byte yes
SMBus Receive Byte yes
SMBus Write Byte yes
SMBus Read Byte yes
SMBus Write Word yes
SMBus Read Word yes
SMBus Process Call yes
SMBus Block Write yes
SMBus Block Read yes
SMBus Block Process Call yes
SMBus PEC yes
I2C Block Write yes
I2C Block Read yes
The standard overlay only looks on the hardware ARM I2C bus for the mux. This overlay adds additional bus entries in /dev for the mux channels on a software (GPIO) I2C bus configured by the install script:
You can check for new devices with 'sudo i2cdetect -y BUS' ( BUS is an integer for the bus to scan )
You should be able to access devices with any code that can use the /dev/i2c-X device to address i2c devices on that bus. The RESET line on the PCA9548 would need code and a GPIO pin to clear hung devices. Reset cannot be implemented in the overlay.
/dev/i2c-3 This is the software GPIO i2c bus
/dev/i2c-4 This is the first mux i2c channel
/dev/i2c-5 This is the second mux i2c channel
.. (other channels)
pi@raspberry:~ cd ~
pi@raspberry:~ wget https://raw.githubusercontent.com/Theoi-Meteoroi/GPIO-pca9548/master/mux-install.sh
pi@raspberry:~ /bin/bash mux-install.sh
After reboot, you should see something like this in dmesg output:
[ 2.168710] systemd-udevd[137]: starting version 215
[ 2.527337] i2c-gpio i2c@0: using pins 23 (SDA) and 24 (SCL)
[ 2.796954] i2c i2c-3: Added multiplexed i2c bus 4
[ 2.799319] i2c i2c-3: Added multiplexed i2c bus 5
[ 2.801690] i2c i2c-3: Added multiplexed i2c bus 6
[ 2.803925] i2c i2c-3: Added multiplexed i2c bus 7
[ 2.808755] i2c i2c-3: Added multiplexed i2c bus 8
[ 2.809316] i2c i2c-3: Added multiplexed i2c bus 9
[ 2.811035] i2c i2c-3: Added multiplexed i2c bus 10
[ 2.812586] i2c i2c-3: Added multiplexed i2c bus 11
[ 2.812604] pca954x 3-0070: registered 8 multiplexed busses for I2C switch pca9548
This is with the default i2c address of 0x70. You can pass another address in the declaration:
dtoverlay=i2c_gpio-pca9548,addr=0x71
You can also list out the configured I2C bus instances with
This is a good reference for general issues:
[https://www.i2c-bus.org/i2c-primer/common-problems/]
cd ~
wget https://raw.githubusercontent.com/Theoi-Meteoroi/GPIO-pca9548/master/i2c_gpio-pca9548.dts
sudo dtc -W no-unit_address_vs_reg -I dts -O dtb -@ -o /boot/overlays/i2c_gpio-pca9548.dtbo ./i2c_gpio-pca9548.dts
vi /boot/config.txt
You will need to have your multiplexer connected with SDA on pin 23 and SCL on pin 24. Use the +3.3v to power the pca9548. Add a 10uf tantalum capacitor near the device if possible to provide for transient spikes. Remember to tie the /RESET pin to +3.3v through a pull-up resistor. The GPIO lines also need a nominal pull-up to 3.3v. 4.7k ohm seems to work well because the Pi pins are capable of sinking a max of 16mA, but be aware the SMbus specification for minimum sink current of 100µA, and a maximum of 350µA, compared to 3mA for the I²C bus. This in turn would determine the lowest acceptable value of the pullup resistor, examples of which are shown in the table below. This will matter for stability of the attached SMbus devices.
| 3V VDD | 5V VDD | |
|---|---|---|
| I²C Bus | > 1k | > 1.6k |
| SMBus | > 8.5k | > 14k |
Address lines should also be connected to GND or +3.3v, either directly or through a pull-up resistor (10k or so is fine.)
If you have it properly connected and powered, after rebooting you will have several new I2C buses available for use, one for each channel of the multiplexer. You can verify the buses are available by going to the Config -> System Information page of Mycodo and viewing the I2C buses. If there's only one, there is an issue. If you see 9 buses (1 from the original bus + 8 new buses from the multiplexer), then everything is working and you can begin connecting devices to your multiplexer channels.
DFRobot DFR0576 PCA9548 Mux breakout - I have tested this board. Comes with a cable that can directly connect to the GPIO pins. At the moment the board sells for US $6.90 which is less than most other breakouts that need alot of work to connect. This board can be up and working with your pi in less than 10 minutes, including installing the mux overlay. [https://www.dfrobot.com/wiki/index.php/Gravity:_Digital_1-to-8_I2C_Multiplexer_SKU:DFR0576]