PCA9535 GPIO Expander

PCA9535 is a GPIO expander with 16 IO lines which has I2C bus interface. It provides additional IOs for I2C bus master such as SBC or MCU that can be used to read push button or sensor, to control LED, etc. PCA9555 is identical to PCA9535 except the presence of internal pull-up resistor which increases power consumption when the IOs are low. PCA9535C is another variation of PCA9535 with open drain outputs. Its operating supply voltage range is 2.3V to 5.5V. There are 3 hardware pins which allow to use 8 different I2C bus addresses, from 0x20 to 0x27.

• Setup
• C++ Example
• Using Device Overlay

Setup

We will test the IC using a Raspberry Pi. Set the I2C address of the PCA9535 to 0x20 by pulling down all three address pins. And connect I2C wires, SDA and SCL, to Raspberry Pi I2C-1 bus. After enabling I2C bus on RPi, we can install i2c utility as follow.

$ sudo apt install i2c-tools 

Then, we can scan the bus using the following command. We should see the address 0x20.

$ i2cdetect -y 1

Once we can see the PAC9535 on the bus, we can read its interrnal registers e.g. we can read port 1 input register at address 0x01 as follow.

$ i2cget -y 1 0x20 0x01

We will also present an example to use a utility lib for I2C bus to control PCA9535 which is connected to RPi. For that install prerequisites as follows.

cd ~
sudo apt -y update
sudo apt -y install build-essential cmake git

# wxWidgets
sudo apt -y install libgtk-3-dev checkinstall
sudo apt install libwxgtk3.0-gtk3-dev

# opencv
sudo apt -y install git pkg-config libavcodec-dev libavformat-dev libswscale-dev
sudo apt -y install libopencv-dev

#jsoncpp
sudo apt -y install libjsoncpp-dev

# ceUtil
git clone --depth 1 https://github.com/yan9a/ceutil.git
cd ceutil
chmod +x ceUtil.sh
./ceUtil.sh c

C++ Example

An example to use PCA9535 with Raspberry Pi can be found at the following link.

https://github.com/yan9a/rpi/blob/master/bus/pca9535_i2c/pca9535_i2c.cpp

It simply read first two register values which holds PCA9535 line input values. We can compile and run it as follows.

$ g++ pca9535_i2c.cpp -o pca9535_i2c
$ ./pca9535_i2c

PCA9535 has 8 internal registers as follows.

• Address 0 = Port 0 input
• Address 1 = Port 1 input
• Address 2 = Port 0 output
• Address 3 = Port 1 output
• Address 4 = Port 0 polarity
• Address 5 = Port 1 polarity
• Address 6 = Port 0 configuration
• Address 7 = Port 1 configuration

As an another example, I will use ceUtil library that we installed in setup section. The source code can be found at the following link.

https://github.com/yan9a/rpi/blob/master/bus/pca9535_cei2c/src/pca9535_cei2c.cpp

I use cmake to build it. Use https://github.com/yan9a/rpi/blob/master/bus/pca9535_cei2c/pca9535_cei2c.sh to build and run it.

$ ./pca9535_cei2c.sh

Using Device Tree Overlay

We can use PCA9535 IO lines as normal GPIO lines by enabling Raspberry Pi device tree overlay support for it [2]. Edit /boot/config.txt and add the following lines.

dtoverlay=pca953x,pca9535,addr=0x20

After rebooting, if you scan the i2c bus 1, you should see UU instead of 0x20 at the corresponding address. And you should see new gpio device when listing dev directory.

i2cdetect -y 1
ls -l /dev/gpio*

We can use old way of controlling gpio using sysfs. But, in this example, we will use newer approach of using libgpiod library for it [3]. Install the library as follow.

sudo apt install gpiod libgpiod-dev

Check gpiochip information using the following commands.

gpiodetect
gpioinfo

If PCA9535 appears as gpio chip 2, its input channel 12 can be read as

gpioget 2 13

Command to set line number 15 is

gpioset 2 15=1

A simple C example to control its GPIO lines using libgpiod is at the following link.

https://github.com/yan9a/rpi/blob/master/bus/gpiodeg/gpiod_eg.cpp

References

[1] ceutil. C++ utilities lib. 2022.
url: https://github.com/yan9a/ceutil.

[2] Raspberry Pi. Firmware boot overlays.
url: https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README.

[3] Jeff Tranter. GPIO Programming: Exploring the libgpiod Library. 2021 Jan 20
url: https://www.ics.com/blog/gpio-programming-exploring-libgpiod-library.