Adafruit PCF8574 I2C GPIO Expander
Created by Kattni Rembor
https://learn.adafruit.com/adafruit-pcf8574
Last updated on 2022-08-03 03:16:01 PM EDT
©Adafruit Industries Page 1 of 21
Table of Contents
Overview
Pinouts
• Power Pins
• I2C Logic Pins
• Expander Pins
• Address Pin and Jumpers
• INT Pin
• On LED and LED Jumper
Python & CircuitPython
• CircuitPython Microcontroller Wiring
• Python Computer Wiring
• Python Installation of PCF8574 Library
• CircuitPython Usage
• Python Usage
• Example Code
Python Docs
Arduino
• Wiring
• Library Installation
• Load Example
3
7
11
16
16
Arduino Docs
Downloads
• Files
• Schematic and Fab Print
20
20
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Overview
GPIO expanders work like this: you have a board with some number of GPIO but not
enough for your project - maybe you need more buttons or LEDs. You could upgrade
to a board with massive number of GPIO like the Grand Central(https://adafru.it/QtD),
or you could pop on one of these boards.
Connect it over I2C and then you can send/receive I2C commands to control the
GPIO pins to write and read them. It's going to be slower than direct GPIO access, but
maybe that doesn't matter if it takes a millisecond instead of a microsecond. You only
need the two I2C pins, and you can even share the I2C port with other sensors and
devices. Heck, you can even add more expanders for massive I/O control!
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The PCF8574 is a common, andslightly unusualI2C expander for folks who are used
to the MCP230xx series(https://adafru.it/10kC):
First up, its very affordable - who doesn't love that?
•
It has 8 I/O pins
•
Three I2C address select jumpers mean up to 8expanders to one bus for 64
•
total GPIO added
Each pin can be an input with light pull-up or an output sink
•
IRQ output will automaticallyalert you when input pins change value
•
This chip does not have a pin direction register. You cannot set the pins to be
•
input or output - instead each pin has two possible states. Basically you can
think of it as an open-drain output with a 100K resistor pull-up built in.
Option one: Lightly pulled up 'input' - by default it will read as a high logic level,
•
but connecting the GPIO to ground will cause it to read as a low logic level.
Option two: Strong 20mA low-driving transistor sink output. This means the
•
output is 'forced' to be low and will always read as a low logic level.
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The pin direction / state thing is a little odd but it actually works fine for many
purposes as long as you know what to expect.
For example, if you want to read abutton or switch, connect one side to the PCF and
the other side to ground. Then set the pin to 'light pull-up input' When the button is
pressed it will read low, when released it will read high.
If you want to control an LED, connect the anodeto positive voltage through a
resistor. When the PCF pin is set to 'light pull-up input' the LED will be off. When the
PCF pin is set to 'strong ground output' the LED will connect to ground and turn on.
If you want to send a GPIO output logic level to some other device or peripheral, the
light pull-up acts as high logic out, the strong ground output acts as low logic out.
If you want to receive a GPIO input logic level, set the pin to light pull-up and then
read the pin to determine if the GPIO input is high or low.
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Basically, the only thing to watch for is you cannot drive an LED that is expecting the
expander GPIO to go high to turn on the LED, or connect a button input to a positive
voltage without adding an additional pull-down resistor. If this is a bit confusing, worry
not - all this stuff is taken care of for you in our Arduino PCF8574 library(https://
adafru.it/10kD) or CircuitPython/Python PCF8574 library(https://adafru.it/10kE) - you
can pretend it has input/output modes and the library will fake out what you are
expecting.
To get you going fast, we spun up a custom-made PCB in theSTEMMA QTform
factor(https://adafru.it/LBQ), making it easy to interface with. TheSTEMMA QT
connectors(https://adafru.it/JqB)on either side are compatible with theSparkFun
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Qwiic(https://adafru.it/Fpw)I2C connectors. This allows you to make solderless
connections between your development board and the PCF8574 or to chain it with a
wide range of other sensors and accessories using acompatible cable(https://
adafru.it/JnB).
Pinouts
The default I2C address is 0x20.
Power Pins
VIN - This is the power pin. To power the
board, give it the same power as the logic
level of your microcontroller - i.e. for a 5V
micro like Arduino, use 5V, or for a 3V
micro like a Feather, use 3V.
GND - This is common ground for power
and logic.
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I2C Logic Pins
SCL - I2C clock pin, connect to your
microcontroller's I2C clock line. This pin is
level shifted so you can use 3-5V logic,
and there's a 10K pullup on this pin.
SDA - I2C data pin, connect to your
microcontroller's I2C data line. This pin is
level shifted so you can use 3-5V logic,
and there's a 10K pullup on this pin.
STEMMA QT(https://adafru.it/Ft4) - These
connectors allow you to connect to
development boards with STEMMA QT
connectors, or to other things, with various
associated accessories(https://adafru.it/
JRA).
Expander Pins
Along the top are the 8 I/O pins for
expanding your project. They are labeled
above the pads, left-to-right as P7 through
P0.
On the far left is the positive voltage for
the expander I/O pins. It is labeled below
the pad with a + in a circle
On the far right is the ground for the
expander I/O pins. It is labeled below the
pad with a - in a circle.
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Address Pin and Jumpers
On the back of the board are three
address jumpers, labeled A0, A1, and A2.
These jumpers allow you to chain up to 8
of these boards on the same pair of I2C
clock and data pins. To do so, you solder
the jumpers "closed" by connecting the
two pads.
On the front of the board is one address
pin, labeled A0. Just like the jumpers, this
pin allows you to change the I2C address
to connect multiple boards by connecting
it to VIN.
The default I2C address is 0x20. The other address options can be calculated by
“adding” the A0/A1/A2to the base of 0x20.
A0 sets the lowest bit with a value of 1, A1 sets the next bit with a value of 2 and A2
sets the next bit with a value of 4. The final address is 0x20 + A2 + A1 + A0 which
would be 0x27.
So for example if A2 is soldered closed and A0 is soldered closed, the address is 0x2
0 + 4 + 1 = 0x25.
If only A0 is soldered closed, the address is 0x20 + 1 = 0x21
If only A1 is soldered closed, the address is 0x20 + 2 = 0x22
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If only A2 is soldered closed, the address is 0x20 + 4 = 0x24
The table below shows all possible addresses, and whether the pin(s) should be high
(closed) or low (open).
INT Pin
The INT pin is the IRQ output, which will
automatically alert you when input pins
change value.
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On LED and LED Jumper
On the left side of the front of the board is
the on LED which lights up when the board
has power. It is labeled above the LED as
on.
Towards the right side of the back of the
board is the LED jumper. It is two pads
connected by a trace, labeled asLEDto
the left of the pads. If you'd rather not
have the on LED lit up when the board has
power, you can cut the trace between the
two pads. To reenable the on LED, you can
bridge the pads again with solder.
Python & CircuitPython
It's easy to use the Adafruit PCF8574 with Python or CircuitPython, and the Adafruit
CircuitPython PCF8574(https://adafru.it/10kE) module. This module allows you to
easily write Python code that enables you to utilise the 8 I/O pins on the expander.
You can use this sensor with any CircuitPython microcontroller board or with a
computer that has GPIO and Python thanks to Adafruit_Blinka, our CircuitPython-for-
Python compatibility library(https://adafru.it/BSN).
CircuitPython Microcontroller Wiring
First wire up a PCF8574 to your board exactly as shown below. These wiring diagrams
include a button and an LED, which are necessary for the example below.
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Here's an example of wiring a Feather RP2040 to the expander with I2C using one of
the handy STEMMA QT(https://adafru.it/Ft4) connectors:
Feather to expander:
Simply use a STEMMA QT cable(https://
adafru.it/Tff) to connect from the STEMMA
QT connector on the microcontroller to the
STEMMA QT connector on the breakout.
Follow the steps below(https://adafru.it/
10kF) to connect the LED and button.
You can also use the standard 0.100" pitch headers to wire it up on a breadboard:
Feather to expander:
Feather 3Vto expander VIN (red wire)
Feather GNDto expander GND (black
wire)
Feather SCLto expander SCL (yellow wire)
Feather SDAto expander SDA (blue wire)
Follow the steps below(https://adafru.it/
10kF) to connect the LED and button.
Connect the LED and the button to the expander as follows:
LED to expander:
LED- to expander P7
LED+ to 470Ω resistor
470Ω resistor to + on top edge of
expander
Button to expander:
One leg of button to - on top edge of
expander
Opposite leg of button to expander P0
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Python Computer Wiring
Since there's dozens of Linux computers/boards you can use, we will show wiring for
Raspberry Pi. For other platforms, please visit the guide for CircuitPython on Linux to
see whether your platform is supported(https://adafru.it/BSN).
These wiring diagrams include a button and an LED, which are necessary for the
example below.
Here's the Raspberry Pi wired to the expander using I2C and a STEMMA QT(https://
adafru.it/Ft4) connector.
Pi to expander:
Pi 3Vtoexpander VIN (red wire)
Pi GNDtoexpander GND (black wire)
Pi SCLtoexpander SCL (yellow wire)
Pi SDAtoexpander SDA (blue wire)
Follow the steps above(https://adafru.it/
10kF) to connect the LED and button.
Finally here is an example of how to wire up a Raspberry Pi to the expander using a
solderless breadboard:
Pi to expander:
Pi 3Vtoexpander VIN (red wire)
Pi GNDtoexpander GND (black wire)
Pi SCLtoexpander SCL (yellow wire)
Pi SDAtoexpander SDA (blue wire)
Follow the steps above(https://adafru.it/
10kF) to connect the LED and button.
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Python Installation of PCF8574 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(https:
//adafru.it/BSN)!
Once that's done, from your command line run the following command:
pip3 install adafruit-circuitpython-pcf8574
•
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 ADXL37x 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 c
ode.py file to your CIRCUITPY drive.
Your CIRCUITPY/lib folder should contain the following folder and file:
adafruit_bus_device/
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adafruit_pcf8574.mpy
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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
Example Code
# SPDX-FileCopyrightText: 2017 Scott Shawcroft, written for Adafruit Industries
# SPDX-FileCopyrightText: Copyright (c) 2022 ladyada for Adafruit Industries
#
# SPDX-License-Identifier: Unlicense
import time
import board
import digitalio
import adafruit_pcf8574
print("PCF8574 digitalio LED + button test")
i2c = board.I2C()
pcf = adafruit_pcf8574.PCF8574(i2c)
# get a 'digitalio' like pin from the pcf
led = pcf.get_pin(7)
button = pcf.get_pin(0)
# Setup pin7 as an output that's at a high logic level default
led.switch_to_output(value=True)
# Setup pin0 as an output that's got a pullup
button.switch_to_input(pull=digitalio.Pull.UP)
while True:
led.value = button.value
time.sleep(0.01) # debounce
Now, press the button to see the LED light up.
The GIF below shows a "lazy" way to connect the button and LED to the expander.
You should always include a resistor when wiring an LED!
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That's all there is to using the PCF8574 with CircuitPython!
Python Docs
Python Docs(https://adafru.it/10hE)
Arduino
Using the PCF8574 with Arduino involves wiring up the sensor to your Arduino-
compatible microcontroller, installing the Adafruit PCF8574(https://adafru.it/10kD)
library and running the provided example code.
Wiring
Wire as shown for a 5V board like an Uno. If you are using a 3V board, like an Adafruit
Feather, wire the board's 3V pin to the PCF8574 VIN.
These wiring diagrams include a button and an LED, which are necessary for the
example below.
Here is an Adafruit Metro wired up to the PCF8574 using the STEMMA QT connector:
©Adafruit Industries Page 16 of 21
Metro to expander:
Use a STEMMA QT to male header pin
cable(https://adafru.it/FA-).
Metro 5Vto expander VIN (red wire)
Metro GNDto expander GND (black wire)
Metro SCLto expander SCL (yellow wire)
Metro SDAto expander SDA (blue wire)
Metro 2 to expander INT (green wire)
Please follow the steps below(https://
adafru.it/10ld) for LED and button wiring.
Here is an Adafruit Metro wired up using a solderless breadboard:
Metro to expander:
Metro 5Vto expander VIN (red wire)
Metro GNDto expander GND (black wire)
Metro SCLto expander SCL (yellow wire)
Metro SDAto expander SDA (blue wire)
Metro 2 to expander INT (green wire
Please follow the steps below(https://
adafru.it/10ld) for LED and button wiring.
Connect the LED and the button to the expander as follows:
LED to expander:
LED- to expander P7
LED+ to 470Ω resistor
470Ω resistor to + on top edge of
expander
Button to expander:
One leg of button to - on top edge of
expander
Opposite leg of button to expander P0
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Library Installation
You can install the PCF8574 library for Arduino using the Library Manager in the
Arduino IDE.
Click theManage Libraries ... menu item, search for PCF8574 ,and select the Adafruit
PCF8574 library:
When asked about dependencies, click "Install all".
Load Example
Open up File -> Examples -> Adafruit PCF8574 -> buttonledirq and upload to your
Arduino wired to the sensor.
#include <Adafruit_PCF8574.h>
/* Example for 1 button that is connected from PCF GPIO #0 to ground,
* and one LED connected from power to PCF GPIO #7
* We also have the IRQ output connected to an Interrupt input pin on the
* Arduino so we are not constantly polling from the PCF8574 expander
*/
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Adafruit_PCF8574 pcf;
#define PCF_BUTTON 0 // on the GPIO expander!
#define PCF_LED 7 // on the GPIO expander!
#define ARDUINO_IRQ 2 // make sure this pin is possible to make IRQ input
void setup() {
while (!Serial) { delay(10); }
Serial.begin(115200);
Serial.println("Adafruit PCF8574 button/led IRQ test");
if (!pcf.begin(0x20, &Wire)) {
Serial.println("Couldn't find PCF8574");
while (1);
}
pcf.pinMode(PCF_BUTTON, INPUT_PULLUP);
pcf.pinMode(PCF_LED, OUTPUT);
pinMode(LED_BUILTIN, OUTPUT);
digitalWrite(LED_BUILTIN, LOW);
// set up the interrupt pin on IRQ signal toggle
pinMode(ARDUINO_IRQ, INPUT_PULLUP);
attachInterrupt(digitalPinToInterrupt(ARDUINO_IRQ), button_detect, CHANGE);
}
// We use a flag to make sure we don't enter the interrupt more than once
volatile bool in_irq = false;
// called when the button is pressed!
void button_detect(void) {
if (in_irq) return; // we are already handling an irq so don't collide!
in_irq = true;
interrupts(); // Arduino UNO seems to require that we turn on interrupts for I2C
to work!
bool val = pcf.digitalRead(PCF_BUTTON);
pcf.digitalWrite(PCF_LED, val);
in_irq = false;
}
void loop() {
delay(100); // we do nothing here!
}
Once loaded, press the button to see the LED light up when the button is pressed.
The GIF below shows a "lazy" way to connect the button and LED to the expander.
You should always include a resistor when wiring an LED!
©Adafruit Industries Page 19 of 21
Arduino Docs
Arduino Docs(https://adafru.it/10hF)
Downloads
Files
PCF8574 Datasheet(https://adafru.it/10la)
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Fritzing object in the Adafruit Fritzing Library(https://adafru.it/10lb)
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EagleCAD PCB files on GitHub(https://adafru.it/10lc)
•
©Adafruit Industries Page 20 of 21
Schematic and Fab Print
©Adafruit Industries Page 21 of 21
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