pcf8574

Control each pin of a PCF8574/PCF8574A/PCF8574P I2C port expander IC.

The PCF8574/PCF8574A is an 8 bit/pin port expander IC, which can be controlled over the I2C-Bus. Each of the 8 pins can be separately used as an input or output. It also offers an interrupt signal, which can be used to detect input changes by the I2C master (e.g. a Raspberry Pi). For more information about the PCF8574/PCF8574A please consult the datasheet from Texas Instruments.

Installation

npm install pcf8574

TypeScript typings are included in this package.

You should be able to use this module on any Linux based OS.

To use the interrupt detection you need a Raspberry Pi or a similar board.

Examples

Note that you need to construct the i2c-bus object and pass it in to the module along with the I2C address of the PCF8574/PCF8574A.

The example blow can be found in the examples directory of this package together with a TypeScript example.

// Require the pcf8574 module
var PCF8574 = require('pcf8574').PCF8574;
 
// Or use ES6 style imports
// import {PCF8574} from 'pcf8574';
 
// Require the i2c-bus module and open the bus
var i2cBus = require('i2c-bus').openSync(1);
 
// Define the address of the PCF8574/PCF8574A
var addr = 0x38;
 
// Init a new PCF8574 with all pins high by default
// Instead of 'true' you can also use a 8-bit binary notation to define each
// pin speratly, e.g. 0b00101010
var pcf = new PCF8574(i2cBus, addr, true);
 
// Enable interrupt detection on BCM pin 17 (which is GPIO.0)
pcf.enableInterrupt(17);
 
// Alternatively you can use for example an interval for manually poll every 250ms
// setInterval(pcf.doPoll.bind(pcf), 250);
 
// Note the missing ; at the end of the following lines.
// This is a Promise chain!
 
// Define pin 0 as inverted output with initally false
pcf.outputPin(0, true, false)
 
// Then define pin 1 as inverted output with initally true
.then(function(){
  return pcf.outputPin(1, true, true);
})
 
// Then define pin 7 as non inverted input
.then(function(){
  return pcf.inputPin(7, false);
})
 
// Delay 1 second
.delay(1000)
 
// Then turn the pin on
.then(function(){
  console.log('turn pin 0 on');
  return pcf.setPin(0, true);
})
 
// Delay 1 second
.delay(1000)
 
// Then turn the pin off
.then(function(){
  console.log('turn pin 0 off');
  return pcf.setPin(0, false);
});
 
// Add an event listener on the 'input' event
pcf.on('input', function(data){
  console.log('input', data);
 
  // Check if a button attached to pin 7 is pressed (signal goes low)
  if(data.pin === 7 && data.value === false){
    // Toggle pin 1
    pcf.setPin(1);
  }
});
 
// Handler for clean up on SIGINT (ctrl+c)
process.on('SIGINT', function(){
  pcf.removeAllListeners();
  pcf.disableInterrupt();
});

API

The API uses Events for detected input changes and Promises for all asyncronous actions.

Input changes can be detected in two ways:

• Using a GPIO to observe the interrupt signal from the PCF8574/PCF8574A IC. Recommended on Raspberry Pi or similar.
• Call doPoll() manually frequently enough to actively read the current states. This leads to a higher load on the I2C-Bus.

If a pin is defined as an input and a changed state is detected, an input Event will be emitted with an object containing the pin number and the new value of this pin.

You can set an inverted flag for each pin separately, which will result in an inverted input or output. If an inverted input has a low level it will be interpreted as true and a high level will be false. An inverted output will write a low level if you set it to true and write a high level if false.

new PCF8574(i2cBus, address, initialState)

constructor(i2cBus:I2cBus, address:number, initialState:boolean|number);

Constructor for a new PCF8574/PCF8574A instance.

• i2cBus - Instance of an opened i2c-bus.
• address - The address of the PCF8574/PCF8574A IC.
• initialState - The initial state of the pins of this IC. You can set a bitmask (e.g. 0b00101010) to define each pin seprately, or use true/false for all pins at once.

Note that you need to construct the i2c-bus object and pass it in to the module.

If you use this IC with one or more input pins, you have to call

• enableInterrupt(gpioPin) to detect interrupts from the IC using a GPIO pin, or
• doPoll() frequently enough to detect input changes with manually polling.

enableInterrupt(gpioPin)

enableInterrupt(gpioPin:PCF8574.PinNumber):void;

Enable the interrupt detection on the specified GPIO pin. You can use one GPIO pin for multiple instances of the PCF8574 class.

• gpioPin - BCM number of the pin, which will be used for the interrupts from the PCF8574/8574A IC.

disableInterrupt()

disableInterrupt():void;

Disable the interrupt detection. This will unexport the interrupt GPIO, if it is not used by an other instance of this class.

doPoll()

doPoll():Promise<{}>;

Manually poll changed inputs from the PCF8574/PCF8574A IC.

If a change on an input is detected, an input Event will be emitted with a data object containing the pin and the new value. This have to be called frequently enough if you don't use a GPIO for interrupt detection. If you poll again before the last poll was completed, the promise will be rejected with an error.

outputPin(pin, inverted, initialValue)

outputPin(pin:PCF8574.PinNumber, inverted:boolean, initialValue?:boolean):Promise<{}>;

Define a pin as an output. This marks the pin to be used as an output pin. Returns a Promise which will be resolved when the pin is ready.

• pin - The pin number. (0 to 7)
• inverted - true if this pin should be handled inverted (true=low, false=high)
• initialValue - (optional) The initial value of this pin, which will be set immediatly.

inputPin(pin, inverted)

inputPin(pin:PCF8574.PinNumber, inverted:boolean):Promise<{}>;

Define a pin as an input. This marks the pin for input processing and activates the high level on this pin. Returns a Promise which will be resolved when the pin is ready.

• pin - The pin number. (0 to 7)
• inverted - true if this pin should be handled inverted (high=false, low=true)

Note that an input is always set to high (pullup) internally.

setPin(pin, value)

setPin(pin:PCF8574.PinNumber, value?:boolean):Promise<{}>;

Set the value of an output pin. If no value is given, the pin will be toggled. Returns a Promise which will be resolved when the new value is written to the IC.

• pin - The pin number. (0 to 7)
• value - The new value for this pin.

setAllPins(value)

setAllPins(value:boolean):Promise<{}>;

Set the given value to all output pins. Returns a Promise which will be resolved when the new values are written to the IC.

• value - The new value for this pin.

getPinValue(pin)

getPinValue(pin:PCF8574.PinNumber):boolean;

Returns the current value of a pin. This returns the last saved value, not the value currently returned by the PCF8574/PCF9574A IC. To get the current value call doPoll() first, if you're not using interrupts.

• pin - The pin number. (0 to 7)

License

Licensed under GPL Version 2

Copyright (c) 2017 Peter Müller peter@crycode.de (https://crycode.de/)