PCF8574
Arduino library for PCF8574 - 8 channel I2C IO expander.
Description
Related to the PCF8575 16 channel IO expander library https://github.com/RobTillaart/PCF8575
This library gives easy control over the 8 pins of a PCF8574 and PCF8574A chip. These chips are identical in behaviour although there are two distinct address ranges.
| type | address-range | notes |
|---|---|---|
| PCF8574 | 0x20 to 0x27 | same range as PCF8575 ! |
| PCF8574A | 0x38 to 0x3F |
So you can connect up to 16 PCF8574 on one I2C bus, giving access to 16 x 8 = 128 IO lines. To maximize IO lines combine 8 x PCF8575 + 8 x PCF8574A giving 128 + 64 = 192 IO lines. Be sure to have a well dimensioned power supply.
The library allows to read and write both single pins or 8 pins at once. Furthermore some additional functions are implemented that are playful and useful.
Interrupts intro
The PCF8574 has an interrupt output line (INT) to notify an MCU that one of the input lines has changed. This can be used to prevent active polling of the PCF8574, which can be more efficient.
From the datasheet:
An interrupt is generated by any rising or falling edge of the port inputs in the input mode. After time, (Tiv), INT is valid. Resetting and reactivating the interrupt circuit is achieved when data on the port is changed to the original setting or data is read from, or written to, the port that generated the interrupt. Resetting occurs in the read mode at the acknowledge bit after the rising edge of the SCL signal, or in the write mode at the acknowledge bit after the high-to-low transition of the SCL signal.
So there are three scenarios how the INT is reset.
- pins revert to original state (lesser known).
- read from the device (well known)
- write to the device (well known)
This implies that polling the PCF8574 can miss an INT in scenario 1. (see #48) In practice if you have faster polling than your signals changes this would not be a problem. E.g. tactile switches and a polling frequency > 100 Hz will work.
Interrupts library
The library cannot handle the PCF8574 interrupts as it has no code for it. The user should catch the interrupt in his own code to set a flag and can use the library to see which line has changed.
There are two examples to show how interrupts can be handled:
- PCF8574_interrupt.ino
- PCF8574_rotaryEncoder.ino
A more advanced interrupt handler would not set a boolean flag in the interrupt routine but increase a counter (uint8_t or larger). Then it would be possible to see that:
- an interrupt occurred. (counter > 0)
- if one or more interrupts are not handled (counter > 1)
A minimal example that shows catching missed interrupts:
- PCF8574_interrupt_advanced.ino
0.4.0 Breaking change
Version 0.4.0 introduced a breaking change. You cannot set the pins in begin() any more. This reduces the dependency of processor dependent Wire implementations. The user has to call Wire.begin() and can optionally set the Wire pins before calling begin().
Related
16 bit port expanders
- https://github.com/RobTillaart/MCP23017_RT
- https://github.com/RobTillaart/MCP23S17
- https://github.com/RobTillaart/PCF8575
8 bit port expanders
- https://github.com/RobTillaart/MCP23008
- https://github.com/RobTillaart/MCP23S08
- https://github.com/RobTillaart/PCF8574
I2C Clock
Tested on UNO with PCF8574_performance showed that the PCF8574 still works at 500 KHz and failed at 600 KHz. These values are outside the specs of the datasheet so they are not recommended. However when performance is needed you can try to overclock the chip.
| clock speed | Read | Write | Notes |
|---|---|---|---|
| 100000 | 236 | 240 | spec datasheet |
| 200000 | 132 | 140 | |
| 300000 | 104 | 108 | |
| 400000 | 96 | 96 | max advised speed |
| 500000 | 92 | 92 | not recommended |
| 600000 | crash | crash |
Interface
#include "PCF8574.h"PCF8574_INITIAL_VALUE is a define 0xFF that can be set compile time or before the include of "pcf8574.h" to overrule the default value used with the begin() call.
Constructor
- PCF8574(uint8_t deviceAddress = 0x20, TwoWire *wire = &Wire) Constructor with optional address, default 0x20, and the optional Wire interface as parameter.
- bool begin(uint8_t value = PCF8574_INITIAL_VALUE) set the initial value (default 0xFF) for the pins and masks.
- bool isConnected() checks if the address set in the constructor or by setAddress() is visible on the I2C bus.
- bool setAddress(const uint8_t deviceAddress) sets the device address after construction. Can be used to switch between PCF8574 modules runtime. Note this corrupts internal buffered values, so one might need to call read8() and/or write8(). Returns true if address can be found on I2C bus.
- uint8_t getAddress() Returns the device address.
Read and Write
- uint8_t read8() reads all 8 pins at once. This one does the actual reading.
- uint8_t read(uint8_t pin) reads a single pin; pin = 0..7
- uint8_t value() returns the last read inputs again, as this information is buffered in the class this is faster than reread the pins.
- void write8(const uint8_t value) writes all 8 pins at once. This one does the actual writing.
- uint8_t write(const uint8_t pin, const uint8_t value) writes a single pin; pin = 0..7; value is HIGH(1) or LOW (0)
- uint8_t valueOut() returns the last written data.
Button
The "button" functions are to be used when you mix input and output on one IC. It does not change / affect the pins used for output by masking these. Typical usage is to call setButtonMask() once in setup as pins do not (often) change during program execution.
- void setButtonMask(const uint8_t mask) sets the (bit) mask which lines are input.
- uint8_t getButtonMask() returns the set buttonMask.
- uint8_t readButton8() use the mask set by setButtonMask to select specific input pins.
- uint8_t readButton8(const uint8_t mask) use a specific mask to select specific input pins. Note this can be a subset of the pins set with setButtonMask() if one wants to process not all.
- uint8_t readButton(const uint8_t pin) read a singe input pin.
Background - https://github.com/RobTillaart/Arduino/issues/38
Special
- void toggle(const uint8_t pin) toggles a single pin
- void toggleMask(const uint8_t mask = 0xFF) toggles a selection of pins, if you want to invert all pins use 0xFF (default value).
- void shiftRight(const uint8_t n = 1) shifts output channels n pins (default 1) pins right (e.g. LEDs ). Fills the higher lines with zero's.
- void shiftLeft(const uint8_t n = 1) shifts output channels n pins (default 1) pins left (e.g. LEDs ). Fills the lower lines with zero's.
- void rotateRight(const uint8_t n = 1) rotates output channels to right, moving lowest line to highest line.
- void rotateLeft(const uint8_t n = 1) rotates output channels to left, moving highest line to lowest line.
- void reverse() reverse the "bit pattern" of the lines, swapping pin 7 with 0, 6 with 1, 5 with 2 etc.
Select
Some convenience wrappers.
- void select(const uint8_t pin) sets a single pin to HIGH, all others are set to LOW. If pin > 7 all pins are set to LOW. Can be used to select one of n devices.
- void selectN(const uint8_t pin) sets pins 0..pin to HIGH, all others are set to LOW. If pin > 7 all pins are set to LOW. This can typical be used to implement a VU meter.
- void selectNone() sets all pins to LOW.
- void selectAll() sets all pins to HIGH.
Miscellaneous
- int lastError() returns the last error from the lib. (see .h file).
Error codes
| name | value | description |
|---|---|---|
| PCF8574_OK | 0x00 | no error |
| PCF8574_PIN_ERROR | 0x81 | pin number out of range |
| PCF8574_I2C_ERROR | 0x82 | I2C communication error |
Operation
See examples.
It is advised to use pull-up or pull-down resistors so the lines have a defined state at startup.
Future
Must
- update documentation.
- keep in sync with PCF8575 (as far as meaningful)
Should
Could
- move code to .cpp
Wont
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