Arduino library for PCF8574 - 8 channel I2C IO expander.
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.
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.
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.
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:
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:
A minimal example that shows catching missed interrupts:
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().
16 bit port expanders
8 bit port expanders
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 |
#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.
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.
Background - https://github.com/RobTillaart/Arduino/issues/38
Some convenience wrappers.
name | value | description |
---|---|---|
PCF8574_OK | 0x00 | no error |
PCF8574_PIN_ERROR | 0x81 | pin number out of range |
PCF8574_I2C_ERROR | 0x82 | I2C communication error |
See examples.
It is advised to use pull-up or pull-down resistors so the lines have a defined state at startup.
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