PCF8573 Arduino Library

Arduino library for the PCF8573 RTC I2C chip with Power Fail Detector.

The library makes it easy to set and read time, set alarm registers and access status flags of the PCF8573 chip.

List of Contents:

1. Installation
2. Connecting the PCF8573 chip and pinout
3. Usage
4. Methods
5. Power Fail Detector
6. Inspiration

1. Installation

Method 1:

In the Arduino IDE:

1. Open LIBRARY MANAGER
2. Search phrase PCF8573
3. Find the PCF8573 by Ignacy110 library in the list
4. Click INSTALL

Method 2:

1. Download the repository as .zip file.
2. In the Arduino IDE, select: Sketch → Include Library → Add .ZIP Library...
3. Select the downloaded .zip file.

2. Connecting the PCF8573 chip and pinout

2.1 Hardware requirements:

The PCF8573 chip requires the following components:

• 32.768 kHz crystal oscillator - connected between OSCI and OSCO pins
  • a trim capacitor can be added to fine-tune the precision of timekeeping - a trimmer is connected between OSCI and VDD pins
• SDA and SCL pull-up resistors (4.7 kΩ) - if your board does not have such resistors
• a 100 nF capacitor between VDD and VSS1

Warning

The 100 nF capacitor can be omitted, but this is not recommended. In the final version of the project with this or other integrated circuits it is worth including it.

2.2 Pinout and how to connect:

Symbol	Pin	Description	I/O	How to connect
A0	1	address input	input	to GND
A1	2	address input	input	to GND
COMP	3	comparator output	output	not connected
SDA	4	I2C SDA	transmission	to the SDA pin in your microcontroller
SCL	5	I2C SCL	transmission	to the SCL pin in your microcontroller
EXTPF	6	enable power fail flag input	input	to GND
PFIN	7	power fail flag input	input	to GND
VSS2	8	GND for the I2C interface	power	to GND
MIN	9	one pulse per minute output	output	not connected
SEC	10	one pulse per second output	output	not connected
FSET	11	oscillator tuning output	output	not connected
TEST	12	test input	input	to GND
OSCI	13	oscillator input	osc. input	Crystal oscillator (32.768 kHz)
OSCO	14	oscillator input/output	osc. input/output	Crystal oscillator (32.768 kHz)
VSS1	15	GND for the clock (RTC)	power	to GND
VDD	16	5V (typical value) common positive supply	power	to VCC

The last column shows the simplest way to connect the PCF8573 chip.

Note

If the outputs are not used they may not be connected.

Caution

Don't leave inputs unconnected to VCC or GND. Leaving inputs floating may cause incorrect system operation!

Depending on your application, you may connect the chip differently. For instance, output pins can be used to read the COMP flag or the one-second/one-minute pulses directly in hardware. Additionally, setting inputs to VCC instead of GND allows you to modify the PCF8573 I2C address or change the behavior of the Power Fail Detector.

The address of the PCF8573 chip is set via inputs A0 and A1. Table of assigned addresses:

A1 input state	A0 input state	PCF8573 address
GND	GND	0x68
GND	VCC	0x69
VCC	GND	0x6A
VCC	VCC	0x6B

3. Usage

3.1 Basic setup

To use this library, your application must include the <Wire.h> library, which enables I2C communication with the chip.

Include this library <PCF8573.h>.

Before using the library methods, you need to create an instance of the PCF8573 class e.g. PCF8573 rtc(0x68); where the parameter is the I2C address set via pins A0 and A1. In short this object represents the physical PCF8573 chip with a given address 0x68 connected via I2C.

To use the PCF8573, you must initialize the I2C communication by calling Wire.begin(); and Wire.setClock(100000); in the setup() function.

Example:

#include <Wire.h> // library required for I2C communication
#define WIRE_CLOCK 100000 // set the wire transmission clock

#include <PCF8573.h> // PCF8573 library
PCF8573 rtc(0x68); // create an rtc object with address of our PCF8573 chip

void setup()
{
  Wire.begin(); // start I2C communication to communicate with PCF8573 chip
  Wire.setClock(WIRE_CLOCK); // set defined wire transmission clock
}

3.2 Use library methods

All available methods and their syntax are described in paragraph 4.

Simple example of use:

This program sets the time to 12:34 and displays it on the Serial Monitor (in Arduino IDE). You can see how minutes and later hours increment.

#include <Wire.h> // library required for I2C communication
#define WIRE_CLOCK 100000 // set the wire transmission clock

#include <PCF8573.h> // PCF8573 library
PCF8573 rtc(0x68); // create an rtc object with address of our PCF8573 chip

void setup()
{
  Serial.begin(115200); // start serial communication for serial monitor

  Wire.begin(); // start I2C communication to communicate with PCF8573 chip
  Wire.setClock(WIRE_CLOCK); // set defined wire transmission clock

  rtc.setTime(PCF8573::time::HOURS, 12);     // setting hours
  rtc.setTime(PCF8573::time::MINUTES, 34);   // setting minutes
}

void loop()
{
  Serial.print("\nHour:\t");
  Serial.println(rtc.readTime(PCF8573::time::HOURS));   // display hours

  Serial.print("Minute:\t");
  Serial.println(rtc.readTime(PCF8573::time::MINUTES)); // display minutes

  delay(5000);
}

Tip

More examples are in the examples folder.

4. Methods

The PCF8573 allows you to:

1. set and read the time
2. set and read the alarm time
3. set and read flags
4. use additional control functions

Important

All methods provided by this library are member functions of the PCF8573 class.
Therefore they must be called using the created object, for example rtc.

You can use the following methods to perform these activities:

4.1 Time operations

4.1.1 Set the time

To set the time, use the setTime() method which requires a time field identifier and your value e.g. setTime(PCF8573::time::HOURS, 7) that means set the hours register to 7.

setTime() parameters:

Function	setTime()	accepted values
Set hours	setTime(PCF8573::time::HOURS, value);	0 to 23
Set minutes	setTime(PCF8573::time::MINUTES, value);	0 to 59
Set days	setTime(PCF8573::time::DAYS, value);	1 to 28 / 29 1 to 30 1 to 31
Set months	setTime(PCF8573::time::MONTHS, value);	1 to 12

Note

The DAYS register accepts values ​​appropriate to the month.

Warning

Incorrect values ​​sent may cause the chip to malfunction!

Important

Leap year support (February 29) must be handled externally because the PCF8573 does not have a year counter.

4.1.2 Read the time

To read the time, use the readTime() method which requires a time field identifier e.g. readTime(PCF8573::time::MINUTES) that means read the minutes register. This method returns a uint8_t value.

readTime() parameters:

Function	readTime()
Read hours	readTime(PCF8573::time::HOURS);
Read minutes	readTime(PCF8573::time::MINUTES);
Read days	readTime(PCF8573::time::DAYS);
Read months	readTime(PCF8573::time::MONTHS);

4.2 Alarm time operations

The alarm works like an alarm clock. When the alarm time matches the current time, the COMP flag is set to 1, indicating that the hour has struck. More details in section 4.3 Flags operation.

4.2.1 Set the alarm time

To set the alarm time, use the setAlarmTime() method which requires a time field identifier and your value e.g. setAlarmTime(PCF8573::time::DAYS, 21) that means set the days alarm register to 21.

setAlarmTime() parameters:

Function	setAlarmTime()	accepted values
Set alarm hours	setAlarmTime(PCF8573::time::HOURS, value);	0 to 23
Set alarm minutes	setAlarmTime(PCF8573::time::MINUTES, value);	0 to 59
Set alarm days	setAlarmTime(PCF8573::time::DAYS, value);	1 to 28 / 29 1 to 30 1 to 31
Set alarm months	setAlarmTime(PCF8573::time::MONTHS, value);	1 to 12

4.2.2 Read the alarm time

To read the alarm time, use the readAlarmTime() method which requires a time field identifier e.g. readAlarmTime(PCF8573::time::MONTHS) that means read the months alarm register. This method returns a uint8_t value.

readAlarmTime() parameters:

Function	readAlarmTime()
Read alarm hours	readAlarmTime(PCF8573::time::HOURS);
Read alarm minutes	readAlarmTime(PCF8573::time::MINUTES);
Read alarm days	readAlarmTime(PCF8573::time::DAYS);
Read alarm months	readAlarmTime(PCF8573::time::MONTHS);

4.3 Flags operation

The PCF8573 chip has 3 flags:

Flag	Description	Method of set (0 to 1)	Method of reset (1 to 0)
POWF	The flag indicates a detected power failure (more information about Power Fail Detector and POWF flag in chapter 5)	When a power failure occurs	By setting the time or alarm time
COMP	The flag indicates that the alarm has been triggered when the alarm time matches the current time.	When alarm time is the same as RTC time	By user
NODA	The flag says to ignore the date when comparing the alarm and time (when comparing it only looks at hours and minutes)	By user	By user

4.3.1 Read the flag

To read the flag, use the readFlag() method which requires a time field identifier e.g. readFlag(PCF8573::flag::COMP) that means read the COMP flag register. This method returns a bool value.

readFlag() parameters:

Function	readFlag()
Read POWF flag	readFlag(PCF8573::flag::POWF);
Read COMP flag	readFlag(PCF8573::flag::COMP);
Read NODA flag	readFlag(PCF8573::flag::NODA);

4.3.2 Reset the flag

To reset the flag, use the resetCOMPflag() or resetNODAflag() method. Reset means changing the state of the flag from 1 to 0.

4.3.3 Set the flag

To set the flag, use the setNODAflag() method. Set means changing the state of the flag from 0 to 1.

Note

Using methods for flags, you can:

• read one of three flags: POWF, COMP, NODA;
• reset one of two flags: COMP, NODA;
• set one: NODA;

4.4 Functions

4.4.1 Reset prescaler - reset seconds

resetPrescaler() method - reset prescaler, including seconds counter, without affecting the minute counter

Tip

It is good to use resetPrescaler() method when you set minutes. Because this method resets the seconds counter so you can start counting minutes precisely from a given moment. In your code:

rtc.setTime(PCF8573::time::HOURS, 9);      // setting hours
rtc.setTime(PCF8573::time::MINUTES, 11);   // setting minutes
rtc.resetPrescaler();   // resetting seconds counter

In this example you set time 09:11:00. Without resetPrescaler() function you can set 09:11:xx where xx has the value at which the chip stopped counting (so random value).

5. Power Fail Detector

Power Fail Detector sets the POWF flag to 1 when a power failure occurs.

There are two types of power failure detection - internal and external.

Internal mode: The flag indicates a detected power failure - VDD-VSS1 voltage reaches approximately 1.2V or less (Voltage of 1.2V due to the design of the PCF8573 chip to work with 1.2 V nickel cadmium battery).

External mode: To use external power failure detection, the EXTPF input must be at logic HIGH (e.g. connected to VCC).

The POWF flag is set based on the PFIN input. If it goes to LOW state, the system interprets it as a power failure.

EXTPF input state	PFIN input state	Function
LOW	LOW	Power Fail Detector - Internal mode
LOW	HIGH	test mode - don't use
HIGH	LOW	Power Fail Detector - External mode - power supply failure detected
HIGH	HIGH	Power Fail Detector - External mode - power supply status correct

Important

Power Fail Detector does not reset the POWF flag to 0. To reset this flag, you must set the time or alarm time.

6. Inspiration

This library was inspired by the PCF8574 Arduino library by MSZ98: https://github.com/MSZ98/pcf8574

It is an independent implementation for the PCF8573 RTC chip.