Interfacing Intersil ISL1208 RTC with Arduino

A real-time clock (RTC) is a circuit that keeps track of time by counting the precise oscillations of a crystal oscillator of known frequency very accurately. For example, a typical frequency of a quartz crystal oscillator used in RTC applications is 32.768 kHz. That frequency is not arbitrary but is exactly 2¹⁵ cycles per second. That means we can use a 15-bit (16-bit in practical) counter to count the oscillations and produce a digital signal every second. A set of such digital counters can be cascaded and be used to count minutes, hours, days and so on. Quartz crystal oscillators (a piezo-electric material) produce very precise oscillations and therefore used in almost all the consumer applications including wrist watches, smartphones, TVs etc. You can learn more about how quartz crystals work and how they're manufactured for time-keeping applications in this video by Steve Mould.

To make the application easier, semiconductor companies produce standalone integrated circuits that implement all the required circuitry for an RTC in a single IC package. These are called RTC ICs. A popular such RTC IC is the DS1307 from Maxim semiconductor. But there are better RTCs available from both Maxim and other manufactures. In this project, I will show you how does a generic RTC IC work and how to interface the ISL1208 RTC IC from Intersil (a subsidiary of Renesas Electronics) with a microcontroller. We'll interface the ISL1208 with an Arduino and create a library for it. The library is released under MIT License.

The Intersil ISL1208 is a low power RTC chip with I2C interface. It uses an external 32.768KHz crystal to keep track of the time and has month-date-hour-min-sec alarm registers. It only consumes around 400nA at battery (VBAT) operation and a maximum of 1.2uA at external supply voltage (VDD). The operating voltage is from 1.8V to 5.5V. What makes this a good candidate are the low power consumption and the month-date alarm feature. Normal RTCs such as DS1307 do not have a month setting in alarm register. I used the month alarm feature of ISL1208 in my Arduino based Birthday Reminder project.

• Real Time Clock/Calendar
• Tracks Time in Hours, Minutes, and Seconds
• Day of the Week, Day, Month, and Year
• 15 Selectable Frequency Outputs
• Single Alarm
• Settable to the Second, Minute, Hour, Day of the Week, Day, or Month
• Single Event or Pulse Interrupt Mode
• Automatic Backup to Battery or Super Capacitor
• Power Failure Detection
• On-Chip Oscillator Compensation
• 2 Bytes Battery-Backed User SRAM
• I2C Interface
• 400kHz Data Transfer Rate
• 400nA Battery Supply Current
• Same Pin Out as ST M41Txx and Maxim DS13xx Devices
• Small Package Options
• 8 Ld MSOP and SOIC Packages
• 8 Ld TDFN Package
• Pb-Free Available (RoHS Compliant)

All RTC ICs have more or less the same internal design. Basically, there will be an internal square wave oscillator that uses an external 32.768 KHz crystal to produce the oscillations. The RTC Divider is simply a set of digital counters that count the oscillations and produce a digital signal at the output each time the counter buffer overflows. In this case, it will include counters that keep track of seconds, minutes, hours, day of week, date. month and year, and the values of the counters are saved to the time registers at a specific rate. There will be logic circuits to apply compensations such as leap year events. The output of the RTC Divider also goes to a Frequency Output section from where we can direct the output to the pin 7 of the RTC which has the function IRQ/FOUT. This pin has dual functions and therefore is multiplexed. It serves as either an interrupt output pin (such as the alarm interrupt) or a frequency output, producing clock signal of desired frequency. The output clock frequency can be programmed via registers.

The RTC Control Logic supervises all the communications via an I2C block and also the read and write of internal time and configuration registers. The ISL1208 will act as a slave on an I2C bus with an address 0x6F. The I2C interface supports data rates up to 400KHz. This block is disabled in the battery backup mode to save power. Therefore, we always need proper voltage level at VDD pin in order to read or write the RTC registers.

Another function of the RTC Control Logic is to check if the current time matches any of the alarm register values. When they do match, it will activate the Alarm block to produce an interrupt signal in interrupt mode or an active low signal for the single event mode at pin 7. The duration of the alarm interrupt signal is 250ms.

There are two ways we can power the RTC - a VDD between 2-5.5VDC or a VBAT of 1.8-5.5V. There is an internal switching circuit that selects the appropriate power source automatically when certain conditions are satisfied. This section also monitor for a total power failure and update the Real Time Clock Failure Bit (RTCF) in the register. These are well described in the Functional Description section of the datasheet.

Since an RTC chip consumes very low current in the ranges of nano amperes, a coin cell is able to operate an RTC for a long time independent of a main supply. 3V Lithium coin cells are used for such applications. Lithum cells have long charge retentivity, up to 10 years.

The register map gives all the details a programmer needs to write software for interfacing the RTC chip. Just like any other I2C devices, you can first send the slave address of the RTC followed by the register address you want to read or write. I've included all these addresses in the Arduino library. Note that, the RTC registers keep the values in BCD (Binary Coded Digits) format. So you need to convert to and from BCD when reading and writing those registers. The library also makes it easy to do this. Full description of the registers can be found on the datasheet.

The ISL1208 needs very few external components to work. When using the IC on an actual application circuit, it's a good practice to place a 0.1uF (100nF) power supply bypass capacitor closer to the VDD pin. The above schematic shows Arduino Nano as example. You can connect the RTC to I2C port of any of the Arduino boards. The R1 and R5 are common pull-up resistors for the I2C bus. For the battery backup, I have included the CR2450 3V Lithium cell which has a capacity of 540mAh and can easily run the RTC circuit for more than 10 years. For demonstrating the interrupt output, I've tied the IRG/FREQ pin of the RTC to digital pin 2 of the Arduino with interrupt capability. R3 is a pull-up for the interrupt pin but you can exclude this and use the internal pull-up instead.

This library supports all official Arduino boards as well as other boards that have the Arduino core avaialable such as ESP8266, ESP32, STM32 Nucleo etc.

Library Details

• Library version : 1.4.2
• Author : Vishnu M Aiea
• Source : https://github.com/vishnumaiea/ISL1208-RTC-Library
• Author's website : www.vishnumaiea.in
• Initial release : IST 11:49:42 AM, 27-05-2018, Sunday
• Last updated : IST 11:06 AM 25-05-2019, Saturday
• License : MIT

Dependancies

1. stdint.h

2. Arduino.h

3. Wire.h

Constants

#define ISL1208_ADDRESS 0x6F  //I2C slave addess of RTC IC

#define ISL1208_SC      0x00  //seconds register

#define ISL1208_MN      0x01  //minutes register

#define ISL1208_HR      0x02  //hours register

#define ISL1208_DT      0x03  //date register

#define ISL1208_MO      0x04  //month register

#define ISL1208_YR      0x05  //year register

#define ISL1208_DW      0x06  //day of the week register

#define ISL1208_SCA     0x0C  //alarm seconds register

#define ISL1208_MNA     0x0D  //alarm minutes register

#define ISL1208_HRA     0x0E  //alarm hours register

#define ISL1208_DTA     0x0F  //alarm date register

#define ISL1208_MOA     0x10  //alarm month register

#define ISL1208_DWA     0x11  //alarm day of the week register

Classes

1. ISL1208_RTC

Member Variables

bool rtc_debug_enable; //enable this to get verbose output at serial monitor

byte yearValue;     //least significant digits of a year (eg. 18 for 2018, range is from 00 to 99)

byte monthValue;    //month (eg. 01 for January, range is 01 to 12)

byte dateValue;     //date (eg. 24, range is 01 to 31)

byte hourValue;     //hours (eg. 06, range is 01 to 12 for 12 hour format)

byte minuteValue;   //minutes (eg. 55, range is 00 to 59)

byte secondValue;   //seconds (eg. 30, range is 00 to 59)

byte periodValue;   //period of the day for 12 hour format (0 = AM, 1 = PM)

byte monthValueAlarm;   //same as time values

byte dateValueAlarm;

byte hourValueAlarm;

byte minuteValueAlarm;

byte secondValueAlarm;

byte periodValueAlarm;

Member Functions

ISL1208_RTC(); //constructor

void begin(); //alternate initializer

bool isRtcActive(); //checks if the RTC is available on the I2C bus

bool updateTime(); //update time registers from variables

bool updateTime(String); //updates time registers from a formatted time string

bool updateAlarmTime(); //updates alarm registers from variables

bool updateAlarmTime(String); //updates alarm registers from a formatted alarm time string

bool fetchTime(); //reads RTC time and alarm registers and updates the variables

int getHour(); //returns the 12 format hour in DEC

int getMinute(); //returns minutes in DEC

int getSecond(); //returns seconds value

int getPeriod(); //returns time period. 0 = AM, 1 = PM

int getDate(); //returns date

int getDay(); //returns day (0 to 6)

int getMonth(); //returns month (0 to 12)

int getYear(); //returns year (00 = 2000, 99 = 2099)

int getAlarmHour();

int getAlarmMinute();

int getAlarmSecond();

int getAlarmPeriod(); //0 = AM, 1 = PM

int getAlarmDate();

int getAlarmDay();

int getAlarmMonth();

String getTimeString(); //returns formatted time string (hh:mm:ss pp)

String getDateString(); //returns formatted date string (DD-MM-YYYY)

String getDayString(); //returns the full name of day

String getDayString(int n); //returns the first n chars of day string (n = 1 to 9)

String getDateDayString(); //returns a formatted date string with day name (DD-MM-YYYY DAY)

String getDateDayString(int n); //returns a formatted date string with n truncated day name

String getTimeDateString(); //returns a formatted time date string

String getTimeDateDayString(); //does what it says!

String getTimeDateDayString(int n); //returns a time, date string with n truncated day string

String getAlarmString();

bool printTime(); //prints time to the serial monitor

bool printAlarmTime(); //prints the alarm time to serial monitor

byte bcdToDec(byte); //converts a BCD value to DEC

byte decToBcd(byte); //converts a DEC value to BCD

1. void begin();

2. bool isRtcActive();

3. bool updateTime();

4. bool updateTime(String);

• T = indicates time information
• YY = least significant digits of a year (eg. 18 for 2018, range is from 00 to 99)
• MM = month (eg. 01 for January, range is 01 to 12)
• DD = date (eg. 24, range is 01 to 31)
• hh = hours (eg. 06, range is 01 to 12 for 12 hour format)
• mm = minutes (eg. 55, range is 00 to 59)
• ss = seconds (eg. 30, range is 00 to 59)
• p = period of the day for 12 hour format (0 = AM, 1 = PM)
• # = delimiter

• T = indicates time information
• 18 = the year 2018
• 01 = month January
• 05 = date
• 08 = hours
• 35 = minutes
• 12 = seconds
• 0 = AM
• # = delimiter

5. bool updateAlarmTime();

6. bool updateAlarmTime(String);

• A = indicates alarm information
• MM = month
• DD = date
• hh = hours
• mm = minutes
• ss = seconds
• p = time period (0 = AM, 1 = PM)
• # = delimiter

7. bool fetchTime();

8. int getHour();

9. int getMinute();

10. int getSecond();

11. int getPeriod();

12. int getDate();

13. int getDay();

14. int getMonth();

15. int getYear();

16. int getAlarmHour();

17. int getAlarmMinute();

18. int getAlarmSecond();

19. int getAlarmPeriod();

20. int getAlarmDate();

21. int getAlarmDay();

22. int getAlarmMonth();

23. String getTimeString();

24. String getDateString();

25. String getDayString();

26. String getDayString(int n);

27. String getDateDayString();

28. String getDateDayString(int n);

29. String getTimeDateString();

30. String getTimeDateDayString();

31. String getTimeDateDayString(int n);

32. String getAlarmString();

33. bool printTime();

34. bool printAlarmTime();

35. byte bcdToDec(byte);

36. byte decToBcd(byte);

Below is an example sketch to demonstrate some functions of this library. Upload it to your Arduino and open any serial monitor with a baudrate 115200.


View this at GitHub

The code is straightforward. When you first open the serial monitor, you'll see the status message.

Then send a command with one or two parameters with each separated with a whitespace.

As I had an SMD version of the ISL1208, I made a small breakout board as shown below. I snatched the crystal from an old quartz clock's PCB.

The current consumption of the ISL1208 in low power mode is only 400nA and at VDD is 1.2uA at max. So it's good for portable applications that are battery powered. Having a wide range of voltage from 1.8-5V means, you can directly interface it with any microcontrollers that work on 5V such as some Arduino boards, or 3.3V ones such as STM32 Nucleo boards. This is for example, not possible with DS1307, because it needs a minimum supply voltage of 4.5V in order to access the registers via I2C, as per the datasheet. Therefore, ISL1208 is a better RTC than DS1307. An alternative option is the DS1308 which has a time-keeping current of 250nA which is better than ISL1208, but has no alarm feature which is a bummer.