DS130X Library Common Reference


arduino rtc software

Code samples and documentation for the DS130X library common to the DS1302 and DS1307 RTCs


This page contains information relevant to both the DS1302 and DS1307 real-time clock (RTC) chips. All information specific to one chip will be pointed out.

View additional chip-specific guides: DS1302, DS1307

This guide uses my DS130X library. Download the library in Zip format

All example code is under File > Examples > DS130X > (your RTC).

Printing Date and Time

After installing the library, go to the DateAndTime > PrintDateTime example.

Creating the RTC Object

The DS1302 constructor takes parameters in the order:

  1. dataPin: Arduino pin that the RTC’s DAT pin is connected to
  2. clockPin: Arduino pin that the RTC’s CLK pin is connected to
  3. cePin: Arduino pin that the RTC’s CE pin is connected to

The DS1307 constructor takes no parameters. Wire.begin must be called to intialize the I2C bus before using the RTC.

Writing Date and Time

DS1302 only: Before writing to the RTC’s registers, we need call setWriteProtect(false); which sets the write protect bit. When write protect is enabled, the write and writeRaw methods won’t do anything, but it is disabled, these methods will write data to the chip.

setTime takes parameters in the order:

  1. hour: Hour to set (0-23)
  2. minute: Minute to set (0-59)
  3. second: Second to set (0-59)

setDate takes parameters in the order:

  1. day: Day of month to set (1-31)
  2. month: Month to set (1-12)
  3. year: Year to set (0-99)
  4. dayOfWk: Day of week to set (use the weekday constants: SUNDAY, MONDAY, etc.)

Reading Date and Time

These functions return parts of the current date/time in predetermined formats:

At the end of each loop iteration, the program pauses for one second so it doesn’t read too quickly.

Reading and Writing Registers

An example of reading registers is at DateAndTime > ReadData.

   Sketch to read unformatted time data from the RTC.
   Created August 28, 2020

   DS1302 connections:
   CLK to 2
   DAT to 3
   CE  to 4

#include <DS130X.h>

DS1302 rtc(3, 2, 4); // DAT, CLK, CE

void setup() {
  // Set time and date
  rtc.setDate(28, 8, 20, FRIDAY); // Friday, 28 August 2020
  rtc.setTime(10, 5, 50);         // 10:05:50
  Serial.begin(9600); // Open serial port

void loop() {
  // Read time from RTC
  uint8_t hours   = rtc.read(HOURS);
  uint8_t minutes = rtc.read(MINUTES);
  uint8_t seconds = rtc.read(SECONDS);

  Serial.println("Hour: "   + String(hours));
  Serial.println("Minute: " + String(minutes));
  Serial.println("Second: " + String(seconds));


  // Read date from RTC
  uint8_t dayOfWeek  = rtc.read(DAY_OF_WEEK);
  uint8_t dayOfMonth = rtc.read(DAY_OF_MONTH);
  uint8_t month      = rtc.read(MONTHS);
  uint8_t year       = rtc.read(YEARS);

  Serial.println("Day of week: " + String(dayOfWeek));
  Serial.println("Day: "         + String(dayOfMonth));
  Serial.println("Month: "       + String(month));
  Serial.println("Year: "        + String(year));

  Serial.println("--------------------"); // Print a separator
  delay(1000); // 1 second pause

Available Registers

The RTC classes have four methods to read and write to registers:

All four methods will require you to specify a register to perform operations on. There are two ways to do this. The first way is to specify a register constant:

You can also specify a register address. For example, with a DS1302, write(0x80) writes to the register 0x80 (the seconds register). (On the DS1307, this register is 0x00.)


DS1302 only:

The read registers are always one higher than the write registers. For example, to write to the minutes register, use write(0x82, x); to write and read(0x83); to read.

If you are using a register constant (like MINUTES), this difference is already handled, and you can use write(MINUTES, x); and read(MINUTES);


Writing to the seconds register will overwrite the halt bit to 0. Keep this in mind when using this method - call setHalt after setting the seconds register (See “Additional Functions”).

Register Operations

read and readRaw both take a single parameter: the register to read from.

read will convert the register data from BCD to base-10 and apply a bitmask, if necessary. readRaw will directly return the register’s data without converting or masking.

write and writeRaw both take two parameters in the order:

  1. reg: Register address to write to
  2. value: Data to write to the register

write expects the value to be in base-10, then converts it to BCD to write. writeRaw expects the value to be in BCD, and directly writes to the register without converting.


When writing to the seconds register with writeRaw, you can explicitly set the halt bit (bit #7, the MSB, in the seconds register).

Reading and Writing RAM

Register operations can be used for operating on the RTC’s RAM. An example is at Other > RAMReadWrite.

   Sketch to demonstrate reading and writing the DS1307's RAM.
   Created August 30, 2020

   DS1302 connections:
   CLK to 2
   DAT to 3
   CE  to 4

#include <DS130X.h>

DS1302 rtc(3, 2, 4); // DAT, CLK, CE

void setup() {

  // Write to all bytes of RAM
  Serial.println("Writing to RAM");
  int value = 0; // Value to hold in RAM
  for (int i = rtc.ramWStart(); i <= rtc.ramWEnd(); i += 2) {
    // Add 2 to counter to skip every other address (the read address)
    // ramWStart() returns the starting address for writing RAM.
    // ramWEnd() returns the ending address for writing RAM.
    rtc.write(i, value); // Write to RAM
    value++; // New value for next address

  // Read from all bytes of RAM
  Serial.println("Reading from RAM");
  for (int i = rtc.ramRStart(); i <= rtc.ramREnd(); i += 2) {
    // Add 2 to counter to skip every other address (the write address)
    // ramRStart() returns the starting address for reading RAM.
    // ramREnd() returns the ending address for reading RAM.
    uint8_t ramValue = rtc.read(i); // Read from RAM
    // Print the value in RAM
    Serial.println("Address 0x" + String(i, HEX) + " has value " + String(ramValue));

void loop() {} // No loop

Addresses and Bounds

Using the below functions as the lower and upper bounds of a for loop, we can iterate through each address.

To operate on a single byte, use read or write and set the register value to the desired address.

DS1302 Specifics

The DS1302 class has four methods to get the lowest and highest RAM addresses, split into read and write:

In each loop, the increment statement is i += 2. Since the addresses alternate between read and write, we have to skip every other one.

RAM byteAddressFunction

DS1307 Specifics

The DS1307 class has two methods to get the lowest and highest RAM addresses:

All addresses can be written to and read from.

Additional Functions

Halt Bit

When the RTC is halted (determined by the halt bit), its oscillator is disabled and goes into low-power mode. This means that it won’t keep track of time until you set it back to normal mode.

The RTC’s setHalt method writes to this halt bit. It takes a Boolean parameter: true to enable the halt bit, false to disable.


DS1307 only: The SQW output won’t work when the RTC is halted.