DS1302 — Trickle-Charge Real-Time Clock Module

Overview

The DS1302 is a classic trickle-charge real-time clock (RTC) chip manufactured by Maxim Integrated (Analog Devices). Included in low-cost Elegoo, SunFounder, and Keyes Arduino starter kits, it is packaged as a compact 5-pin breakout module featuring an onboard $32.768\text{ kHz}$ tuning fork crystal and a CR2032 coin cell battery holder.

Communicating over a simple 3-wire synchronous serial interface (CE, I/O, SCLK), the DS1302 maintains full BCD timekeeping registers (seconds, minutes, hours, date, month, day-of-week, and year with leap-year compensation up to 2100) along with 31 bytes of battery-backed static RAM.

Quick reference


Operating voltage (`VCC2`)	2.0 V to 5.5 V DC (3.3 V or 5.0 V nominal)
Primary backup voltage (`VCC1`)	2.0 V to 5.5 V DC (CR2032 3V coin cell battery)
Interface	3-Wire Serial Interface (`CE` / `RST`, `I/O` / `DAT`, `SCLK` / `CLK`)
Crystal frequency	$32.768\text{ kHz}$
Backup power draw	$< 0.3\ \mu\text{A}$ at $V_{CC1} = 2.0\text{V}$
Static RAM	31 bytes battery-backed SRAM for user data storage
Timekeeping format	BCD (Seconds, Minutes, 12/24-Hour, Date, Month, Day, Year)
Trickle Charger	Software-selectable charging diode & resistor network for rechargeable batteries

Pinout

Breakout module 5-pin 0.1" (2.54 mm) connector header:

text

        ┌───────────────────────────────┐
        │  [DS1302 IC]  [32.768kHz]     │
        │  [CR2032 Coin Cell Holder]    │
        └─┬─────┬─────┬─────┬─────┬─────┘
         VCC   GND   CLK   DAT   RST
          1     2     3     4     5

Pin	Module Label	Name	Type	Description
1	`VCC`	`VCC2`	Power	Main system power supply input (+2.0 V to +5.5 V DC)
2	`GND`	`GND`	Power	Ground reference (0 V)
3	`CLK`	`SCLK`	Digital Input	Serial Clock input pin
4	`DAT`	`I/O`	Digital Input / Output	Bi-directional Serial Data line
5	`RST`	`CE`	Digital Input	Active-High Chip Enable / Reset control line

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Main Supply Voltage	$V_{CC2}$	2.0	3.3 / 5.0	5.5	V	System operating power
Backup Battery Voltage	$V_{CC1}$	2.0	3.0	3.5	V	CR2032 coin cell input
Active Supply Current	$I_{CC2}$	—	0.3	1.0	mA	SPI serial data transfer active
Backup Current	$I_{CC1}$	—	0.3	1.0	µA	Main power off, $V_{CC1}=3.0\text{V}$
Clock Transfer Speed	$f_{SCLK}$	0	—	2.0	MHz	At $V_{CC2} = 5.0\text{V}$
Internal SRAM	$RAM$	—	31	—	Bytes	Battery-backed non-volatile RAM

3-Wire Serial Timing Protocol

Chip Enable (CE / RST): Drive CE High before initiating serial transfers. Drive CE Low to end a transaction.
Clock (SCLK): Input data bits on I/O are sampled on the rising edge of SCLK; output data bits on I/O change on the falling edge of SCLK.
Data (I/O): First byte sent is the Command Byte ( $R/\bar{W}$ bit, Register Address, and Magic Bit 7 High).

text

 Command Byte Read:  [ 1 | A6 | A5 | A4 | A3 | A2 | A1 | R/W=1 ]
 Command Byte Write: [ 1 | A6 | A5 | A4 | A3 | A2 | A1 | R/W=0 ]

Wiring

DS1302 Pin	Arduino Uno	ESP32	Notes
`VCC`	5V / 3.3V	3.3V	Power rail
`GND`	GND	GND	System ground
`CLK`	Digital D6	GPIO 18	`SCLK` clock line
`DAT`	Digital D7	GPIO 19	`I/O` bi-directional data line
`RST`	Digital D8	GPIO 5	`CE` active-high chip enable

Example (Arduino `RtcByMakuna` Library)

cpp

#include <ThreeWire.h>  
#include <RtcDS1302.h>

// Pins: DAT (IO), CLK (SCLK), RST (CE)
ThreeWire myWire(7, 6, 8); 
RtcDS1302<ThreeWire> Rtc(myWire);

void setup() {
  Serial.begin(115200);
  Rtc.Begin();

  if (!Rtc.IsDateTimeValid()) {
    Serial.println("RTC lost confidence in the DateTime!");
    // Set compilation time
    RtcDateTime compiled = RtcDateTime(__DATE__, __TIME__);
    Rtc.SetDateTime(compiled);
  }

  if (Rtc.GetIsWriteProtected()) {
    Rtc.SetIsWriteProtected(false);
  }

  if (!Rtc.GetIsRunning()) {
    Rtc.SetIsRunning(true);
  }
}

void loop() {
  RtcDateTime now = Rtc.GetDateTime();

  char datestring[20];
  snprintf_P(datestring, 
             countof(datestring),
             PSTR("%02u/%02u/%04u %02u:%02u:%02u"),
             now.Month(), now.Day(), now.Year(),
             now.Hour(), now.Minute(), now.Second());

  Serial.println(datestring);
  delay(1000);
}

Common mistakes

Enabling trickle charger with primary CR2032 lithium batteries: The DS1302 includes a software-selectable trickle charger circuit. Do not enable the trickle charger when using standard non-rechargeable CR2032 batteries, as charging non-rechargeable lithium cells causes coin cell swelling and explosion hazards.
Confusing 3-wire serial with SPI or $I^2C$ : The DS1302 uses a custom 3-wire protocol with active-high CE. Standard hardware SPI libraries or $I^2C$ scanners cannot communicate with the DS1302.

Notes

DS1302 vs DS1307 vs DS3231: DS1302 uses a 3-wire serial interface; DS1307 uses $I^2C$ ; DS3231 includes an internal TCXO crystal for $\pm 2\text{ ppm}$ precision.