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Analog Devices (Maxim) · Integrated circuit

DS1307 Real-Time Clock

The cheap I2C clock chip. Keeps time on a coin cell for years and drifts minutes a month, because the crystal is bare and uncompensated.

Cheat sheet (SVG)
Logic level
4.5-5.5 V
Package
PDIP-8 / SOIC-8

1.Pinout diagram

Highlight a signalhover to preview, click to keep

Pins 1-4

DS1307

Pins 5-8

Pin detail

Hover a pin for its mappings and gotchas. Click to keep it here.

Logic level
4.5-5.5 V
Package
PDIP-8 / SOIC-8

Fig. 1. DS1307 Real-Time Clock, PDIP-8. Hover or tap a signal to light up every pin that carries it.

2.Watch out

  • The crystal is external and uncompensated, so accuracy is whatever your 32.768 kHz part and its stray capacitance give you. Expect minutes of drift per month, and more as the temperature swings. The DS3231 has the crystal and a temperature compensator on-die and drifts minutes per year.
  • 5 V only. It will not run from 3.3 V, which rules it out on an ESP32, a Pico or any modern board without a level shifter and a 5 V rail.
  • Fixed I2C address 0x68, not selectable. That clashes with an MPU6050, which also sits at 0x68.
  • The crystal must be a 32.768 kHz part with 12.5 pF load capacitance, mounted as close to pins 1 and 2 as possible, with no ground pour underneath. Stray capacitance here is the main reason a hand-built DS1307 runs fast or slow.
  • SQW/OUT is open drain and can emit 1 Hz, 4.096 kHz, 8.192 kHz or 32.768 kHz, which makes a free interrupt tick. It needs a pull-up.
  • There are 56 bytes of battery-backed RAM above the clock registers, which is handy for storing a few settings.
  • On power-up the clock halt bit is set and the chip is NOT running. Clear bit 7 of register 0 or it sits there reading the same time forever, which is the usual first-time confusion.

3.Minimum circuit

What has to go around the chip before it will run.

VCCGNDDS1307CR2032 cell between VBAT (3) and GNDCR2032cell4.7 kOhm between SDA (5) and VCC4.7 kOhm4.7 kOhm between SCL (6) and VCC4.7 kOhm100 nF between VCC (8) and GND100 nFGND (4) to GNDVCC (8) to VCC32.768 kHz crystal between X1 (1) and X2 (2)32.768 kHz crystalX1 1X2 2VBAT 3GND 4SDA 5SCL 6VCC 8VCC 8
Fig. 2. Minimum circuit, drawn from Table 1. Crossing wires are not connected.
Table 1. External parts, and why each one is there.
PartBetween
32.768 kHz crystalX1 (1) to X2 (2)
CR2032 cellVBAT (3) to GND
4.7 kOhmSDA (5) to VCC
4.7 kOhmSCL (6) to VCC
100 nFVCC (8) to GND

Stray capacitance around the crystal is the main reason a hand-built DS1307 runs fast or slow. The DS3231 puts the crystal on-die and avoids the problem.

4.Full pin reference

Every pin, alias and mapping. This is the table that prints.

Table 2. Pin assignments.

Complete pin reference for DS1307 Real-Time Clock
PinNameFunctions

Primary sources

Found a wrong pin? The whole board is one file, data/boards/ds1307.json.

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