DS1307

The DS1307 is a low-power serial real-time clock and calendar from Maxim Integrated (now part of Analog Devices) that combines a BCD clock, a crystal oscillator circuit and 56 bytes of battery-backed nonvolatile SRAM in an 8-pin DIP or 150-mil SOIC. It counts seconds, minutes, hours, day of week, date, month and year, automatically corrects months with fewer than 31 days, and applies leap-year compensation valid through 2100. The 12-hour mode carries an AM/PM flag; the 24-hour mode is the norm in industrial designs. Address and data move over a two-wire I2C bus from the fixed address 0x68, and time is captured into a second register set on START so a read never straddles a seconds rollover.

The device needs an external 32.768 kHz watch crystal whose specified load capacitance matches the internal oscillator's 12.5 pF. This is the single most important sourcing and design decision around the DS1307: there is no temperature-compensated oscillator inside, so accuracy is set by the crystal's initial tolerance and by its parabolic drift with temperature — a typical ±20 ppm crystal produces roughly ±1 minute per week at room temperature and noticeably more across an industrial temperature range. If your product must hold ±2 ppm, use a DS3231 or DS3232 with an integrated TCXO instead. Power-fail detection switches the clock and RAM to VBAT when VCC drops below about 1.25 × VBAT, and battery-backup current is 300 nA typical and 500 nA maximum with the oscillator running, which gives more than ten years on a 48 mAh lithium cell at 25 °C.

Other details that catch integrators out: the interface is standard-mode I2C only, so SCL must stay at or below 100 kHz even though most MCUs happily run 400 kHz; SDA and SQW/OUT are open-drain and need pull-ups; VCC is 4.5 V to 5.5 V while VBAT accepts 2.0 V to 3.5 V; the clock-halt bit in the seconds register must be cleared after first programming because power-on register states are undefined; and the SQW/OUT pin provides a programmable 1 Hz, 4.096 kHz, 8.192 kHz or 32.768 kHz square wave that can wake an MCU. There is no internal trickle charger, so a supercapacitor must be charged through an external diode and resistor.

The DS1307 is available in commercial (0 °C to +70 °C) and industrial (−40 °C to +85 °C) grades and is UL recognized for use with lithium backup cells. It is heavily cloned on module boards; the failure mode is almost always crystal load-capacitance mismatch or a marginal cell holder rather than the die. Verify marking, measure backup current, and audit the module's crystal specification. MOQ is 1 reel or 100 pcs loose, with 1-2 day dispatch from Shenzhen stock and 4-8 weeks through the ADI authorized channel.