DS1307

DS1307 Serial Real-Time Clock (RTC) Technical Summary

Overview

The DS1307 is a low-power, full binary-coded decimal (BCD) clock/calendar with an integrated 56-byte nonvolatile SRAM. This serial real-time clock (RTC) provides accurate timekeeping functionality for embedded systems, consumer electronics, and industrial applications. Communication with the DS1307 is accomplished via a simple 2-wire (I²C) interface, making it easy to integrate with microcontrollers and other digital systems.

The device operates in either 24-hour or 12-hour format with an AM/PM indicator and automatically adjusts for months with fewer than 31 days, including leap year compensation up to the year 2100. With its ultra-low power consumption and battery backup support, the DS1307 is ideal for applications requiring reliable timekeeping even during power outages.

Key Features

• Real-Time Clock (RTC) Counts: Seconds, minutes, hours, day, date, month, and year with leap-year compensation
• 56-Byte Nonvolatile SRAM: For general-purpose data storage
• Two-Wire Serial Interface: I²C-compatible (up to 400kHz)
• Programmable Square-Wave Output: Configurable output signal at 1Hz, 4kHz, 8kHz, or 32kHz
• Low Power Operation: Consumes less than 500nA in battery backup mode
• Automatic Power-Fail Detect: Switches to battery backup when main power fails
• Operating Voltage: 4.5V to 5.5V (2V to 5.5V in battery backup mode)
• Industrial Temperature Range: -40°C to +85°C
• Clock Accuracy: ±2ppm from 0°C to +40°C (±23ppm from -40°C to +85°C)

Applications

The DS1307 is widely used in various electronic systems requiring accurate timekeeping:

• Embedded systems and microcontroller-based projects
• Point-of-sale (POS) terminals
• Digital cameras and camcorders
• Medical equipment
• Security systems
• Data loggers
• Automotive electronics
• Smart meters and utility monitoring
• Industrial automation systems
• Consumer electronics (VCRs, DVRs, etc.)

Electrical Characteristics

Absolute Maximum Ratings

• VCC to GND: -0.5V to +6.0V
• Input Voltage to GND: -0.5V to VCC + 0.5V
• Operating Temperature: -40°C to +85°C
• Storage Temperature: -65°C to +150°C

DC Electrical Characteristics (VCC = 4.5V to 5.5V, TA = -40°C to +85°C)

• Supply Current (Active Mode): 500µA max at 5V
• Supply Current (Battery Backup Mode): 300nA typical at 3V
• Input Low Voltage: 0.3VCC max
• Input High Voltage: 0.7VCC min
• Output Low Voltage (IOL = 3mA): 0.4V max

AC Electrical Characteristics

• I²C Bus Frequency: 100kHz standard, 400kHz fast mode
• Clock Input Frequency (Square Wave Output): 32.768kHz
• Start Condition Hold Time: 4.0µs min
• Stop Condition Setup Time: 4.0µs min

Functional Description

The DS1307 operates as a slave device on the I²C bus, with a fixed device address of 1101000 (0xD0 for write, 0xD1 for read). The time and calendar information is stored in registers that can be read or written by the master device. The clock operates in either 24-hour or 12-hour format with AM/PM indicator, and all date fields are stored in BCD format.

The device includes a power-sense circuit that detects power failures and automatically switches to the backup supply. When operating from the backup supply, the DS1307 maintains timekeeping operation while minimizing power consumption.

Register Map

The DS1307 contains the following register set:

• 00h: Seconds (bit 7 = CH clock halt)
• 01h: Minutes
• 02h: Hours (bit 6 = 12/24 hour mode)
• 03h: Day of week (01-07)
• 04h: Date (01-31)
• 05h: Month (01-12)
• 06h: Year (00-99)
• 07h: Control (SQW output control)
• 08h-3Fh: 56-byte general-purpose RAM

Package Information

The DS1307 is available in an 8-pin 300-mil DIP or SOIC package with the following pin configuration:

• Pin 1 (X1): 32.768kHz Crystal Connection
• Pin 2 (X2): 32.768kHz Crystal Connection
• Pin 3 (VBAT): +3V Battery Input
• Pin 4 (GND): Ground
• Pin 5 (SDA): Serial Data I/O
• Pin 6 (SCL): Serial Clock Input
• Pin 7 (SQW/OUT): Square Wave/Output Driver
• Pin 8 (VCC): +5V Power Supply

Design Considerations

When implementing the DS1307 in a design, several factors should be considered:

• A 32.768kHz crystal with 12.5pF load capacitance is recommended for timekeeping
• Backup battery should be a 3V lithium cell or equivalent
• Pull-up resistors (typically 4.7kΩ) are required on SDA and SCL lines
• PCB layout should minimize stray capacitance on the crystal connections
• Power supply decoupling (0.1µF capacitor) should be placed close to VCC pin

Conclusion

The DS1307 provides a complete, low-power solution for adding accurate timekeeping functionality to electronic systems. Its simple 2-wire interface, integrated nonvolatile RAM, and battery backup capability make it one of the most popular RTC solutions for embedded designs. With its industrial temperature range and reliable performance, the DS1307 is suitable for a wide range of applications from consumer products to industrial equipment.