DS1308 Serial Real-Time Clock (RTC) Technical Summary

Overview

The DS1308 is a low-power serial real-time clock (RTC) that maintains accurate timekeeping while consuming minimal power. This component provides full binary-coded decimal (BCD) clock/calendar functionality with additional non-volatile SRAM for data storage. Communication with the device is accomplished through a simple 2-wire serial interface, making it easy to integrate into various microcontroller-based systems.

Designed for reliability and precision, the DS1308 operates over a wide voltage range and includes automatic power-fail detection and switch circuitry. When primary power fails, the device seamlessly switches to battery backup while consuming less than 500nA of current, ensuring continuous timekeeping without data loss.

Key Features

• Real-time clock counts seconds, minutes, hours, day of week, date, month, and year with automatic leap year compensation
• 56 bytes of general-purpose non-volatile SRAM for data storage
• Simple 2-wire serial interface (I²C-compatible)
• Ultra-low battery backup current (typically less than 500nA)
• Automatic power-fail detect and switch circuitry
• Programmable square wave output signal
• Operating voltage range: 2.0V to 5.5V
• Industrial temperature range: -40°C to +85°C
• Available in 8-pin DIP and SOIC packages

Applications

The DS1308 is ideal for a wide range of timekeeping applications including:

• Embedded systems requiring accurate time/date stamps
• Data logging equipment
• Point-of-sale terminals
• Medical devices
• Industrial automation systems
• Building automation controllers
• Consumer electronics
• Battery-powered devices
• Security systems

Electrical Characteristics

Absolute Maximum Ratings

• Voltage on VCC relative to GND: -0.5V to +6.0V
• Storage Temperature Range: -65°C to +150°C
• Soldering Temperature: 260°C for 10 seconds

Recommended Operating Conditions

• Operating Voltage Range (VCC): 2.0V to 5.5V
• Operating Temperature Range: -40°C to +85°C
• Battery Voltage (VBAT): 1.8V to 5.5V

DC Electrical Characteristics

• Active Supply Current (VCC = 5.0V): 300µA max at 25°C
• Battery Backup Current (VBAT = 3.0V): 500nA max at 25°C
• Input Low Voltage: -0.5V to 0.3 × VCC
• Input High Voltage: 0.7 × VCC to VCC + 0.5V

AC Electrical Characteristics

• Serial Clock Frequency: 100kHz max (standard mode)
• Serial Clock Frequency: 400kHz max (fast mode)
• Start Condition Hold Time: 4.0µs min
• Stop Condition Setup Time: 4.0µs min

Functional Description

Timekeeping Registers

The DS1308 contains seven registers for timekeeping data in BCD format. These registers store seconds, minutes, hours, day of week, date, month, and year information. The clock operates in either 24-hour or 12-hour format with AM/PM indicator, configurable through the hours register.

Control Register

The control register is used to enable or disable the square wave output and select its frequency. Available output frequencies include 1Hz, 4.096kHz, 8.192kHz, and 32.768kHz.

Non-Volatile SRAM

The 56 bytes of general-purpose RAM are non-volatile when powered from the backup battery. This storage area can be used for system parameters, calibration data, or other information that must be preserved during power loss.

Package Information

8-Pin DIP (Dual In-line Package)

• Package Dimensions: 9.81mm × 6.35mm × 3.3mm
• Lead Pitch: 2.54mm
• Lead Count: 8
• Material: Molded Plastic

8-Pin SOIC (Small Outline Integrated Circuit)

• Package Dimensions: 5.00mm × 4.00mm × 1.50mm
• Lead Pitch: 1.27mm
• Lead Count: 8
• Material: Molded Plastic

Pin Configuration

• Pin 1 (X1): Connection for 32.768kHz crystal
• Pin 2 (X2): Connection for 32.768kHz crystal
• Pin 3 (VBAT): Battery input for backup power
• Pin 4 (GND): Ground
• Pin 5 (SDA): Serial Data Line (open drain)
• Pin 6 (SCL): Serial Clock Input
• Pin 7 (SQW/OUT): Square Wave/Output Driver
• Pin 8 (VCC): Primary Power Supply

Design Considerations

Crystal Selection

The DS1308 requires an external 32.768kHz quartz crystal for timekeeping. For optimal performance, use a crystal with a specified load capacitance of 12.5pF and an equivalent series resistance (ESR) of 35kΩ maximum.

Power Supply Decoupling

A 0.1µF ceramic capacitor should be placed as close as possible between VCC and GND pins to filter power supply noise. When using battery backup, a 1µF capacitor is recommended between VBAT and GND.

PCB Layout Guidelines

• Keep crystal traces as short as possible
• Minimize parallel trace runs to reduce capacitive coupling
• Provide a solid ground plane under the device
• Route I²C signals away from high-frequency or high-current traces

Conclusion

The DS1308 provides a robust, low-power solution for accurate timekeeping in embedded systems. With its simple 2-wire interface, non-volatile storage, and wide operating voltage range, it offers designers a reliable timing component suitable for a variety of applications. The device's automatic power-fail protection ensures continuous operation even during main power interruptions, making it particularly valuable for battery-backed systems.