DS1308U-3+

Real Time Clock Overview

The DS1308U-3+ is a low-power serial real-time clock (RTC) and 56-byte NV SRAM in an 8-pin uSOP package. It features an I2C interface, automatic power-fail detection with backup switching, and leap-year compensation valid to 2400. Operating voltage ranges from 2.7V to 5.5V, with battery support from 1.3V to 5.5V. Timekeeping current is typically 250nA at VBAT=3V. The device supports 12/24-hour formats, programmable square-wave outputs, and external clock synchronization.

Real Time Clock Feature Summary

• I2C Serial Interface: Slave address D0h; supports standard and fast-mode up to 400kHz.
• Automatic Power-Fail Switching: Detects VCC drop below trip point (VPF ~2.55V for -3 variant) and switches to VBAT.
• Low-Power Operation: Typical standby current of 250nA at VBAT=3V; data retention current 100nA max.
• Leap-Year Compensation: Automatically adjusts for months with fewer than 31 days through year 2400.

Real Time Clock Applications

• Handheld devices (GPS, POS terminals)
• Consumer electronics (Set-top boxes, digital recorders)
• Office equipment (Fax machines, printers)
• Medical devices (Glucometers, medicine dispensers)
• Telecommunications infrastructure (Routers, servers)

Real Time Clock Procurement Notes

Verify the ordering suffix 'U' indicates the uSOP-8 package and '+T&R' denotes tape-and-reel packaging. Confirm the '-3' suffix specifies the 2.7V–5.5V operating voltage range with a typical power-fail trip point of 2.55V. Ensure compatibility with I2C bus pull-up voltages up to 5.5V regardless of VCC level.

Real Time Clock Selection Notes

Select the DS1308U-3+ when a wide supply voltage range (2.7V–5.5V) and low standby current are required. The device supports external clock input via SQW/CLKIN for synchronization with 50Hz/60Hz powerlines or GPS signals. Verify crystal load capacitance requirements (CL=6pF) and ESR limits (up to 100kΩ) for oscillator stability.

Real Time Clock Part Context

The DS1308U-3+ belongs to the Maxim Integrated DS1308 family of low-current I2C RTCs. It shares functional similarities with the DS1338 but offers broader voltage tolerance and specific power-fail characteristics. The 'U' suffix distinguishes the compact uSOP-8 package from other form factors like SOIC or DIP.

Real Time Clock Replacement Considerations

Publicly confirmed substitutes include the DS1338U-33+T&R and DS1339U-3+. Note that the DS1338 operates at 3.0V–5.5V with a higher VPF (~2.82V), while the DS1339 lacks NV RAM. Verify pinout compatibility and I2C address alignment before substitution.

Real Time Clock FAQ

What is the default state of the oscillator upon first power application?

Upon initial power-up, the clock halt (CH) bit in the seconds register is set to 1, disabling the oscillator. The time/date registers reset to 01/01/00 01 00:00:00.

How does the device handle external clock synchronization?

The SQW/CLKIN pin accepts external references (e.g., 50Hz/60Hz, GPS 1PPS). The internal 1Hz signal is digitally adjusted to lock onto the external reference if within ±0.8% tolerance.

What is the function of the Freshness Seal Mode?

Freshness Seal Mode prevents battery consumption during storage/shipping. Battery backup power is only enabled after VCC exceeds VPF for the first time post-manufacturing.