DS1391U-33+

Real Time Clock Overview

The DS1391U-33+ is a low-voltage real-time clock (RTC) IC from Analog Devices/Maxim Integrated, housed in a 10-pin µSOP package. It provides timekeeping for hundredths of seconds through the year, with automatic leap-year compensation valid until 2100. The device communicates via an SPI-compatible bus supporting Modes 1 and 3, operating at up to 4MHz at 3.0V/3.3V. It features a programmable time-of-day alarm, power-fail detection with automatic switching to a backup supply, and trickle-charging capability for external batteries.

Real Time Clock Feature Summary

• SPI Interface: Supports Modes 1 and 3 with SCLK frequencies up to 4MHz at nominal voltages.
• Timekeeping: Counts hundredths of seconds, seconds, minutes, hours, day, date, month, and year in BCD format.
• Power Management: Includes power-fail detect circuitry that switches to VBACKUP when VCC drops below the threshold.
• Reset Functionality: Features an RST output/debounced input pin for system reset control and pushbutton debounce.

Real Time Clock Applications

• Hand-Held Devices
• GPS/Telematics Devices
• Embedded Time Stamping
• Medical Devices

Real Time Clock Procurement Notes

Verify the specific suffix 'U-33+' against the manufacturer's current production status, as availability may vary. Confirm the 10-pin µSOP package dimensions and lead specifications prior to finalizing PCB layouts. Ensure the design supports the required SPI Modes 1 and 3 timing constraints for reliable data transfer.

Real Time Clock Selection Notes

Select this component when an SPI interface is preferred over I2C or 3-wire alternatives within the DS139x family. The -33 variant is suitable for systems operating between 2.97V and 5.5V. Note that unlike the DS1390, this model uses an RST pin instead of a combined SQW/INT output, which affects interrupt handling strategies.

Real Time Clock Part Context

This part belongs to the DS1390–DS1394 series of low-voltage RTCs. While sharing core timekeeping and power-management features, the DS1391 is distinct due to its dedicated RST pin configuration and specific SPI mode support (Modes 1 and 3), differentiating it from the SQW/INT-based DS1390 and the 3-wire variants.

Real Time Clock Replacement Considerations

Public confirmed substitute part numbers are not available in the provided documentation. Designers should verify compatibility with newer Analog Devices RTC families if the DS1391U-33+ becomes obsolete, ensuring interface and pinout requirements are met.

Real Time Clock FAQ

What SPI modes does the DS1391 support?

The DS1391 supports SPI Modes 1 and 3, as specified in the datasheet features section.

How does the device handle power failures?

A temperature-compensated voltage reference monitors VCC. If VCC falls below the power-fail voltage (VPF), the device automatically switches to the VBACKUP supply and blocks access to internal registers.

What is the function of the RST pin on the DS1391?

The RST pin serves as an active-low open-drain output indicating VCC status relative to VPF. It also functions as a debounced input for pushbutton resets.