Analog Devices Inc./Maxim Integrated DS1321

Overview

DS1321 from Maxim Integrated is a flexible nonvolatile SRAM controller with integrated lithium battery monitoring and power-fail detection. It converts up to four CMOS SRAMs into nonvolatile memory, provides automatic battery switchover (VBAT to VCCO), unconditionally write-protects SRAM on VCCI out-of-tolerance, and delivers open-drain RST and BW signals. Used in industrial data loggers requiring long-term RAM retention during mains loss.

For engineers reviewing the DS1321 datasheet, DS1321 pinout, DS1321 application, or DS1321 equivalent, this page delivers verified functional roles, real-world timing behavior (tREC = 125 ms, tRPU = 200 ms), battery test interval (24 hr), and precise trip points (VCCTP = 4.62 V typ, VBTP = 2.6 V typ) - all confirmed from Maxim's official DS1321 datasheet Rev 6/12.

Technical Context

The DS1321 implements dual-voltage monitoring: a precision comparator detects VCCI drop below VCCTP (4.5–4.75 V or 4.25–4.5 V depending on TOL pin state) to trigger SRAM write-protection and RST assertion; simultaneously, it performs periodic loaded-battery voltage measurement using a 1 MΩ internal test load and factory-trimmed 2.6 V reference to detect end-of-life lithium cells.

Its architecture supports three memory configurations via MODE pin latching at power-up: 4 banks × 1 SRAM, 2 banks × 2 SRAMs, or 1 bank × 4 SRAMs. CEO outputs are actively held low for ≤1.5 µs after CEI deactivation during power failure to ensure in-progress memory accesses complete before write-protection engages.

Key Specifications

Pinout & Package

DS1321 is available in three packages: 16-pin PDIP (300 mil), 16-pin SO (150 mil), and 20-pin TSSOP. Pin functions are consistent across variants except for NC placements; all share identical electrical interface and memory control logic.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar nonvolatile SRAM controller applications.

Compared with DS1321, DS1230Y offers simpler implementation but no battery health intelligence, while MAX6900 adds RTC capability at the cost of configurable memory mapping and advanced lithium monitoring - making DS1321 optimal for systems prioritizing long-term SRAM reliability and predictive battery maintenance.

Availability

DS1321 is available at Aetrix Electronics and suitable for industrial data loggers, medical patient monitors, programmable logic controllers, and smart energy meters requiring stable component supply and long-lifecycle support.

Supply support for DS1321 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Maxim Integrated (now part of Analog Devices) designs precision analog and mixed-signal ICs for industrial, automotive, and communications applications, with emphasis on power management, sensing, and reliability-critical functions.

The DS1321 belongs to Maxim's nonvolatile memory controller product line, engineered specifically to eliminate data loss in SRAM-based systems during power interruption while extending lithium backup battery service life through intelligent, low-power monitoring.

FAQ

What is the primary function of the DS1321 in an SRAM-based system?

The DS1321 serves as a nonvolatile SRAM controller that automatically switches memory power from VCCI to VBAT during power failure, unconditionally inhibits chip enables to prevent writes, and asserts RST for system reset. In the DS1321, this is achieved with sub-200 ns fail-detect latency and <0.2 V power-switch dropout - ensuring zero data corruption during brownouts.

How does the DS1321 perform lithium battery monitoring without draining the cell?

The DS1321 performs battery monitoring by applying a 1 MΩ internal resistive load to VBAT for exactly 1 second every 24 hours (tBTCN), then comparing the resulting voltage against a 2.6 V factory-trimmed reference. This intermittent method draws only 100 nA in standby, enabling >10-year battery life - a core design feature of the DS1321.

What happens to the CEO outputs during a power failure event in the DS1321?

During a DS1321 power failure, CEO1–CEO4 are actively driven high (within 0.2 V of VCCO) to disable SRAM writes. If a CEI input is low when failure occurs, the corresponding CEO remains low for up to 1.5 µs to allow ongoing memory access to complete - a timing-critical behavior documented in DS1321 AC specs as tREC enforcement.

Can the DS1321 support different SRAM memory configurations, and how is this controlled?

Yes, the DS1321 supports three SRAM topologies via the MODE pin: 4 banks × 1 SRAM (MODE = GND), 2 banks × 2 SRAMs (MODE = VCCO), or 1 bank × 4 SRAMs (MODE floating). The state is latched at VCCI = VCCTP during power-up - a hardware-configured feature unique to the DS1321 that eliminates firmware overhead.

What is the purpose of Freshness Seal Mode in the DS1321, and when is it active?

Freshness Seal Mode prevents VCCO output activation when VCCI is absent - allowing lithium battery installation during manufacturing without discharge. It remains active until VCCI first exceeds VCCTP, ensuring zero battery drain during storage and shipping. This mode is intrinsic to DS1321 operation and requires no external control.