Analog Devices Inc./Maxim Integrated DS1321S

Overview

DS1321S 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 ≥ 2.0V), unconditionally write-protects SRAM on VCCI out-of-tolerance (4.50–4.75V trip), and delivers open-drain RST and BW outputs. Used in industrial data loggers requiring persistent memory during AC power loss.

For engineers reviewing the DS1321S datasheet, DS1321S pinout, DS1321S application, or DS1321S equivalent, key selection considerations include its 16-pin SO package, 4.5–5.5V VCCI operating range, 24-hour periodic battery voltage test, 200ms power-on reset pulse, and support for three SRAM bank configurations via MODE pin.

Technical Context

The DS1321S implements precision voltage monitoring using internal comparators for both VCCI (with selectable 5% or 10% tolerance via TOL pin) and VBAT (2.50–2.70V trip point). Its battery test circuit applies a 1MΩ load for 1 second every 24 hours to measure loaded voltage-enabling early end-of-life warning without excessive battery drain.

It integrates dual-function outputs: RST provides power-on reset (200ms nominal hold time) and power-fail reset (15µs response), while BW signals low-battery condition via open-drain output. The device supports three SRAM memory configurations (1×4, 2×2, or 4×1 banks) latched at power-up based on MODE pin state (GND/VCCO/floating).

Key Specifications

Pinout & Package

DS1321S is housed in a 16-pin small-outline (SO) package (150-mil width), RoHS-compliant, with 75°C/W junction-to-ambient thermal resistance.

Key Features

Applications

Equivalent & Alternatives

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

Compared with DS1230Y and MAX690T, the DS1321S uniquely integrates SRAM write protection, battery-backed power switchover, programmable memory configuration, and predictive lithium cell monitoring in a single 16-pin SO package-eliminating need for discrete supervisors, logic, and analog monitoring circuits.

Availability

DS1321S is available at Aetrix Electronics and suitable for industrial data loggers, medical device memory backup, point-of-sale terminals, and telecom base station controllers requiring stable component supply and long-term lifecycle support.

Supply support for DS1321S 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, mixed-signal, and power management ICs for industrial, medical, and communications applications.

The DS1321S belongs to Maxim's nonvolatile memory controller product line, engineered specifically to eliminate external circuitry needed to convert standard SRAMs into battery-backed nonvolatile memory with intelligent health monitoring.

FAQ

What is the primary function of the DS1321S in a memory subsystem?

The DS1321S serves as a flexible nonvolatile SRAM controller that automatically converts up to four CMOS SRAMs into nonvolatile memory. It monitors VCCI for power failure, switches to battery backup, inhibits chip enables to prevent writes, and asserts RST and BW outputs. The DS1321S handles all critical functions-including voltage threshold detection, SRAM power routing, and battery health monitoring-without external components.

How does the DS1321S determine which SRAM memory configuration to use?

The DS1321S latches the MODE pin state at the moment VCCI crosses the VCCTP threshold during power-up. If MODE = GND, it configures four independent banks (1 SRAM each); if MODE = VCCO, it configures two banks (2 SRAMs each); if MODE is floating, it configures one bank (4 SRAMs). This configuration is fixed for the power cycle and cannot be changed dynamically. The DS1321S uses this setting to route CEI inputs to the appropriate CEO outputs.

What happens to the DS1321S outputs during a power failure event?

During a power failure, the DS1321S immediately activates RST (open-drain, active-low) and forces CEO1–CEO4 high (within 15µs) to disable SRAM writes. VCCO remains powered by VBAT, preserving SRAM data. CEI inputs are sampled: if any CEI is low when failure occurs, the corresponding CEO stays low for up to 1.5µs to complete the ongoing access, then goes high. The DS1321S maintains these states until VCCI recovers above VCCTP and tREC (125ms) expires.

Can the DS1321S monitor battery health while the system is powered down?

No-the DS1321S performs battery voltage monitoring only when VCCI is nominal (≥ VCCTP). It applies a 1MΩ internal load for 1 second every 24 hours to measure loaded voltage-but this test is suspended during power failure or VCCI undervoltage. Systems using DS1321S battery monitoring must be powered up periodically (at least once every few months) to ensure timely detection of battery degradation; otherwise, the cell may deplete fully without warning.

What are the key electrical differences between the DS1321S and the DS1321E package variants?

The DS1321S (16-pin SO) and DS1321E (20-pin TSSOP) share identical electrical specifications, functionality, and timing parameters-including VCCI trip points, battery test intervals, RST pulse width, and CEO propagation delay. The only differences are mechanical: pin count (16 vs. 20), package type (SO vs. TSSOP), thermal resistance (75°C/W vs. 73.8°C/W θJA), and pin assignment (e.g., DS1321E has dedicated A/B pins, while DS1321S multiplexes A/CEI3 and B/CEI4). The DS1321S does not support the extra NC pins present in the TSSOP variant.