STMicroelectronics M41ST85WMX6

Overview

M41ST85WMX6 from STMicroelectronics is a 3.0/3.3 V I²C combination real-time clock (RTC), microprocessor supervisor, and NVRAM supervisor in a 28-lead SOIC package with embedded crystal (SOX28). It integrates a 32.768 kHz oscillator with 12.5 pF load capacitance, supports 400 kHz I²C interface, delivers 500 nA max battery current, and provides programmable watchdog (62.5 ms–128 s), power-fail warning (1.25 V reference), and alarm repeat modes - used in industrial control panels requiring long-term timekeeping and brownout resilience.

For engineers reviewing the M41ST85WMX6 datasheet, M41ST85WMX6 pinout, M41ST85WMX6 application, or M41ST85WMX6 equivalent, key selection criteria include its integrated crystal oscillator stability, dual reset inputs (RSTIN1/RSTIN2), battery-backed 44-byte NVRAM, and automatic switchover at VSO = 2.50 V with VPFD = 2.60 V for external LPSRAM write protection.

Technical Context

The M41ST85WMX6 implements a BCD-encoded RTC with century bit support and automatic leap-year correction through year 2100. Its oscillator operates from 2.5 to 5.5 V and integrates fixed 12.5 pF load capacitance, eliminating external capacitors and enabling high crystal series resistance tolerance.

The microprocessor supervisor includes an open-drain reset output (RST), two debounced reset inputs (RSTIN1/RSTIN2), and a steerable watchdog that can drive RST directly. The NVRAM supervisor uses ECON (conditioned chip-enable) and EX (external chip-enable) signals to automatically deselect and write-protect external low-power SRAM during power fail.

Key Specifications

Pinout & Package

Package: SOX28 - 28-lead plastic small outline, 300 mil, with embedded 32.768 kHz crystal. No separate battery top required; user-supplied backup capacitor or coin cell needed for non-volatile operation.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar RTC + supervisor applications.

Compared with DS3231SN#T&R and PCF8563T, the M41ST85WMX6 uniquely integrates RTC, dual-reset supervisor, and NVRAM protection in one SOX28 package - reducing design risk, board space, and qualification effort for industrial and safety-critical applications requiring coordinated power management.

Availability

M41ST85WMX6 is available at Aetrix Electronics and suitable for industrial HMIs, medical diagnostics, smart energy meters, and railway signaling controllers requiring stable component supply, long-term lifecycle support, and RoHS-compliant manufacturing.

Supply support for M41ST85WMX6 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

STMicroelectronics is a global semiconductor leader designing and manufacturing microcontrollers, analog ICs, and power management solutions for industrial, automotive, and consumer markets.

The M41ST85WMX6 belongs to ST's TIMEKEEPER® family - engineered specifically for robust, battery-backed timekeeping and system supervision in harsh environments where power reliability and data integrity are critical.

FAQ

What is the function of the ECON and EX pins on the M41ST85WMX6?

ECON (Conditioned Chip Enable) controls internal 44-byte NVRAM access, while EX (External Chip Enable) drives the chip-select line of an external low-power SRAM. Both are automatically deasserted when VCC falls below VSO = 2.50 V, ensuring write-protection during power failure without host intervention.

Does the M41ST85WMX6 require external load capacitors for its integrated crystal?

No. The M41ST85WMX6's SOX28 package embeds a 32.768 kHz crystal with 12.5 pF integrated load capacitance. This eliminates external capacitors, reduces layout sensitivity, and improves oscillator stability across temperature and aging - confirmed in ST's DocID7531 Rev 12, Section 3.1.

How does the power-down timestamp (HT bit) operate during a power failure?

The HT (Halt Time) bit is set automatically when VCC drops below VSO, capturing the exact RTC value at power-loss onset. It remains latched until cleared by software, enabling precise forensic logging of outage start time - detailed in Section 3.1 and Table 6 of the official datasheet.

Can the M41ST85WMX6's watchdog timer be configured to reset the system without CPU involvement?

Yes. The watchdog timeout can be steered directly to the open-drain RST output via internal configuration, allowing autonomous system reset on firmware hang. This bypasses IRQ/FT/OUT and requires no host polling - verified in Figure 4 (Block Diagram) and Section 3.5 of the datasheet.