STMicroelectronics M41ST85WMX6TR

Overview

M41ST85WMX6TR from STMicroelectronics is a 28-pin SOIC real-time clock (RTC) IC integrating I²C serial RTC, microprocessor supervisor, and NVRAM supervisor functions in a single low-power CMOS device. It delivers BCD-formatted timekeeping (seconds to century), programmable alarm, watchdog timer, 1 Hz–32 kHz squarewave output, and automatic battery switchover at VSO = 2.50 V (nom) for external LPSRAM write-protection during power failure.

For engineers reviewing the M41ST85WMX6TR datasheet, M41ST85WMX6TR pinout, M41ST85WMX6TR application, or M41ST85WMX6TR equivalent, this part supports time-critical embedded systems requiring guaranteed data retention, precise calendar operation across temperature, and dual reset inputs with early power-fail warning (PFI/PFO) at 1.25 V reference.

Technical Context

The M41ST85WMX6TR implements a crystal-controlled 32.768 kHz oscillator with integrated 12.5 pF load capacitance, enabling stable timekeeping up to +85°C without external components. Its I²C interface operates at 400 kHz with automatic address incrementing and open-drain SDA/SCL signaling compliant with standard two-wire bus timing.

Supervision logic includes dual reset inputs (RSTIN1/RSTIN2), programmable watchdog timeout (62.5 ms–128 s), and a dedicated power-fail comparator (VPFD = 2.60 V nom) that triggers PFO assertion and initiates battery switchover before system brownout. The NVRAM supervisor asserts ECON to gate external SRAM chip-enable during backup mode.

Key Specifications

Pinout & Package

Package: 28-lead SOIC (SOX28), 300 mil width, with embedded 32.768 kHz crystal. Pin-compatible with SNAPHAT® SOH28 variant but uses internal crystal instead of external top-mount battery/crystal module.

Key Features

Applications

Equivalent & Alternatives

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

Compared with DS1339U+T&R and PCF8563T/1,118, the M41ST85WMX6TR uniquely integrates RTC, supervisor, and NVRAM control in one SOIC package-reducing BOM count, eliminating timing skew between functions, and guaranteeing synchronized switchover behavior under brownout conditions.

Availability

M41ST85WMX6TR is available at Aetrix Electronics and suitable for industrial PLC time stamping, medical device power monitoring, smart meter calendar management, and POS terminal transaction logging requiring stable component supply across extended product lifecycles.

Supply support for M41ST85WMX6TR 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 headquartered in Geneva, Switzerland, designing and manufacturing analog, MCU, power, and sensing solutions for industrial, automotive, and consumer markets.

The M41ST85W belongs to ST's TIMEKEEPER® family-engineered specifically for high-reliability embedded systems demanding guaranteed timekeeping, deterministic power supervision, and non-volatile memory protection during voltage transients.

FAQ

What is the function of the ECON pin?

ECON (Conditioned Chip Enable Output) is an active-low, open-drain output that automatically disables external low-power SRAM during battery backup mode. When VCC drops below VSO = 2.50 V, ECON asserts to gate the SRAM's chip-enable line, preventing writes and ensuring data integrity. It remains asserted until VCC rises above VPFD (2.60 V) plus trec delay, providing hysteresis against supply noise.

Does the M41ST85WMX6TR require external load capacitors for the crystal?

No. The M41ST85WMX6TR integrates a 32.768 kHz crystal oscillator with built-in 12.5 pF load capacitance, eliminating the need for external tuning capacitors. This is confirmed in the datasheet Section 1 (Description) and Figure 4 (Block diagram), where the crystal is shown as embedded within the SOX28 package and internally compensated for optimal stability across –40°C to +85°C.

How does the power-fail warning (PFI/PFO) differ from the reset function?

PFI is an input that monitors VCC against a 1.25 V precision reference; when VCC falls below VPFD = 2.60 V, PFO asserts as an early warning signal-typically 10–100 ms before reset activation. The RST output, however, is triggered only after VCC drops further or during power-on, providing a hard reset. PFO enables proactive system response (e.g., data save), while RST enforces hardware-level recovery.

Can the M41ST85WMX6TR operate without a backup battery?

Yes, but with functional limitations: RTC and NVRAM lose state during power loss; supervisor functions (watchdog, PFI/PFO, reset) remain active as long as VCC is present. Battery is required only for timekeeping continuity and external SRAM write-protection during outage. The device draws 500 nA from VBAT in backup mode, supporting >10-year coin-cell life per ST application note AN1012.