STMicroelectronics M41ST85WMH6E

Overview

M41ST85WMH6E 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 communication, delivers 500 nA max battery current, and provides programmable watchdog (62.5 ms–128 s), power-fail detection (VPFD = 2.60 V), and automatic external LPSRAM write-protection during switchover (VSO = 2.50 V). It is used in industrial control panels requiring long-term timekeeping and system-level power monitoring.

For engineers reviewing the M41ST85WMH6E datasheet, M41ST85WMH6E pinout, M41ST85WMH6E application, or M41ST85WMH6E equivalent, key selection considerations include its integrated crystal option (SOX28), dual reset inputs (RSTIN1/RSTIN2), open-drain IRQ/FT/OUT and RST outputs, battery-backed 44-byte NVRAM, and calibrated RTC with century bit and tenths/hundredths-of-second resolution.

Technical Context

The device implements a BCD-formatted RTC with automatic leap-year correction through year 2100, using an internal 32.768 kHz crystal oscillator with built-in 12.5 pF load capacitance for stable operation across –40°C to +85°C. Its alarm subsystem supports repeat modes (daily, monthly, weekday) via dedicated registers and generates interrupts on IRQ/FT/OUT.

Microprocessor supervision includes a programmable watchdog timer steerable to the open-drain RST output, early power-fail warning (PFI/PFO) referenced to a 1.25 V precision bandgap, and dual independent reset inputs (RSTIN1, RSTIN2) with debouncing. The NVRAM supervisor autonomously deselects external LPSRAM via ECON when VCC drops below VSO (2.50 V), preventing corruption during brownout.

Key Specifications

Pinout & Package

Package: 28-lead plastic SOIC, 300 mil width, embedded crystal (SOX28). Designed for low-profile applications where crystal replacement is not required and surface-mount reflow compatibility is critical.

Key Features

Applications

Equivalent & Alternatives

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

Compared with DS3231M and PCF2129AT, M41ST85WMH6E uniquely combines RTC, watchdog, dual-reset inputs, and NVRAM supervisor in one SOX28 package - making it optimal for space-constrained industrial controllers where board area and BOM count must be minimized.

Availability

M41ST85WMH6E is available at Aetrix Electronics and suitable for industrial HMI panels, smart energy meters, and medical infusion pumps requiring stable component supply across extended product lifecycles.

Supply support for M41ST85WMH6E 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, specializing in automotive, industrial, and power management ICs with broad analog and mixed-signal portfolio coverage.

M41ST85WMH6E belongs to ST's TIMEKEEPER® family - designed specifically for embedded systems demanding integrated timekeeping, power supervision, and non-volatile memory control in a single chip.

FAQ

What is the function of the ECON pin on M41ST85WMH6E?

ECON is a conditioned chip-enable output that automatically deactivates external low-power SRAM during battery switchover. When VCC drops below VSO (2.50 V), ECON goes high-impedance or logic-low (depending on configuration), disabling the SRAM's chip-select path to prevent write corruption. This function is hardware-controlled and requires no firmware intervention.

Does M41ST85WMH6E require external load capacitors for the crystal?

No. The M41ST85WMH6E in SOX28 package integrates 12.5 pF load capacitance directly into the oscillator circuit. This eliminates the need for external ceramic capacitors, simplifies PCB layout, improves start-up reliability across temperature, and reduces bill-of-materials count by two components.

How does the HT (halt-timestamp) bit improve system diagnostics?

The HT bit is set automatically during power-down events and retained in battery-backed memory. Firmware reads this flag at boot to determine whether the last shutdown was intentional (e.g., controlled reset) or due to power loss. This enables accurate outage logging, predictive maintenance alerts, and validation of power-integrity design margins.

Can the SQW pin generate a 1 Hz output synchronized to the RTC second boundary?

Yes. The SQW pin supports 1 Hz output with edge-aligned synchronization to the RTC's second rollover. This is achieved by configuring the square-wave register (address 0x14) with appropriate control bits and ensures precise timing for wake-up interrupts or LED blink cycles without software overhead.