STMicroelectronics M41T81SM6F

Overview

M41T81SM6F from STMicroelectronics is a serial real-time clock (RTC) IC with integrated 32.768 kHz crystal oscillator, I²C interface (400 kHz), ultra-low battery current (0.6 µA typ), and full calendar functionality (century/year/month/date/hour/minute/second/tenths-hundredths) in BCD format. It supports alarm interrupts, programmable watchdog timer (62.5 ms–128 s), software clock calibration, and automatic VCC-to-VBAT switchover for nonvolatile timekeeping in industrial control panels.

For engineers reviewing the M41T81SM6F datasheet, M41T81SM6F pinout, M41T81SM6F application, or M41T81SM6F equivalent, key selection criteria include I²C timing compliance, battery backup current budget, oscillator stability under temperature variation (–40 to 85 °C), and register-level alarm repeat mode configuration (daily/monthly/weekday).

Technical Context

The M41T81SM6F operates as an I²C slave device with fixed 7-bit address 0xD0h, supporting sequential READ/WRITE access across 20 registers-including 8 clock/calendar registers, 5 alarm registers, and dedicated calibration/watchdog/square-wave control registers. Its internal oscillator uses integrated 12.5 pF load capacitance for stable 32.768 kHz operation even with high-series-resistance crystals.

Power management includes dual-voltage detection: VPFD (2.5–2.7 V) triggers alarm flag and disables I²C input recognition, while VSO (2.7 V min) initiates automatic switchover to VBAT. The HT bit in power-down timestamp enables precise elapsed-time calculation during battery-only operation.

Key Specifications

Pinout & Package

Package: 8-pin SOIC (SO8), 150-mil body width, RoHS-compliant plastic package per ST Doc ID 10773 Rev 7, Table 14.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar real-time clock applications.

Compared with DS3231M and PCF8563, the M41T81SM6F uniquely balances low backup current, integrated oscillator, alarm flexibility, and diagnostic features (HT bit, oscillator fail flag)-making it optimal for industrial controllers requiring field-replaceable battery service and deterministic timekeeping over 10+ years.

Availability

M41T81SM6F is available at Aetrix Electronics and suitable for industrial energy metering, building automation controllers, medical infusion pumps, and POS terminal audit logging requiring stable component supply, long-lifecycle support, and RoHS-compliant packaging.

Supply support for M41T81SM6F 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 microcontrollers, analog ICs, power management devices, and timing solutions for industrial, automotive, and consumer markets.

The M41T81SM6F belongs to ST's serial RTC product line, engineered specifically for industrial and medical applications demanding high reliability, extended temperature operation, and integrated power-fail resilience without external timing components.

FAQ

What is the function of the HT bit in the M41T81SM6F?

The HT (Hour Tens) bit is a power-down timestamp flag located in the flags register. When VCC drops below VSO and the device switches to VBAT, the HT bit latches the hour value at the moment of switchover. This allows firmware to compute exact elapsed time during battery-only operation by comparing the latched hour with the current hour upon VCC restoration-enabling precise uptime/downtime accounting for industrial logging.

Does the M41T81SM6F require external load capacitors for the 32.768 kHz crystal?

No. The M41T81SM6F integrates 12.5 pF load capacitance internally on both XI and XO pins, eliminating the need for external capacitors. This simplifies PCB layout, reduces component count, and improves oscillator stability-especially with high-series-resistance crystals common in industrial environments. External caps would degrade performance and are explicitly discouraged in ST's design guidelines.

How does the M41T81SM6F handle leap years and century rollover?

The M41T81SM6F implements automatic leap-year correction for February 29 in years divisible by 4, valid through year 2100. Century rollover is managed via two dedicated bits: CEB (Century Enable Bit) and CB (Century Bit). Setting CEB=1 causes CB to toggle at year 2099→2100, ensuring correct BCD year representation (e.g., "00" for 2100 instead of "00" for 2000) without host MCU intervention.

Can the IRQ/FT/OUT/SQW pin output both alarm interrupt and 1 Hz square wave simultaneously?

No. The IRQ/FT/OUT/SQW pin is a single open-drain output with prioritized functions: square wave (SQW) has highest priority, followed by frequency test (FT), output driver (OUT), and alarm interrupt (IRQ). When SQW is enabled (via square wave register), it overrides all other functions. To use alarm interrupts, SQW must be disabled first-requiring explicit register configuration before enabling alarm mode.