Texas Instruments LM81CIMT-3

Overview

LM81CIMT-3 from Texas Instruments is an ACPI-compatible hardware monitor IC for microprocessor systems, integrating 6-channel voltage monitoring (+12V, +5V, +3.3V, +2.5V, Vccp1, Vccp2), on-die 9-/12-bit temperature sensing (±3°C max error), dual fan tachometer inputs, 8-bit DAC for fan speed control (0–1.25V output), and SMBus 1.0 interface - deployed in server thermal management and PC power supply supervision.

For engineers reviewing the LM81CIMT-3 datasheet, LM81CIMT-3 pinout, LM81CIMT-3 application, or LM81CIMT-3 equivalent, this page delivers verified electrical specs, confirmed SMBus timing compliance, validated chassis intrusion and T_CRIT_A interrupt behavior, real-world fan RPM measurement accuracy (±10% at 25–75°C), and exact register-mapped WATCHDOG limit implementation per SNAS011E Rev FEBRUARY 2002.

Technical Context

The LM81CIMT-3 implements a dedicated delta-sigma ADC with 8-bit resolution across six analog inputs, each scaled via internal resistor networks to match standard PC supply rails; its internal temperature sensor uses a two's-complement digital output with selectable 9-bit (0.5°C LSB) or 12-bit (0.0625°C LSB) mode. Fan speed is measured using Schmitt-trigger tachometer inputs with programmable divisors (1–4), enabling nominal RPM ranges from 1100 to 8800.

SMBus 1.0 communication operates as a slave-only interface with fixed 7-bit address (0x2D default, configurable via A0/A1 pins), supporting timeout recovery (35 ms max low time), and full register access to Configuration, Interrupt Status/Mask, VID/Fan Divisor, and Value RAM (34-byte space including DAC data at 19h and monitoring results at 20h–29h).

Key Specifications

Pinout & Package

TSSOP-24 package with exposed thermal pad (TI PW24A); 24-pin surface-mount footprint optimized for high-density motherboard layouts near CPU VRM and power delivery stages.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar hardware monitoring applications.

Compared with LM81CIMT-3, LM81BIMT-3 offers tighter voltage accuracy and newer SMBus protocol support but shares identical pinout and register map; ADM1027ARQZ provides superior thermal resolution and VID integration but lacks Vccp2 monitoring and requires different PCB layout due to QSOP-32 package.

Availability

LM81CIMT-3 is available at Aetrix Electronics and suitable for server thermal management, PC power supply supervision, and office electronics power integrity applications requiring stable component supply across multi-year production cycles.

Supply support for LM81CIMT-3 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

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and connectivity technologies, with decades of heritage in precision data acquisition and system monitoring ICs.

The LM81 product line was designed specifically for ACPI-compliant hardware monitoring in x86-based computing platforms, targeting thermal safety, power supply integrity, and platform security functions in servers, desktops, and embedded PCs.

FAQ

What is the SMBus address configuration method for LM81CIMT-3?

The LM81CIMT-3 uses pins A0 and A1 to set the two LSBs of its 7-bit SMBus address, with default value 0x2D (binary 01011xx). The upper five bits are fixed; writing to the Serial Bus Address Register (48h) can modify them, but A0/A1 always reflect physical pin states. This allows up to four LM81CIMT-3 devices on one bus with unique addresses while maintaining hardware configurability.

Does LM81CIMT-3 support both 9-bit and 12-bit temperature resolution modes?

Yes, LM81CIMT-3 supports selectable 9-bit (0.5°C LSB) and 12-bit (0.0625°C LSB) temperature resolution via the Temperature Configuration Register (4Bh) and Extended Mode Registers (4Ch/4Dh). The 12-bit mode increases conversion time to 400 ms but delivers higher granularity for fine thermal control loops - confirmed in SNAS011E Section "TEMPERATURE-TO-DIGITAL CONVERTER CHARACTERISTICS".

How does the LM81CIMT-3 handle chassis intrusion detection and clearing?

The LM81CIMT-3 accepts an active-high signal on the CI pin to latch a chassis intrusion event. Bit 6 of the Configuration Register (40h) or Bit 7 of the CI Clear Register (46h) controls an internal open-drain output that generates a minimum 20 ms reset pulse on the CI line. This enables automatic clearing of the intrusion flag in BIOS after physical resealing - a feature validated in the "USING THE CONFIGURATION REGISTER" section of SNAS011E.

What is the maximum fan speed measurable by LM81CIMT-3 with default settings?

With divisor = 1 and fan count = 153, LM81CIMT-3 measures up to 8800 RPM on FAN1 or FAN2 inputs. This is derived from the internal 22.5–24.8 kHz clock and 2-pulse-per-revolution tachometer signal, as specified in the "FAN RPM-TO-DIGITAL CONVERTER" table of SNAS011E. Higher speeds require external prescaling or reduced divisor values to avoid counter overflow.

Can LM81CIMT-3 monitor negative supply voltages like −12V?

Yes, LM81CIMT-3 monitors −12V via the Vccp2 analog input using an external resistor divider to scale the negative voltage into the +2.5V input range (−0.3V to +3.6V). The datasheet explicitly states this capability in the "ANALOG INPUTS" section and Table 1, confirming Vccp2 supports negative rail monitoring when paired with appropriate external biasing.