SBAS475A June   2009  – January 2023 AMC6821-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Timing Requirements
    7. 7.7 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Functional Block Diagram
    2. 8.2 Feature Description
      1. 8.2.1 ADC Converter
      2. 8.2.2 Temperature Sensor
        1. 8.2.2.1 Series Resistance Cancellation
        2. 8.2.2.2 Reading Temperature Data
        3. 8.2.2.3 Temperature Out-of-Range Detection
        4. 8.2.2.4 Remote Temperature Sensor Failure Detection
      3. 8.2.3 PWM Output
      4. 8.2.4 PWM Waveform Setting
      5. 8.2.5 Fan Speed Measurement
        1. 8.2.5.1 Tach-Data Register
          1. 8.2.5.1.1 Reading the Tach Data Register
          2. 8.2.5.1.2 RPM Measurement Rate
          3. 8.2.5.1.3 Select Number of Pulses/Revolution
          4. 8.2.5.1.4 Tach Mode Selection
          5. 8.2.5.1.5 Fan RPM Out-of-Range Detection
      6. 8.2.6 Fan Failure Detection
      7. 8.2.7 FAN-FAULT Pin
      8. 8.2.8 Fan Control
        1. 8.2.8.1 THERM Pin and External Hardware Control
          1. 8.2.8.1.1 THERM Pin as an Output
          2. 8.2.8.1.2 THERM Pin as an Input
        2. 8.2.8.2 Fan Spin-Up
        3. 8.2.8.3 Normal Fan Speed Control
          1. 8.2.8.3.1 Software DCY Control Mode
          2. 8.2.8.3.2 Software-RPM Control Mode (Fan Speed Regulator)
          3. 8.2.8.3.3 Auto Temperature Fan Mode
      9. 8.2.9 Interrupt
        1. 8.2.9.1 OVR Pin
        2. 8.2.9.2 SMBALERT Pin
        3. 8.2.9.3 SMBALERT Interrupt Behavior
        4. 8.2.9.4 Handling SMBALERT Interrupts
    3. 8.3 Device Functional Modes
    4. 8.4 Programming
      1. 8.4.1 SMBus Interface
        1. 8.4.1.1 Communication Protocols
      2. 8.4.2 SMBus Alert Response Address (ARA)
      3. 8.4.3 Power-On Reset and Start Operation
    5. 8.5 Register Map
      1. 8.5.1 Register Description
        1. 8.5.1.1 Device Configuration Registers
        2. 8.5.1.2 Device Status Registers
        3. 8.5.1.3 Fan Controller Registers
        4. 8.5.1.4 Temperature Data Registers
        5. 8.5.1.5 Temperature Limit Registers
          1. 8.5.1.5.1 Tach-Data Register
          2. 8.5.1.5.2 Tach Setting Register
          3. 8.5.1.5.3 Tach Low Limit Register
          4. 8.5.1.5.4 Tach High Limit Register
  9. 9 Application and Implementation
    1. 9.1 Power Supply Recommendations
  10. 10Device and Documentation Support
    1. 10.1 Receiving Notification of Documentation Updates
    2. 10.2 Support Resources
    3. 10.3 Trademarks
    4. 10.4 Electrostatic Discharge Caution
    5. 10.5 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.2.5 Fan Speed Measurement

The AMC6821-Q1 monitors the fan speed (RPM) via the TACH pin, as illustrated in Figure 8-7. The TACH-EN bit of Configuration Register 2 (bit 2, 0x01) enables the fan speed measurement. When TACH-EN is cleared ('0'), the measurement is disabled. The measurement is enabled when the TACH-EN bit is set to '1'. This section describes the device behavior when TACH-EN is set ('1').

The on-chip fan-speed counter does not count the fan tach output pulses directly because of the low RPM of the fan. Instead, the period of the fan revolution is measured by gating an on-chip clock (100kHz). The result is stored in the TACH-DATA Register that contains two bytes (16 bits total). RPM monitoring is disabled when the START bit of Configuration Register 1 or the TACH-EN bit of Configuration Register 2 is cleared ('0'), and is enabled when START = 1 and TACH-EN = 1.

If the TACH-MODE bit is cleared, RPM monitoring stops and the TACH-DATA register is not updated when the duty cycle is less than 7% for the software duty cycle mode and auto-temperature-fan control modes. In software-RPM mode, RPM monitoring is always performed and updated after each monitoring. If the TACH mode = '1' the RPM monitoring is always performed, and the TACH data are always updated after each monitoring.