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.