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FAN SPEED MEASUREMENT

AMC6821

SBAS386C – MAY 2006 – REVISED JULY 2007

Table 12. PWM Frequency

PWM2

PWM1

PWM0

PWM FREQUENCY

When the PWM-MODE Pin is Floating or Tied to VDD

10Hz

15Hz

23Hz

30Hz (default)

38Hz

47Hz

62Hz

94Hz

When the PWM-MODE Pin is Tied to GND

1kHz

10kHz

20kHz

25kHz (default)

30kHz

40kHz

The AMC6821 monitors the fan speed (RPM) via the TACH pin, as illustrated in Figure 17. 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.

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