AMC6821 Datasheet, PDF (14/47 Pages) Burr-Brown (TI) – Analog Monitor and Control Circuit

AMC6821
SBAS386A – MAY 2006 – REVISED MAY 2006
Table 5. PWM Frequency
PWM2
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
PWM1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
PWM0
PWM FREQUENCY
When the PWM-MODE Pin is Floating or Tied to V_DD
0
10Hz
1
15Hz
0
23Hz
1
30Hz (default)
0
38Hz
1
47Hz
0
62Hz
1
94Hz
When the PWM-MODE Pin is Tied to GND
0
1kHz
1
10kHz
0
20kHz
1
25kHz (default)
0
30kHz
1
40kHz
0
40kHz
1
40kHz
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FAN SPEED MEASUREMENT
The AMC6821 monitors the fan speed (RPM) via the TACH pin (Figure 8 illustrates this process). 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%. If the TACH mode is equal to '1' the RPM monitoring is always performed, and the
TACH data are always updated after each monitoring.
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