AMC6821SDBQG4 Datasheet, PDF (17/56 Pages) Texas Instruments – Intelligent Temperature Monitor and PWM Fan Controller

AMC6821
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PWM2
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
SBAS386C – MAY 2006 – REVISED JULY 2007
PWM1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
Table 12. PWM Frequency
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
FAN SPEED MEASUREMENT
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.
Copyright © 2006–2007, Texas Instruments Incorporated
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