AMC6821 Datasheet, PDF (15/47 Pages) Burr-Brown (TI) – Analog Monitor and Control Circuit

AMC6821
www.ti.com
SBAS386A – MAY 2006 – REVISED MAY 2006
TACH-DATA Register
Two fan tach pulse periods (PSPR = 0) or four tach pulse periods (PSPR = 1) are measured and the result is
stored in the TACH-DATA Register, as shown in Figure 8. Counting stops if the counter is over-range; the
measurement cycle repeats until monitoring is disabled, and the fan speed (RPM) can be calculated as shown in
Equation 2:
(10000 x 60)
RPM =
(Value of TACH-DATA Register)
(2)
Reading the TACH Data Register
To read the fan speed, both TACH-DATA-LByte and TACH-DATA-HByte must be read. TACH-DATA-LByte
must be read first. This reading causes TACH-DATA-HByte to be frozen until both the high and low byte
registers have been read from; this method prevents erroneous TACH readings.
RPM Measurement Rate
The TACH-FAST bit of Configuration Register 4 determines the rate. When TACH-FAST = 1, the TACH-DATA
Register is updated every 250ms (fast monitoring). When TACH-FAST = 0 (default), the reading is updated
every second (standard monitoring period).
Select Number of Pulses/Revolution
The speed sensor of most common fans provides two or four TACH pulses per revolution. The PSPR bit of
Configuration Register 4 specifies how many pulses per revolution are generated. PSPR = 1 indicates four
pulses/revolution and PSPR = 0 (default) indicates two pulses/revolution.
TACH Mode Selection
The TACH-MODE bit of Configuration Register 2 specifies the TACH pulse output mode of the fan. Some fans
(such as three- and two-wire) are powered directly by the PWM, and must be PWM-On to provide a TACH pulse
output. When the PWM-Out pin switches these fans ON/OFF directly, the PWM-Out must be kept ON to power
the fan during the measurement. In this case, the TACH-MODE bit of Configuration Register 2 must be cleared
('0'). When TACH-MODE = 0, the PWM-Out pin is kept ON during the measurement period. Clearing the TACH
mode ('0') also enables the internal correction circuitry to correct the error caused by the extra duty cycle applied
in the measurement period. The power-on default value of the PWM mode is 0.
Clock
PWM
TACH Data
+5V
Signal Conditioning
FAN Speed Counter
TACH
TACH
Output
+5V
START
TACH-EN
PWM ^PWM-Out
Control
AMC6821
a) Block Diagram of Fan Speed Monitoring
TACH Pulse
Measurement Period
for 2 Pulses/Revolution
Measurement Period for 4 Pulses Per Revolution
RPM Measurement for TACH-MODE = 0
Clock
TACH Pulse
Measurement
Starts
Measurement Period
for 2 Pulses/Revolution
Measurement Period for 4 Pulses Per Revolution
RPM Measurement for TACH-MODE = 1
b) Measuring the Period of TACH Pulses to Determine the Fan Speed
Figure 8. Fan Speed Measurement
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