Datasheet

TACH-DATA Register
RPM=
(100,000x60)
(ValueofTACH-DATARegister)
(2)
Clock
TACHPulse
PWM
MeasurementPeriod
for2Pulses/Revolution
TACHPulse
Measurement
Starts
Clock
MeasurementPeriodfor4PulsesPerRevolution
MeasurementPeriod
for2Pulses/Revolution
RPMMeasurementforTACH-MODE=0
RPMMeasurementforTACH-MODE=1
b)MeasuringthePeriodofTACHPulsestoDeterminetheFanSpeed
SignalConditioning
FANSpeedCounter
TACH
PWM
Control
PWM-Out
TACHData
TACH
Output
+5V
+5V
START
TACH-EN
AMC6821
a)BlockDiagramofFanSpeedMonitoring
MeasurementPeriodfor
4PulsesPerRevolution
AMC6821-Q1
SBAS475 – JUNE 2009 ...................................................................................................................................................................................................... www.ti.com
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 17 . 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 :
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, preventing TACH reading errors.
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 critical tach edges of 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'.
Figure 17. Fan Speed Measurement
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