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TACH-DATA Register

RPM=

(100,000x60)

(ValueofTACH-DATARegister)

(2)

Clock

TACHPulse

PWM

MeasurementPeriod

for2Pulses/Revolution

TACHPulse

Measurement

Starts

Clock

MeasurementPeriodfor4PulsesPerRevolution

MeasurementPeriod

for2Pulses/Revolution

RPMMeasurementforTACH-MODE=0

RPMMeasurementforTACH-MODE=1

b)MeasuringthePeriodofTACHPulsestoDeterminetheFanSpeed

SignalConditioning

FANSpeedCounter

TACH

PWM

Control

PWM-Out

TACHData

TACH

Output

+5V

START

TACH-EN

AMC6821

a)BlockDiagramofFanSpeedMonitoring

MeasurementPeriodfor

4PulsesPerRevolution

AMC6821

SBAS386C – MAY 2006 – REVISED JULY 2007

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

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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