NCT7491RQR2G【PDF 30/80 页】IC REMOTE THERMAL MONITOR 24QSOP

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NCT7491

http://onsemi.com

TACH Measurement Overview

The fan counter does not count the fan TACH output

pulses directly because the fan speed could be less than 1000

RPM, and it takes several seconds to accumulate a

reasonably large and accurate count. Instead, the period of

the fan revolution is measured by gating an onchip 78 kHz

oscillator into the input of a 16bit counter for N periods of

the fan TACH output (see Figure 21) so the accumulated

count is actually proportional to the fan tachometer period

and inversely proportional to the fan speed. N, the number

of pulses counted, is determined by the settings of the TACH

pulses per revolution register (0x7B). This register contains

two bits for each fan, allowing one, two (default), three, or

four TACH pulses to be counted.

Figure 21. Fan Speed Measurement

Fan Speed Measurement Registers

The fan tachometer registers are 16bit values consisting

of a 2byte read from the NCT7491.

" Register 0x28, TACH1 Low Byte

" Register 0x29, TACH1 High Byte

" Register 0x2A, TACH2 Low Byte

" Register 0x2B, TACH2 High Byte

" Register 0x2C, TACH3 Low Byte

" Register 0x2D, TACH3 High Byte

" Register 0x2E, TACH4 Low Byte

" Register 0x2F, TACH4 High Byte

Reading Fan Speed from the NCT7491

The measurement of fan speeds involves a 2register read

for each measurement. The low byte should be read first.

This causes the high byte to be frozen until both high and low

byte registers have been read, preventing erroneous TACH

readings. The fan tachometer reading registers report back

the number of 12.82 us period clocks (78 kHz oscillator)

gated to the fan speed counter, from the rising edge of the

first fan TACH pulse to the rising edge of the third fan TACH

pulse (assuming two pulses per revolution are being

counted). Because the device is essentially measuring the

fan TACH period, the higher the count value, the slower the

fan is actually running. A 16bit fan tachometer reading of

0xFFFF indicates that either the fan has stalled or is running

very slowly (<100 RPM).

Because the actual fan TACH period is being measured,

falling below a fan TACH limit by 1 sets the appropriate

status bit and can be used to generate an SMBALERT

Fan TACH Limit Registers

The fan TACH limit registers are 16bit values consisting

of two bytes.

" Register 0x54, TACH1 Minimum Low Byte = 0xFF

default

" Register 0x55, TACH1 Minimum High Byte = 0xFF

default

" Register 0x56, TACH2 Minimum Low Byte = 0xFF

default

" Register 0x57, TACH2 Minimum High Byte = 0xFF

default

" Register 0x58, TACH3 Minimum Low Byte = 0xFF

default

" Register 0x59, TACH3 Minimum High Byte = 0xFF

default

" Register 0x5A, TACH4 Minimum Low Byte = 0xFF

default

" Register 0x5B, TACH4 Minimum High Byte = 0xFF

default

Fan Speed Measurement Rate

The fan TACH readings are normally updated once every

second. When set, the FAST bit (Bit 3) of Configuration

250 ms.

DC Bits

If any of the fans are not being driven by a PWM channel

but are powered directly from 5.0 V or 12 V, their associated

dc bit in Configuration Register 3 (0x78) should be set. This

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