ADM1024ARU datasheet(16/28 Pages) AD

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ADM1024

–16–

REV. 0

If the fan has a strong pull-up (less than 1 k

Ω) to 12 V, or a

totem-pole output, then a series resistor can be added to limit the

Zener current, as shown in Figure 17c. Alternatively, a resistive

attenuator may be used, as shown in Figure 17d.

R1 and R2 should be chosen such that:

2 V < VPULL-UP

× R2/(R

PULL-UP + R1 + R2) < 5 V

The fan inputs have an input resistance of nominally 160 k

Ω to

ground, so this should be taken into account when calculating

resistor values.

With a pull-up voltage of 12 V and pull-up resistor less than 1 k

Ω,

suitable values for R1 and R2 would be 100 k

Ω and 47 kΩ. This

will give a high-input voltage of 3.83 V.

12V

TACHO

OUTPUT

FAN1 OR

FAN2

VCC

FAN SPEED

COUNTER

ZD1*

ZENER

*CHOOSE ZD1 VOLTAGE APPROX. 0.8

VCC

PULL-UP

TYP. <1k

OR TOTEM-POLE

10k

160k

Figure 17c. Fan with Strong Tach. Pull-Up to >VCC or

Totem-Pole Output, Clamped with Zener and Resistor

12V

<1k

TACHO

OUTPUT

FAN1 OR

FAN2

VCC

*SEE TEXT

R2*

R1*

FAN SPEED

COUNTER

160k

Figure 17d. Fan with Strong Tach. Pull-Up to >VCC or

Totem-Pole Output, Attenuated with R1/R2

FAN SPEED MEASUREMENT

The fan counter does not count the fan tach output pulses directly,

because the fan speed may be less than 1000 rpm and it would

take several seconds to accumulate a reasonably large and accu-

rate count. Instead, the period of the fan revolution is measured

by gating an on-chip 22.5 kHz oscillator into the input of an 8-bit

counter for two periods of the fan tach output, as shown in Fig-

ure 18; the accumulated count is actually proportional to the fan

tach period and inversely proportional to the fan speed.

22.5kHz

CLOCK

CONFIG.

REG. 1 BIT 0

FAN1

INPUT

FAN2

INPUT

START OF

MONITORING

CYCLE

FAN1

MEASUREMENT

PERIOD

FAN2

MEASUREMENT

PERIOD

Figure 18. Fan Speed Measurement

The monitoring cycle begins when a one is written to the Start

Bit (Bit 0), and a zero to the

INT_Clear Bit (Bit 3) of the

Conﬁguration Register.

INT_Enable (Bit 1) should be set to

one to enable the

INT output. The measurement begins on the

rising edge of a fan tach pulse, and ends on the next-but-one

rising edge. The fans are monitored sequentially, so if only one

fan is monitored the monitoring time is the time taken after the

Start Bit for it to produce two complete tach cycles or for the

counter to reach full scale, whichever occurs sooner. If more

than one fan is monitored, the monitoring time depends on the

speed of the fans and the timing relationship of their tach pulses.

This is illustrated in Figure 19. Once the fan speeds have been

measured, they will be stored in the Fan Speed Value Registers

and the most recent value can be read at any time. The measure-

ments will be updated as long as the monitoring cycle continues.

To accommodate fans of different speed and/or different num-

bers of output pulses per revolution, a prescaler (divisor) of

1, 2, 4, or 8 may be added before the counter. The default value

is 2, which gives a count of 153 for a fan running at 4400 rpm

producing two output pulses per revolution.

The count is calculated by the equation:

Count = (22.5

× 103 × 60) /(rpm × Divisor)

For constant speed fans, fan failure is normally considered to have

occurred when the speed drops below 70% of nominal, which

would correspond to a count of 219. Full scale (255) would be

reached if the fan speed fell to 60% of its nominal value. For

temperature-controlled variable speed fans the situation will be

different.

Table V shows the relationship between fan speed and time

per revolution at 60%, 70%, and 100% of nominal rpm for fan

speeds of 1100, 2200, 4400, and 8800 rpm, and the divisor that

would be used for each of these fans, based on two tach pulses

per revolution.

Table V. Fan Speeds and Divisors

Time

per

Nominal Rev

70%

Rev (70%) 60%

Rev (60%)

Divisor rpm

(ms)

rpm (ms)

rpm

(ms)

÷1

8800

6.82

6160 9.74

5280

11.36

÷2

4400

13.64

3080 19.48

2640

22.73

÷4

2200

27.27

1540 38.96

1320

45.44

÷8

1100

54.54

770

77.92

660

90.90

FAN1 and FAN2 Divisors are programmed into Bits 4 to 7 of

the VID 0–3/Fan Divisor Register.

LIMIT VALUES

Fans in general will not overspeed if run from the correct voltage,

so the failure condition of interest is underspeed due to electrical

or mechanical failure. For this reason only, low-speed limits are

programmed into the limit registers for the fans. It should be noted

that, since fan period rather than speed is being measured, a fan

failure interrupt will occur when the measurement exceeds the

limit value.

MONITORING CYCLE TIME

The monitoring cycle time depends on the fan speed and number

of tach output pulses per revolution. Two complete periods of the

fan tach output (three rising edges) are required for each fan

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