EVAL-ADM1026EB datasheet(26/56 Pages) AD

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ADM1026

Rev. A | Page 26 of 56

The fan speed counter starts counting as soon as the fan

channel has been switched to. If the fan tach count reaches 0xFF,

the fan has failed or is not connected. If a fan is connected and

running, the counter is reset on the second tach rising edge, and

oscillator pulses are actually counted from the second rising

tach edge to the fourth rising edge. The measurement then

switches to the next fan channel. Here again, the counter begins

counting and is reset on the second tach rising edge, and

oscillator pulses are counted from the second rising edge to the

fourth rising edge. This is repeated for the other six fan

channels.

Note that fan speed measurement does not occur until 1.8

seconds after the monitor bit has been set. This is to allow the

fans adequate time to spin up. Otherwise, the ADM1026 could

generate false fan failure interrupts. During the 1.8 second fan

spin-up time, all fan tach registers read 0x00.

To accommodate fans of different speed and/or different

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

1, 2, 4, or 8 may be added before the counter. Divisor values for

Fans 0 to 3 are contained in the Fan 0–3 divisor register

(Address 02h) and those for Fans 4 to 7 in the Fan 4–7 divisor

count of 153 for a fan running at 4400 RPM producing two

output pulses per revolution. The count is calculated by the

equation:

Divisor

RPM

Count

For constant-speed fans, fan failure is typically considered to

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

corresponding to a count of 219. Full scale (255) is reached if

the fan speed fell to 60% of its nominal value. For temperature-

controlled, variable-speed fans, the situation is different.

Table 8 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.

Limit Values

Fans generally do not over-speed if run from the correct

voltage, so the failure condition of interest is under speed 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 because fan period rather than speed is

being measured, a fan failure interrupt occurs when the

measurement exceeds the limit value.

Fan Monitoring Cycle Time

The fan speeds are measured in sequence from 0 to 7. The

monitoring cycle time depends on the fan speed, the number

of tach output pulses per revolution, and the number of fans

being monitored.

If a fan is stopped or running so slowly that the fan speed

counter reaches 255 before the second tach pulse after initializa-

tion, or before the fourth tach pulse during measurement, the

measurement is terminated. This also occurs if an input is con-

figured as GPIO instead of fan. Any channels connected in this

manner time out after 255 clock pulses.

The worst-case measurement time for a fan-configured channel

occurs when the counter reaches 254 from start to the second

tach pulse and reaches 255 after the second tach pulse. Taking

into account the tolerance of the oscillator frequency, the worst-

case measurement time is

509 × D × 0.05 ms

where:

509 is the total number of clock pulses.

D is the divisor: 1, 2, 4, or 8.

0.05 ms is the worst-case oscillator period in ms.

The worst-case fan monitoring cycle time is the sum of the

worst-case measurement time for each fan.

Although the fan monitoring cycle and the analog input

monitoring cycle are started together, they are not synchronized

in any other way.

Table 8. Fan Speeds and Divisors

Time Per

Divisor RPM

Nominal Rev

RPM (ms)

70% RPM

Rev 70% (ms)

60% RPM

Rev 60% (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

45.45

÷ 8

1100

54.54

770

77.92

660

90.9

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