ADM1026
Limit Values
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.
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.
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.
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.
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
register (Address 03h). 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:
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
22.5×103 ×60
RPM × Divisor
509 × D × 0.05 ms
Count =
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.
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.
The worst-case fan monitoring cycle time is the sum of the
worst-case measurement time for each fan.
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.
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
8800
4400
2200
±±00
RPM (ms)
6.82
±3.64
27.27
54.54
70% RPM
6±60
3080
±540
770
Rev 70% (ms)
9.74
±9.48
38.96
77.92
60% RPM
5280
2640
±320
660
Rev 60% (ms)
±±.36
22.73
45.45
90.9
÷ ±
÷ 2
÷ 4
÷ 8
Rev. A | Page 26 of 56
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