ADM1024
If the fan has a strong pull-up (less than 1 kΩ) 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.
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.
R1 and R2 should be chosen such that:
2 V < VPULL-UP × R2/(RPULL-UP + R1 + R2) < 5 V
The fan inputs have an input resistance of nominally 160 kΩ 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 kΩ,
suitable values for R1 and R2 would be 100 kΩ and 47 kΩ. This
will give a high-input voltage of 3.83 V.
V
CC
12V
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.
PULL-UP
TYP. <1k⍀
OR TOTEM-POLE
FAN1 OR
FAN2
R1
10k⍀
FAN SPEED
COUNTER
TACHO
OUTPUT
160k⍀
ZD1*
ZENER
The count is calculated by the equation:
Count = (22.5 × 103 × 60) /(rpm × Divisor)
*CHOOSE ZD1 VOLTAGE APPROX. 0.8 ؋
V
CC
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.
Figure 17c. Fan with Strong Tach. Pull-Up to >VCC or
Totem-Pole Output, Clamped with Zener and Resistor
12V
V
CC
FAN1 OR
FAN2
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.
<1k⍀
R1*
FAN SPEED
COUNTER
TACHO
OUTPUT
160k⍀
R2*
*SEE TEXT
Figure 17d. Fan with Strong Tach. Pull-Up to >VCC or
Totem-Pole Output, Attenuated with R1/R2
Table V. Fan Speeds and Divisors
Time
per
Time
per
Time
per
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.
Nominal Rev
Divisor rpm
70% Rev (70%) 60% Rev (60%)
(ms) rpm (ms)
rpm (ms)
÷1
÷2
÷4
÷8
8800
4400
2200
1100
6.82 6160 9.74
5280 11.36
2640 22.73
1320 45.44
13.64 3080 19.48
27.27 1540 38.96
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.
22.5kHz
CLOCK
LIMIT VALUES
CONFIG.
REG.1 BIT 0
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.
FAN1
INPUT
FAN2
INPUT
FAN1
MEASUREMENT
PERIOD
FAN2
MEASUREMENT
PERIOD
MONITORING CYCLE TIME
START OF
MONITORING
CYCLE
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
Figure 18. Fan Speed Measurement
REV. 0
–16–
ADI [ ADI ]
