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| 型號： | ADM1024EVB |
| 廠商： | ON Semiconductor |
| 文件頁數(shù)： | 10/30頁 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR ADM1024 |
| 標準包裝： | 1 |
| 其它名稱： | EVAL-ADM1024EB EVAL-ADM1024EB-ND |
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ADM1024
http://onsemi.com
18
Figure 29. Fan Speed Measurement
22.5kHz
CLOCK
CYCLE
PERIOD
START OF
MONITORING
MEASUREMENT
FAN1
PERIOD
MEASUREMENT
FAN2
INPUT
FAN1
INPUT
FAN2
REG. 1 BIT 0
CONFIG.
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 tachometer pulse, and ends on the
nextbutone 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 tachometer 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 tachometer pulses.
This is illustrated in Figure 30. 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 measurements will be updated as long as the monitoring
cycle continues.
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. 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:
(eq. 8)
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 temperaturecontrolled variable
speed fans, the situation will be different.
Table 9 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 tachometer pulses per revolution.
FAN1 and FAN2 Divisors are programmed into Bits 4 to 7
of the VID0–3/Fan Divisor Register.
Table 9. FAN SPEEDS AND DIVISORS
Time Per
Divisor RPM
Nominal RPM
Rev (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
45.44
8
1100
54.54
770
77.92
660
90.90
Limit Values
Fans in general will 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, 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 tachometer output pulses per revolution. Two
complete periods of the fan tachometer output (three rising
edges) are required for each fan measurement. Therefore, if
the start of a fan measurement just misses a rising edge, the
measurement can take almost three tachometer periods. In
order to read a valid result from the fan value registers, the
total monitoring time allowed after starting the monitoring
cycle should, therefore, be three tachometer periods of
FAN1 plus three tachometer periods of FAN2 at the lowest
normal fan speed.
Although the fan monitoring cycle and the analog input
monitoring cycle are started together, they are not
synchronized in any other way.
Fan Manufacturers
Manufacturers of cooling fans with tachometer outputs
are listed below:
NMB Tech
9730 Independence Ave.
Chatsworth, California 91311
Phone: 8183413355; Fax: 8183418207
Model
Frame Size
Airflow
CFM
2408NL
2.36 in sq 0.79 in; (60 mm sq 20 mm)
9–16
2410ML
2.36 in sq 0.98 in; (60 mm sq 25 mm)
14–25
3108NL
3.15 in sq 0.79 in; (80 mm sq 20 mm)
25–42
3110KL
3.15 in sq 0.98 in; (80 mm sq 25 mm)
25–40
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