ADM1029
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14
If two fans are installed, Bit 0 would be 1 by default and
Pin 18 would be tied high* to set Bit 1. If only one fan is
installed, it would normally be Fan 1 and Pin 18 would be
tied low* to clear Bit 1. However, both of these bits can be
modified by writing to the register, so it is possible to have
Fan 2 installed and not Fan 1, or even have no fans installed.
* Note that Pin 18 also sets TMIN for automatic fan speed control. If
this function is used, Pin 18 would be set to some other level
according to Table 13.
FAULT Inputs/Outputs
The ADM1029 can be used with fans that have a fault
output which indicates if the fan has stalled or failed. If one
or both of the FAULT
inputs (Pin 2 or Pin 23) goes low, both
INT
and CFAULT will be asserted.
Events on the fault inputs are also reflected in Bits 2 and
3 of the corresponding Fan Status Registers at addresses 10h
and 11h. Bit 2 reflects the inverse state of the FAULT
pin (0
if FAULT
is high, 1 if FAULT is low), while Bit 3 is latched
high if a FAULT
input goes low. It must be cleared by writing
a zero to it.
If the fan(s) being used do not have a FAULT
output, the
FAULT
input(s) on the ADM1029 should be pulled high to
V
CC
.
The FAULT
pins can also be configured as open-drain
outputs by setting Bit 5 of the corresponding Fan Fault
Action Register (18h or 19h). If a FAULT
pin is configured
as an output, it will still function as an input. This means that
when a fault input occurs it will be latched low by the fault
output, even if the fault input is removed. The fault output
can be used to drive a fan failure indicator such as an LED.
If the FAULT
pin is used as an output, any input to the
FAULT
pin should also be open-drain. This will avoid the
fault input trying to source a high current into the FAULT
pin
if the fault input goes high while the fault output is low.
Fan Present Inputs
The fan PRESENT signal is implemented by a shorting
link to ground in the fan connector. When the fan is plugged
in, the corresponding PRESENT
input (Pin 4 or Pin 21) on
the ADM1029 is pulled low. If the fan is unplugged, the
PRESENT
input will be pulled high. INT and CFAULT will
be asserted (unless masked) and the event will be reflected
in Bits 0 and Bit 1 of the corresponding Fan Status Register.
Appearance or disappearance of a PRESENT
input signal
during normal operation signals to the ADM1029 that a fan
has been hot-plugged or unplugged. INT
and CFAULT will
be asserted (unless masked). When a fan is hot-plugged,
Bit 7 of the corresponding Fan Status Register will be set
and a Fan Free Wheel Test commences automatically.
Fan Speed Measurement
The fan counter does not count the fan tach output pulses
directly, because at low fan speeds it would take several
seconds to accumulate a reasonably large and accurate
count. Instead, the period of the fan revolution is measured
by gating an onchip oscillator into the input of an 8-bit
counter.
The fan speed measuring circuit is initialized on the first
rising edge of a fan tach pulse after monitoring is enabled by
setting Bit 4 of the Configuration Register. It then starts
counting on the rising edge of the second tach pulse and
counts for four fan tach periods, until the rising edge of the
sixth tach pulse, or until the counter overranges if the fan
tach period is too long.
After the speed of the first fan has been measured, the
speed of the second fan (if installed) will be measured in the
same way. The measurement cycle will repeat until
monitoring is disabled. The fan speed measurements are
stored in the Fan Tach Value registers at addresses 70h and
71h.
If both fans are installed, Fan 1 will be measured first. If
only one fan is installed, the ADM1029 will still try to
measure both fans, starting with Fan 1, but the measurement
on the noninstalled fan will time out when the Fan Tach
Value count overranges.
The fan speed count is given by:
(eq. 2)
Count + f 4 60ńRńN
where:
f is oscillator frequency in Hz
factor 4 is because 4 tach periods are counted
factor 60 is to convert minutes to seconds
R = fan speed in RPM
N is number of tach pulses per revolution
The frequency of the oscillator can be adjusted to suit the
expected frequency range of the fan tach pulses, which
depends on the fan speed and the number of tach pulses
produced for each revolution of the fan, which is either 1, 2,
or 4. The oscillator frequency is set by Bits 7 and 6 of the Fan
Configuration Registers (68h for Fan 1 and 69h for Fan 2).
Table 8. OSCILLATOR FREQUENCIES
Bit 7 Bit 6 Oscillator Frequency (Hz)
0 0 Measurement Disabled
0 1 470
1 0 940
1 1 1880
Figure 24. Fan Speed Measurement
CLOCK
CONFIG
REG. BIT 4
FAN 1
TACH
FAN 1
MEASUREMENT
PERIOD
FAN 2
MEASUREMENT
PERIOD
START OF
MONITORING
CYCLE
FAN 2
TACH
