ADM1029ARQZ-R7 datasheet(14/50 Pages) ONSEMI

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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 VIII.

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 VCC.

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 dur-

ing 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 on-

chip 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:

Count = f

60/R/N

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 Regis-

ters (68h for Fan 1 and 69h for Fan 2).

Table III. Oscillator Frequencies

Bit 7

Bit 6

Oscillator Frequency (Hz)

Measurement Disabled

470

940

1880

CLOCK

CONFIG

REG. BIT 4

FAN 1

TACH

FAN 1

MEASUREMENT

PERIOD

FAN 2

MEASUREMENT

PERIOD

START OF

MONITORING

CYCLE

FAN 2

TACH

Figure 8. Fan Speed Measurement

FAN SPEED LIMITS

Fans generally do not overspeed if run from the correct voltage,

so the failure condition of interest is under-speed due to electri-

cal or mechanical failure. For this reason only low-speed limits

are programmed into the Tach Limit Registers for the fans.

These registers are at address 78h for Fan 1 and 79h for Fan

2. It should be noted that, since fan period rather than speed is

being measured, the fan speed count will be larger the slower

the fan speed. Therefore a fan failure fault will occur when the

measurement exceeds the limit value.

ADM1029

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