adm9240 Analog Devices, Inc., adm9240 Datasheet - Page 12

• Manufacturer Part Number: adm9240
• Description: Low Cost Microprocessor System Hardware Monitor
• Manufacturer: Analog Devices, Inc.

ADM9240
FAN INPUTS
Two inputs are provide for monitoring the condition of cooling
fans. Signal conditioning in the ADM9240 accommodates the
slow rise and fall times typical of fan tachometer outputs. The
maximum input signal range is 0 to V
inputs are supplied from fan outputs that exceed 0 to V
either resistive attenuation of the fan signal or diode clamping
must be included to keep inputs within an acceptable range.
Figures 6a to 6c show circuits for most common fan tacho
outputs.
If the fan tacho output has a resistive pull-up to V
connected directly to the fan input, as shown in Figure 6a.
If the fan output has a resistive pull-up to +12 V (or other
voltage greater than V
a Zener diode, as shown in Figure 6b. The Zener voltage
should be chosen so that it is greater than V
V
about 0.8
If the fan has a strong pull-up (less than 1 k ) to +12 V, or a
totem-pole output, a series resistor can be added to limit the
zener current, as shown in Figure 6c. Alternatively, a resistive
attenuator may be used, as shown in Figure 6d.
R1 and R2 should be chosen such that:
If the value of the pull-up resistor is not known, the value of R1
and R2 should be made fairly large, but not so large that the
input leakage current will cause a large voltage drop across
them.
With a pull-up voltage of 12 V and pull-up resistor less than
1 k , suitable values for R1 and R2 would be 100 k and
47 k . This will give a high input voltage of 3.83 V.
Figure 6b. Fan with Tachometer Pull-Up to Voltage >V
(e.g., 12 V) Clamped with Zener Diode
CC
Figure 6c. Fan with Strong Tachometer Pull-Up to
>V
Resistor
, allowing for the voltage tolerance of the Zener. A value of
Figure 6a. Fan with Tachometer Pull-Up to +V
CC
+12V
+12V
2 V < V
+12V
or Totem-Pole Output, Clamped with Zener and
OR TOTEM-POLE
V
*CHOOSE ZD1 VOLTAGE APPROX. 0.8
CC
*CHOOSE ZD1 VOLTAGE APPROX. 0.8
TYP. < 1k
PULL-UP
PULL-UP
PULL-UP
PULL-UP
is suitable.
4.7k
4.7k
TYP.
TYP.
CC
), the fan output can be clamped with
R2/(R
TACHO
OUTPUT
TACHO
OUTPUT
TACHO
OUTPUT
ZENER
10k
R1
ZD1*
PULL-UP
OR FAN2
OR FAN2
OR FAN2
ZD1*
ZENER
FAN1
FAN1
FAN1
CC
+ R1 + R2) < V
. In the event that these
IH
V
V
CC
CC
FAN SPEED
FAN SPEED
FAN SPEED
COUNTER
but less than
COUNTER
COUNTER
V
V
V
CC
CC
CC
CC
it can be
CC
CC
CC
,
CC
–12–
INPUT CURRENT LIMITING
If the fans are powered while the ADM9240 is unpowered, the
inputs of the ADM9240 will try to clamp the fan output volt-
age. In this case the input current must be limited to less than
the maximum value in the Absolute Maximum Ratings table.
The pull-up resistor of the fan tacho output may provide this
current limiting but, if its value is too low, it may be necessary
to add additional resistance in series with the fan input pins.
FAN SPEED MEASUREMENT
The fan counter does not count the fan tacho 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 accurate 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 tacho out-
put, as shown in Figure 7, so the accumulated count is actually
proportional to the fan tacho period and inversely proportional
to the fan speed.
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 Con-
figuration 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 tacho pulse, and ends on the next-but-one rising
edge. Once the fan speeds have been measured, they will be
stored in the Fan Speed Value Registers and can be read at any
time. The measurements will be updated as long as the moni-
toring cycle continues.
REG. BIT 0
CONFIG
Figure 6d. Fan with Strong Tachometer Pull-Up to
>V
22.5kHz
CLOCK
INPUT
INPUT
FAN1
FAN2
CC
+12V
MONITORING
START OF
or Totem-Pole Output, Attenuated with R1/R2
CYCLE
Figure 7. Fan Speed Measurement
< 1k
MEASUREMENT
PERIOD
FAN1
TACHO
OUTPUT
R1*
*SEE TEXT
OR FAN2
R2*
FAN1
MEASUREMENT
PERIOD
FAN SPEED
COUNTER
FAN2
V
CC
REV. 0