LPC47B272-MS SMSC, LPC47B272-MS Datasheet - Page 121

• Manufacturer Part Number: LPC47B272-MS
• Description: IC CTRLR SUPER I/O LPC 100-QFP
• Manufacturer: SMSC

Specifications of LPC47B272-MS

• Controller Type: I/O Controller
• Interface: LPC
• Voltage - Supply: 3.3V
• Current - Supply: 15mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 100-QFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 638-1019

The fan tachometer input signal and clock source is shown below.
The counter is reset by the rising edge of each pulse (and by writing the preload register). The counter
does not wrap; if it reaches 0xFF, it remains at 0xFF until it is reset by the next pulse.
The 2 MSBs of the count are sampled and a PME or SMI is generated (if enabled through the
PME_EN1 enable register or the SMI_EN5 enable register - see the “Runtime Registers” section) when
these two bits are set. This corresponds to a count value of 192.
The fan count is determined according to the following equation:
Count =
Term 1 in the equation above is determined by multiplying the clock source of 32.768kHz by 60sec/min
and dividing by the product of the revolutions per minute times the divisor. The default divisor, located
in the Fan Control Register, is 2. This results in a value for Term 1 in Equation 1 of 111 for a 4400
RPM.
The divisor for each fan is programmable via the Fan Control Register, Logical Device A, 0x58. The
choices for the divisor are 1, 2, 4 and 8. The default value is 2. The factor of ½ in Term 1 corresponds
to two pulses per revolution.
The preload value is programmable via the FAN1 Preload Register and FAN2 Preload Register. The
preload is the initial value for the fan count which is used to adjust the count such that the value of 192
corresponds to the “lower limit” of the RPM. By setting the preload value and divisor properly, the PME
or SMI will be generated when the RPM reaches the desired percentage of the nominal RPM to indicate
a fan failure.
A PME or SMI is generated, if enabled through the PME or SMI enable register, at a count of 192,
which corresponds to the “upper limit” for the fan count. This value is made to correspond to the “lower
SMSC LPC47B27x
Fan
Tachometer
Input
Clock Source
for Counter
1
2
x
(Term 1)
RPM x Divisor
1.966 x 10
6
T
P
+ Preload
DATASHEET
T
R
- 121 -
T
T
F = 32.786kHz ÷ Divisor
R
P
(Equation 1)
= Pulse Time = T
= Revolution Time = 60/RPM (sec)
(Two Pulses Per Revolution)
R
/2
Rev. 04-17-07