Fan Speed Control Summary
The following table illustrates the different modes for the fans.
SMSC DS – LPC47M192
Page 122
Rev. 03/30/05
DATASHEET
Table 56 - Different Modes for Fan
FANx
Clock
Control
Bit
(Note 1)
0
0
0
0
0
0
0
0
0
1
FANx
Clock
Multiplier
Bit
(Note 2)
X
0
0
0
0
1
1
1
1
X
FANx
Clock
Source
Select Bit
(Note 3)
X
0
0
1
1
0
0
1
1
X
FANx
Clock
Select Bit
(Note 4)
F
out
6-Bit Duty
Cycle
Control
bits[6:1]
(DCC)
0
1-63
-
Duty Cycle
(%)
X
0
1
0
1
0
1
0
1
X
0Hz – LOW
15.625kHz
23.438kHz
40Hz
60Hz
31.25kHz
46.876kHz
80Hz
120Hz
0Hz – HIGH
-
(DCC/64)
x 100
-
Note 1:
This is FANx Register Bit 0
Note 2:
This is Fan Control Register Bit 2 or 3
Note 3:
This is Fan Control Register Bit 0 or 1
Note 4:
This is FANx Register Bit 7
FANx Registers
The FAN1 and FAN2 Registers are located at 0x56 and 0x57 from base I/O in Logical Device A. The bits are defined
below. See the register description in the “Runtime Registers” section.
Fan x Clock Select Bit, D7
The Fan x Clock select bit in the FANx registers is used with the Fan x Clock Source Select and the Fan x
Clock Multiplier bits in the Fan Control register to determine the fan speed F
OUT
. See Table 56 above.
Duty Cycle Control for Fan x, Bits D6 – D1
The Duty Cycle Control (DCC) bits determine the fan duty cycle. The LPC47M192 has
≈
1.56% duty cycle
resolution. When DCC = “000000” (min. value), F
OUT
is always low. When DCC is “111111” (max. value),
F
OUT
is almost always high; i.e., high for 63/64
and low for 1/64
of the F
OUT
period. Generally, the F
OUT
duty cycle (%) is (DCC/64) x 100.
Fan x Clock Control, Bit D0
The Fan x Clock Control bit D0 is used to override the Duty Cycle Control for Fan x bits and force F
OUT
always high. When D0 = “0”, the DCC bits determine the F
OUT
duty cycle. When D0 = 1, F
OUT
is always
high, regardless of the state of the DCC bits.
Fan Control Register
The Fan Control Register is located at 0x58 from base I/O in Logical Device A. The bits are defined below. See the
register description in the “Runtime Registers” section.
Fan x Count Divisor, Bits D7-D6 / D5-D4
Fan x Count Divisor bit in Fan Control Register is used to determine fan tachometer count. The choices for
the divisor are 1, 2, 4 and 8. See “Fan Tachometer Input” section.
Fan x Clock Multiplier, Bits D3 / D2
The Fan x Clock Multiplier bit is used with the Fan x Clock Source Select bit in the Fan Control Register and
the Fan x Clock Select bit in Fan register to determine the F
OUT
. When the Fan x Clock Multiplier bit = “0”,
no clock multiplier is used. When the Fan x Clock Multiplier bit = “1”, the clock speed determined by the Fan
x Clock Source Select bit is doubled.
Fan x Clock Source Select, Bits D1 / D0
The Fan x Clock Source Select and the Fan x Clock Multiplier bits in the Fan Control register is used with
the Fan x Clock Select bit in the Fan x registers to determine the fan speed F
OUT
. See Table 56 above.
7.15.2 FAN SPEED MONITORING
The LPC47M192 monitors the speed of the fans by utilizing fan tachometer input signals from fans equipped with
tachometer outputs. There are two pins that can be configured as the fan tachometer inputs (pins 51 and 52) which