
SMSC DS – LPC47M14X 
Page 119 
Rev. 03/19/2001 
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 57 above. 
Duty Cycle Control for Fan x, Bits D6 – D1 
The Duty Cycle Control (DCC) bits determine the fan duty cycle.  The LPC47M14x 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
th
 of the F
OUT 
period. 
Generally, the F
OUT 
duty cycle (%) is (DCC 
÷
 64) 
×
 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 57 above.   
6.17.2 
Fan Tachometer Inputs 
The LPC47M14x implements fan tachometer inputs for signals from fans equipped with tachometer outputs.  The part 
can generate both a PME and an SMI when the fan speed drops below a predetermined value.  See description 
below. 
The clock source for the tachometer count is the 32.768kHz oscillator.  The Fan Tachometer Inputs gate a divided 
down version of the 32.768kHz oscillator for one period of the Fan signal into an 8-bit counter (maximum count is 
255).