
SMSC LPC47B27x
- 121 -
Rev. 08-10-04
DATASHEET
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 58
above.
Fan Tachometer Inputs
The LPC47B27x 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).
The clock source is determined by the CLK32_PRSN bit in the CLOCKI32 register in logical device A. It
is either the 32.768kHz clock from the CLKI32 pin or an internal 32.765kHz clock derived from the
14MHz clock.