SCH3112I-NE SMSC [SMSC Corporation], SCH3112I-NE Datasheet - Page 216

• Manufacturer Part Number: SCH3112I-NE
• Description: LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
• Manufacturer: SMSC [SMSC Corporation]

Rev 0.2 (09-28-04)
23.13.3
23.13.3.1
Following a VCC Power-On-Reset (exiting sleep mode) the following steps must be taken. These steps
are required for most systems in order to prevent improper fan start-up due to the reset of the Top
Temperature and zone low limit registers to their default values on active PWRGD_PS.
1. Set the ramp rate to the min value [registers 62h and 63h].
2. Clear the start bit (bit 0 of register 40h) to stop monitoring
3. Set the Top Temperature Remote 1, 2, Ambient registers [AEh, AFh, B3h] to their initial values
4. Set the zone temperature low limit registers [67-69h] to their initial values
5. Set the start bit (bit 0 of register 40h) to start monitoring
6. Set the lock bit (bit 1of register 40h) to lock the limit and parameter registers (optional)
Note: If not locked, the ramp rate can be set to a new value at a later time if desired [registers 62h
PWM Fan Speed Control
The following description applies to PWM1, PWM2, and PWM3.
Note: The PWM output pins are held low when VCC=0. The PWM pins will be forced to “spinup”
The PWM pin reflects a duty cycle that is determined based on 256 PWM duty cycle intervals. The
minimum duty cycle is “off”, when the pin is low, or “full on” when the pin is high for 255 intervals and
low for 1 interval. The INVERT bit (bit 4 of the PWMx Configuration registers at 80h-82h) can be used
to invert the PWM output, however, the default operation (following a VCC POR) of the part is based
on the PWM pin active high to turn the fans “on”. When the INVERT bit is set, as long as power is not
removed from the part, the inversion of the pin will apply thereafter.
When describing the operation of the PWMs, the terms “Full on” and “100% duty cycle” means that
the PWM output will be high for 255 clocks and low for 1 clock (INVERT bit = 0). The exception to this
is during fan spin-up when the PWM pin will be forced high for the duration of the spin-up time.
The SCH311X can control each of the PWM outputs in one of two modes:
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These modes are described in sections that follow.
Manual Fan Control Operating Mode (Test Mode)
When operating in Manual Fan Control Operating Mode, software controls the speed of the fans by
directly programming the PWM duty cycle. The operation of the fans can be monitored based on
reading the temperature and tachometer reading registers and/or by polling the interrupt status
registers. The SCH311X offers the option of generating an interrupt indicated by the nHWM_INT signal.
To control the PWM outputs in manual mode:
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To monitor the fans:
Fans equipped with Tachometer outputs can be monitored via the FANTACHx input pins. See
23.14.2, "Fan Speed Monitoring," on page
Manual Fan Control Operating Mode: software controls the speed of the fans by directly
programming the PWM duty cycle.
Auto Fan Control Mode: the device automatically adjusts the duty cycle of the PWM outputs based
on temperature, according to programmed parameters.
To set the mode to operate in manual mode, write ‘111’ to bits[7:5] Zone/Mode, located in Registers
5Ch-5Eh: PWMx Configuration.
The speed of the fan is controlled by the duty cycle set for that PWM output. The duty cycle must
be programmed in Registers 30h-32h: Current PWM Duty
and 63h].
when PWRGD_PS goes active. See “Spin Up” on page 204.
DATASHEET
LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
200
217.
SMSC SCH311X
Datasheet
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