SCH3112I-NE SMSC [SMSC Corporation], SCH3112I-NE Datasheet - Page 222

• 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)
RRX-
[2:0]
000
001
010
100
101
011
110
Internally, the PWM Ramp Rate Control Logic will increment/decrement the internal PWM Duty cycle
by ‘1’ at a rate determined by the Ramp Rate Control Register (see ). The actual duty cycle output is
changed once per the period of the PWM output, which is determined by the frequency of the PWM
output. (See
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■
Note: The calculated PWM Duty cycle reacts immediately to a change in the temperature reading
PWM Output Transition from OFF to ON
When the calculated PWM Duty cycle generated by the auto fan control logic transitions from the ‘OFF’
state to the ‘ON’ state (i.e., Current PWM duty cycle>00h), the internal PWM duty cycle in the Ramp
Rate Control Logic is initialized to the calculated duty cycle without any ramp time and the PWMx
Current Duty Cycle register is set to this value. The PWM output will latch the current duty cycle value
in the Ramp Rate Control block to control the PWM output.
PWM Output Transition from ON to OFF
Each PWM output has a control bit to determine if the PWM output will transition immediately to the
OFF state (default) or if it will gradually step down to Off at the programmed Ramp Rate. These control
bits (SZEN) are located in the PWMx Options registers at offsets 94h-96h.
PWM RAMP TIME (SEC)
(TIME FROM 33% DUTY
CYCLE TO 100% DUTY
If the period of the PWM output is less than the step size created by the PWM Ramp Rate, the
PWM output will hold the duty cycle constant until the Ramp Rate logic increments/decrements the
duty cycle by ‘1’ again. For example, if the PWM frequency is 87.7Hz (1/87.7Hz = 11.4msec) and
the PWM Step time is 206msec, the PWM duty cycle will be held constant for a minimum of 18
periods (206/11.4 = 18.07) until the Ramp Logic increments/decrements the actual PWM duty cycle
by ‘1’.
If the period of the PWM output is greater than the step size created by the PWM Ramp Rate, the
ramp rate logic will force the PWM output to increment/decrement the actual duty cycle in
increments larger than 1/255. For example, if the PWM frequency is 11Hz (1/11Hz = 90.9msec)
and the PWM Step time is 5msec, the PWM duty cycle output will be incremented 18 or 19 out of
255 (i.e., 90.9/5 = 18.18) until it reaches the calculated duty cycle. Note: The step size may be
less if the calculated duty cycle minus the actual duty cycle is less than 18.
value. The temperature reading value may be updated once in TBDmsec (default) (see
Table 23.2, “ADC Conversion Sequence,” on page
generated by the Ramp Rate control logic gradually ramps up/down to the calculated duty cycle
at a rate pre-determined by the value programmed in the PWM Ramp Rate Control bits . The
PWM output latches the internal duty cycle generated by the Ramp Rate Control Block every
1/(PWM frequency) seconds to determine the actual duty cycle of the PWM output pin.
CYCLE)
17.6
11.8
7.0
4.4
3.0
1.6
35
Figure 23.7 Illustration of PWM Ramp Rate Control on page
Table 23.4 PWM Ramp Rate
PWM RAMP TIME (SEC)
CYCLE TO 100% DUTY
(TIME FROM 0% DUTY
DATASHEET
LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
CYCLE)
17.595
10.455
52.53
26.52
6.63
4.59
2.55
206
(PWM STEP SIZE =
190). The internal PWM duty cycle
PWM STEP
TIME PER
206 msec
104 msec
69 msec
41 msec
26 msec
18 msec
10 msec
1/255)
207.)
RAMP RATE
SMSC SCH311X
14.49
24.39
38.46
55.56
PWM
4.85
9.62
(HZ)
100
Datasheet