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EMC2300 Datasheet(PDF) 36 Page - SMSC Corporation

Part No. EMC2300
Description  Fan Control Device with High Frequency PWM and Temperature Monitors
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Maker  SMSC [SMSC Corporation]
Homepage  http://www.smsc.com
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EMC2300 Datasheet(HTML) 36 Page - SMSC Corporation

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Fan Control Device with High Frequency PWM and Temperature Monitors
Datasheet
Revision 0.2 (06-14-06)
36
SMSC EMC2300
DATASHEET
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 Register 62h, 63h: PWM Ramp
Rate Control on page 63). 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 Figure 7.6 Illustration of PWM
Ramp Rate Control on page 37.)
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.
Notes:
The step size may be less if the calculated duty cycle minus the actual duty cycle is less than 18.
The calculated PWM Duty cycle reacts immediately to a change in the temperature reading value.
The temperature reading value may be updated once in 624msec, once in 78msec, once in
223msec (default), or once in 447msec (see Table 6.2, “Conversion Cycle Timing,” on page 21).
The internal PWM duty cycle 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.
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.
Note 7.5
This event will only occur if the MInimum PWM for a particular PWM driver is programmed
to be 00h and is either changed (if the PWM driver is configured for Manual Operating
Mode) or the temperature rises above the low limit (if the PWM driver is configured for
Automatic Operating Mode).
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.
This transition will only occur in Manual Mode if the user sets the PWM duty cycle to 0% from a non-
zero value.
Table 7.1 PWM Ramp Rate
RRX-[2:0]
PWM RAMP TIME (SEC)
(TIME FROM 33%
DUTY CYCLE TO
100% DUTY CYCLE)
PWM RAMP TIME (SEC)
(TIME FROM MIN DUTY
CYCLE TO 100% DUTY
CYCLE)
TIME PER
PWM STEP
(PWM STEP SIZE =
1/255)
PWM
RAMP
RATE
(HZ)
000
35
52.53
206 msec
4.85


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