EMC2101-R-ACZL-TR datasheet(48/55 Pages) SMSC

48 / 55 page

SMBus Fan Control with 1°C Accurate Temperature Monitoring

Datasheet

Revision 2.53 (03-13-07)

SMSC EMC2101

DATASHEET

Appendix A Advanced PWM Options

The PWM Frequency Register determines the number of clocks (set by the CLK_SEL bit or the

PWM_D register settings) represent 1/2 of the period of the final PWM output waveform. Therefore,

as the PWM Frequency Register is updated, the PWM frequency is likewise updated. However, it also

directly affects the PWM Resolution and PWM duty cycle.

The PWM frequency is set according to Equation [8] or Equation [9] or, if the PWM Divide Register is

used, Equation [5].

The PWM Frequency Register does not affect the Fan Setting (either the Fan Setting Register or the

Fan Setting entries in the Fan Control Look-up Table Registers).

The Fan Setting Register determines the number of clocks that the PWM output is high for is always

based on 64 time steps for a PWM cycle. As the PWM Frequency Register changes (or the Fan Setting

changes) the effective duty cycle will vary according to Equation [6] and the PWM resolution will vary

according to Equation [7]. This is a result of the “on” time determined by Fan Setting changing with

respect to the overall PWM period determined by the PWM Frequency Register.

APPLICATION NOTE: If the Fan Setting is set at a value that is higher than 2x the PWM Frequency Register

settings, the PWM output will be at 100% duty cycle.

Table 6.34 shows the effective resolution, duty cycle, and frequency as the PWM Frequency Register

setting is changed.

Where: PWM_F is the setting of the PWM

Frequency Register (4Dh)

[6]

Where: PWM_F is the setting of the PWM

Frequency Register (4Dh)

[7]

Where: PWM_F is the setting of the

PWM Frequency register (4Dh)

PWM_D is the setting of the PWM

Frequency Divide Register (4Eh)

[8]

CLK_SEL = ‘0’

CLK_SEL = ‘1’

[9]

EFFECTIVE_DUTY_CYCLE

FAN_SETTING

PWM_F

--------------------------------------------

⎝⎠

⎛⎞

100%

EFFECTIVE_RESOLUTION

100%

PWM_F

--------------------------------

PWM_FREQUENCY

360k

2xPWM_F

-----------------------------

PWM_FREQUENCY

1.4k

2xPWM_F

-----------------------------

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