SMBus Fan Control with 1°C Accurate Temperature Monitoring
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.
FAN_SETTING
PWM_F × 2
Where: PWM_F is the setting of the PWM
Frequency Register (4Dh)
⎛
⎝
⎞
⎠
--------------------------------------------
EFFECTIVE_DUTY_CYCLE =
× 100%
[6]
[7]
100%
PWM_F × 2
Where: PWM_F is the setting of the PWM
Frequency Register (4Dh)
--------------------------------
EFFECTIVE_RESOLUTION =
Where: PWM_F is the setting of the
PWM Frequency register (4Dh)
PWM_D is the setting of the PWM
Frequency Divide Register (4Eh)
360k
2xPWM_F
-----------------------------
PWM_FREQUENCY =
PWM_FREQUENCY =
[8]
[9]
CLK_SEL = ‘0’
CLK_SEL = ‘1’
1.4k
-----------------------------
2xPWM_F
Revision 2.53 (03-13-07)
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