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Fan Speed Control

This chapter describes a generic Fan Speed Control unit. One or more Fan Speed Control units are included in the Fan

Speed Control and Monitor (FSCM) module. For the specific implementation in this device, see Section 3.15.1 on page 67.

7.1 OVERVIEW

The Fan Speed Control is a programmable Pulse Width Modulation (PWM) generator. The PWM generator output is used

to control the fan power voltage, which is correlated to the fan speed. Converting a 0 to 100% duty cycle PWM signal to an

analog voltage range is performed by an external driver circuit, as shown in Figure 17. Some newer fans accept direct PWM

input without any external driver.

Figure 17. Fan Speed Control - System Conﬁguration

7.2 FUNCTIONAL DESCRIPTION

The PWM generator operation is based on a PWM counter and two registers: the Fan Speed Control Pre-Scale register

(FCPSR), used to determine the overall cycle time (or the frequency) of the FANPWM output, and the Fan Speed Control

Duty Cycle register (FCDCR), used to determine the duty cycle of the FANPWM between 0 to 100%.

The PWM counter is an 8-bit, free-running counter that runs continuously in a cyclic manner, i.e., its cycle equals 256 clock

periods. The PWM output is high, as long as the count is lower than the FCDCR value and it flips to low as the counter ex-

ceeds that value. The duty cycle (expressed as a percentage) is therefore (FCDCR/256)*100. In particular, the PWM output

is continuously low when FCDCR=0; it is continuously high when FCDCR=FFh. The FANOUT output may be inverted by an

external configuration bit (FANCTL_INV in FSCMCF1 or FSCMCF2 register; see Section 3.15.3 on page 68 and Section

3.15.4 on page 69), in which case the FANPWM duty cycle is ([256-FCDCR]/256)*100.

The PWM counter clock is generated by dividing the input clock (either 24 MHz or 200 KHz), using a clock divider. The di-

vision factor, which must be between 1 and 124, is defined as PRE-SCALE +1, where PRE-SCALE is the binary value stored

in bits 6 to 0 of the FCPSR register. The resulting PWM output frequency is therefore:

(24 MHz or 200 kHz) / ([PRE-SCALE + 1]

* 256).

The default selection of 24 MHz input clock allows a programmable FANPWM frequency in the range of 756 Hz to

93.75 KHz. For lower frequencies, selecting the 200 KHz input clock allows a frequency range of 6 Hz to 781 Hz (see

Figure 18).

The FANPWM frequency must be selected, according to the fan type’s specific requirements, prior to enabling the Fan

Speed Control. The only register that must be changed at runtime to control the fan speed is the FCDCR register.

Warning! The contents of the FCPSR register must not be changed while the Fan Speed Control is enabled.

Fan Speed

Control

Fan

FANPWM

Control

External

Driver

24 MHz

(1-124)

PWM

FCDCR Register

(FCDCR > Counter)

FANPWM

FCPSR Register

Figure 18. Fan Speed Control - Simpliﬁed Diagram

200 KHz

Bit 7

Bits 6-0

PWM Output -

Counter

Comparator

FANCTL_INV

Clock

Divider

CLK_SEL

PRE-SCALE

DUTY-CYC

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