The PWM duty cycle controls the cooling fan speed. To spin up a fan from a stopped state or under-speed status, the spin-up process is applied to overcome the fan inertia. During the first third of spin-up, the duty cycle of the PWM gradually increases from 33.3% to 100%, and then maintains at 100% through the rest of the process. At the end of the spin-up process, the duty cycle is adjusted to 33.3%. After starting, the fan speed is controlled normally. Figure 8-14 shows the spin-up process. The bits [STIME2:STIME0] (bits 2:0 of 0x20) define the spin-up time, from 0.2 seconds to 8 seconds, as shown in Table 8-3. Fan speed is monitored immediately after the spin-up process.

Spin-up is disabled by setting the FSPD bit of the Fan Characteristics Register to '1'. If disabled, the spin-up process is not applied when the fan stops or an RPM is detected below the minimum speed. The TACH low limit register defines the minimum speed. After power-on or reset, the FSPD bit is cleared and the spin-up is always performed, regardless of the state of the FANS bit (bit 1 of 0x02).

Note that no FANS (fan-slow) detections are performed during spin-up. This bit is cleared ('0') only after reading it, and reasserts '1' in the next monitoring if a fan-slow condition is detected. After spin-up, FANS is set ('1') even if the TACH data are less than the TACH low limit until the flag is read.

Figure 8-14 Spin-Up Process