The larger value of the TACH data corresponds to a slower speed. When the TACH data are larger than the TACH-Low-Limit, the fan runs at a speed below the predefined minimum RPM, and the FANS bit in Status Register 1 is set to '1'. Note that no FANS (fan-slow) detections are made during spin-up. The FANS bit is cleared ('0') only after reading this register and reasserted ('1') in the next monitoring if a fan-slow is detected. After spin-up, FANS is set ('1') even if the TACH data are less than the TACH-Low-Limit until the register is read.

When the TACH data are less than the TACH-High-Limit, the fan runs at a speed above the predefined maximum RPM, and the RPM-ALARM bit in Status Register 1 is set ('1'). Note that the RPM-ALARM bit is cleared when reading the register. Once cleared, this bit is not reasserted in the next monitoring cycle even if the condition persists. This bit may be reasserted only if the RPM drops below the allowed maximum speed.

When FANS = 1 or RPM-ALARM = 1, there is a fan-out-of-range interrupt and FAN-ORN is generated if the FANIE bit in Configuration Register 1 is set ('1'). This interrupt makes the SMBALERT pin go low.

Figure 8-8 RPM Out-of-Range Detection