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RPMAlarm (TACHislessthanthehighlimit)

(RPMout-of-range)

FANIE

(FAN-TACHinterruptenabled)

FANS (TACHisgreaterthanthelowlimit)

FAN-ORNInterrupt

AMC6821

SBAS386C – MAY 2006 – REVISED JULY 2007

Some fans (such as the JMC™ four-wire fan) are powered directly by dc power, instead of being powered by the

PWM. In this case, the TACH mode must be set to '1'. When TACH-MODE = 1, the PWM-Out pin is not forced

ON; instead, the status is controlled completely by the DCY register, just as in normal operation. Setting

TACH-MODE to '1' also disables the internal correction circuit because no extra duty cycle is applied. Setting the

TACH mode to '1' allows TACH reading continuously, regardless of the status of the PWM-Out pin.

The selection of the TACH mode affects the RPM monitoring and control. When the TACH-MODE bit is equal to

'1', the duty cycle of the PWM-Out pin is always determined by the calculated value; the TACH data are always

updated at every RPM monitoring. However, when the TACH-MODE bit is equal to '0', in the Software-RPM

Control mode the PWM-Out pin is forced to 30% if the calculated duty cycle is less than 30%; in other modes,

the PWM-Out pin is forced to 0% and the TACH data are not updated if the calculated duty cycle is less than 7%.

FAN RPM Out-of-Range Detection

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

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 18. RPM Out-of-Range Detection

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