MEC1322 datasheet(292/456 Pages) MICROCHIP

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MEC1322

DS00001719D-page 292

 2014 - 2015 Microchip Technology Inc.

Using RPM based Fan Control Algorithm - in this mode of operation, the user determines a target tachometer

reading and the drive setting is automatically updated to achieve this target speed.

26.8.3

RPM BASED FAN CONTROL ALGORITHM

The RPM-PWM Interface includes an RPM based Fan Control Algorithm.

The fan control algorithm uses Proportional, Integral, and Derivative terms to automatically approach and maintain the

system’s desired fan speed to an accuracy directly proportional to the accuracy of the clock source. Figure 26-2, "RPM

based Fan Control Algorithm" shows a simple flow diagram of the RPM based Fan Control Algorithm operation.

The desired tachometer count is set by the user inputting the desired number of 32.768KHz cycles that occur per fan

revolution. The user may change the target count at any time. The user may also set the target count to FFh in order to

disable the fan driver.

For example, if a desired RPM rate for a 2-pole fan is 3000 RPMs, the user would input the hexadecimal equivalent of

1312d (52_00h in the TACH Target Registers). This number represents the number of 32.768KHz cycles that would

occur during the time it takes the fan to complete a single revolution when it is spinning at 3000RPMs (see Section

26.9.11, "TACH Target Register" and Section 26.9.12, "TACH Reading Register").

The RPM-PWM Interface’s RPM based Fan Control Algorithm has programmable configuration settings for parameters

such as ramp-rate control and spin up conditions. The fan driver automatically detects and attempts to alleviate a

stalled/stuck fan condition while also asserting the interrupt signal. The RPM-PWM Interface works with fans that oper-

ate up to 16,000 RPMs and provide a valid tachometer signal. The fan controller will function either with an externally

supplied 32.768KHz clock source or with its own internal 32KHz oscillator depending on the required accuracy.

TABLE 26-6:

FAN CONTROLS ACTIVE FOR OPERATING MODE

Manual Mode

Algorithm

Fan Driver Setting (read / write)

Fan Driver Setting (read only)

EDGES[1:0] (Fan Configuration)

UPDATE[2:0] (Fan configuration)

LEVEL (Spin Up Configuration)

SPINUP_TIME[1:0] (Spin Up Configuration)

Fan Step

Fan Minimum Drive

Valid TACH Count

TACH Target

TACH Reading

RANGE[2:0] (Fan Configuration 2)

DRIVE_FAIL_CNT[2:0] (Spin Up Config) and

Drive Fail Band

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