EMC2302 datasheet(16/70 Pages) MICROCHIP

16 / 70 page

EMC2301/2/3/5

DS20006532A-page 16

 2021 Microchip Technology Inc.

When the FSC algorithm is enabled, ramp rate control

is automatically used. When the FSC is not active, then

ramp rate control can be enabled by asserting the

ENRCx bit (see Register 6-14).

The Update Time (UDT) bits and Fan Step Register

settings operate independently of the RPM-based FSC

algorithm and will always limit the fan drive setting.

That is, if the programmed fan drive setting (either as

determined by the RPM-based FSC algorithm or by

manual settings) exceeds the current fan drive setting

by greater than the Fan Step Register setting, the

EMC2301/2/3/5 will limit the fan drive change to the

value of the Fan Step Register. It will use the Update

Time to determine how often to update the drive

settings.

If the Fan Speed Control Algorithm is used, the default

settings in Register 6-14 will cause the maximum fan

step settings to be ignored.

4.3.3.4

Minimum Drive Setting

The Fan Minimum Drive Register (see Register 6-18)

stores the minimum drive setting for each RPM-based

Fan Speed Control algorithm. The RPM-based Fan

Speed Control algorithm will not drive the fan at a level

lower than the minimum drive unless the target Fan

Speed is set at FFh.

During normal operation, if the fan stops for any reason

(including low drive), the RPM-based Fan Speed Con-

trol algorithm will attempt to restart the fan. Setting the

Fan Minimum Drive Register to a setting that will main-

tain fan operation is a useful way to avoid potential fan

oscillations as the control circuitry attempts to drive it at

a level that cannot support fan operation.

4.4

Tachometer Measurement

The tachometer measurement circuitry is used in con-

junction with the RPM-based Fan Speed Control Algo-

rithm to update the fan driver output. Additionally, it can

be used in Direct Setting mode as a diagnostic for host

based fan control.

This method monitors the TACHx signal in real time. It

constantly updates the tachometer measurement by

reporting the number of clocks between a user pro-

grammed number of edges (EDGX bits in Register 6-

13) on the TACHx signal (see Table 4-7).

The tachometer measurement provides fast response

times for the RPM-based Fan Speed Control algorithm

and the data is presented as a count value that

represents the fan RPM period.

4.4.1

TACHOMETER COUNTS TO RPM

The TACH Reading Registers describe the current

tachometer reading for each of the fans. By default, the

data represents the fan speed as the number of 32 kHz

clock periods that occur for a single revolution of the

fan.

Equation 4-2 shows the detailed conversion from

TACH measurement (COUNT) to RPM, while

Equation 4-3 shows the simplified translation of TACH

Reading Register count to RPM assuming a two-pole

fan, measuring five edges, with a frequency of

32.768 kHz. These equations are solved and tabulated

for ease of use in AN17.4 RPM to TACH Counts Con-

version.

EQUATION 4-2:

TACHOMETER COUNTS

TO RPM

Note:

The tachometer measurement method

works independently of the drive settings.

If the device is put into Direct Setting

mode and the fan drive is set at a level that

is lower than the fan can operate (includ-

ing zero drive), then the tachometer mea-

surement may signal a Stalled Fan

condition and assert an interrupt.

TABLE 4-7:

MINIMUM EDGES FOR FAN

ROTATION

EDGX[1:0]

Minimum

TACH

Edges

Number

of Fan

Poles

Effective TACH

Multiplier

(Based on Two

Pole Fans)

0.5

52 (default)

1.5

RPM

poles



-----------------

–



COUNT

----



------------------------------



fTACH



Where:

poles = number of poles on the fan (typically

two)

fTACH = the tachometer measurement

frequency (typically 32.768 kHz)

n = number of edges measured (typically

five for a two-pole fan)

m = the multiplier defined by the RANGE

bits

COUNT = TACH Reading Register value (in

decimal)

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