AMC6821 datasheet(48/54 Pages) TI

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AMC6821

SBAS386C – MAY 2006 – REVISED JULY 2007

TACH-DATA Register

TACH-DATA-LByte Register (Address 0x08, Power-On Default = 0x00)

Bit 7 (MSB)

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0 (LSB)

TACH-DATA7

TACH-DATA6

TACH-DATA5

TACH-DATA4

TACH-DATA3

TACH-DATA2

TACH-DATA1

TACH-DATA0

TACH-DATA-HByte Register (Address 0x09, Power-On Default = 0x00)

Bit 7 (MSB)

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0 (LSB)

TACH-DATA15

TACH-DATA14

TACH-DATA13

TACH-DATA12

TACH-DATA11

TACH-DATA10

TACH-DATA9

TACH-DATA8

Bits [TACH-DATA15:TACH-DATA0] are the number of clock pulses counted during one fan revolution and

represents the period of the fan revolution (refer to the Fan Speed Measurement section). Reading the TACH

data register involves a two-register read. The low byte should be read first. This method causes the high byte to

be frozen until both the high and low byte registers have been read from, preventing erroneous TACH readings.

TACH Setting Register

TACH-SETTING-LByte Register (Address 0x1E, Power-On Default = 0xFF)

Bit 7 (MSB)

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0 (LSB)

TACH-SETTING7

TACH-SETTING6

TACH-SETTING5

TACH-SETTING4

TACH-SETTING3

TACH-SETTING2

TACH-SETTING1

TACH-SETTING0

TACH-SETTING-HByte Register (Address 0x1F, Power-On Default = 0xFF)

Bit 7 (MSB)

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0 (LSB)

TACH-

SETTING15

SETTING14

SETTING13

SETTING12

SETTING11

SETTING10

SETTING9

SETTING8

Bits [TACH-SETTING15:TACH-SETTING0] represent the period of the fan revolution (in the number of clock

pulses counted during one revolution), which is equal to the reciprocal of the target fan speed. Refer to the Fan

Speed Measurement section. Software writes this register to set the target RPM in the Software-RPM Control

mode. When the TACH-MODE bit (bit 1, 0x02) is cleared ('0'), the TACH setting must be not greater than the

value corresponding to the RPM for a 30% duty cycle. When the TACH mode is equal to '1', the TACH setting

must be not greater than the value corresponding to the allowed minimum RPM at which the fan properly runs.

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