- 28 -
© Andigilog, Inc. 2006
www.andigilog.com
October 2006 - 70A06010
aSC7621
Register 04-07h: Fan Tachometer Measurement Configuration
Register
Address
Read/
Write
Register
Name
Bit 7
(MSB)
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(LSB)
Default
Value
Lock
04h
R/W
Tach 1
Configuration
3-Wire
Enable
1
3-Wire
Enable
0
Meas
Blank
1
Meas
Blank
0
Meas
Dwell
1
Meas
Dwell
0
Meas
Duration
1
Meas
Duration
0
36
X
05h
R/W
Tach 2
Configuration
3-Wire
Enable
1
3-Wire
Enable
0
Meas
Blank
1
Meas
Blank
0
Meas
Dwell
1
Meas
Dwell
0
Meas
Duration
1
Meas
Duration
0
36
X
06h
R/W
Tach 3
Configuration
3-Wire
Enable
1
3-Wire
Enable
0
Meas
Blank
1
Meas
Blank
0
Meas
Dwell
1
Meas
Dwell
0
Meas
Duration
1
Meas
Duration
0
36
X
07h
R/W
Tach 4
Configuration
3-Wire
Enable
1
3-Wire
Enable
0
Meas
Blank
1
Meas
Blank
0
Meas
Dwell
1
Meas
Dwell
0
Meas
Duration
1
Meas
Duration
0
36
X
The Fan Tachometer Configuration registers contain the settings that define the modes of measurement of the Tachometer
input signals. The user is allowed to disable a tachometer measurement or to request PWM stretching, in the case of a 3-
wire fan. Also, the rate, start-up and period of measurements within a fan rotation cycle may be selected. The table below
describes the controls.
This register becomes read-only when the Ready/Lock/Start/Override register Lock bit is set. Any further attempts to write to
this register shall have no effect.
Bit
Name
R/W
Default
Description
1:0
Measurement
Duration
R/W
10
The amount of fan rotation used for the tach measurement.
Assumes 2 pulse periods per rotation of fan.
00: ¼ Rotation – Tach Count x4 = Reported Value
01: ½ Rotation – Tach Count x2 = Reported Value
10: 1 Rotation – Tach Count x1 = Reported Value (default)
11: 2 Rotation – Tach Count x1 = Reported Value
3:2
Measurement
Dwell
R/W
01
Delay between Tach Measurements
00: 100 ms
01: 300 ms (default)
10: 500 ms
11: 728 ms
5:4
Measurement
Blank
R/W
11
In 3-wire fan mode, a delay is needed to assure that the tach
input has stabilized after the PWM has been set to 100%
00: 11.1 µs
01: 22.2 µs
10: 33.3 µs
11: 44.4 µs (default)
7:6
3-Wire Enable
R/W
00
For 3-Wire mode, the PWM output will be forced to 100% when
the tach measurement is being processed. Each fan has a 3-
Wire Mode control that will behave as indicated in this table:
HLFRQ
(5Fh-61h)
3-Wire Enable (7:6)
3-Wire
Mode
0
0
0
Enabled
0
0
1
Enabled
0
1
0
Enabled
0
1
1
Disabled
1
0
0
Disabled
1
0
1
Disabled
1
1
0
Enabled
1
1
1
Disabled
Table 15 Tachometer Configuration Register
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