Fan Control Device with Hardware Monitoring and Acoustic Noise Reduction Features
Datasheet
Notes:
■
Whenever the programmed number of edges is detected, the edge detection ends and the state
machine is reset. The tachometer reading register is updated with the tachometer count value at
this time. See Note 6.2 below Table 6.2 for the one exception to this behavior.
■
In the case where the programmed number of edges occurs during the stretching time, the
tachometer value is latched when the last required edge is detected. The table below shows the
maximum values that may appear in the reading register.
Table 6.2 Max Tachometer Count Values for Different Stretch Periods
MAXIMUM TACHOMETER COUNT AT END OF PERIOD
PWM STRETCHING PERIOD
(Note 6.1)
50ms
100ms
200ms
400ms
600ms
800ms
950ms
1194h
2328h
4650h
8CA0h
D2F0h
FFFFh (Note 6.2)
FFFFh (Note 6.2)
Note 6.1 The maximum tachometer count does not take into account the guard time.
Note 6.2 This max value will be FFFFh if the programmed number of edges is detected when the
count reaches FFFFh or if no edges are detected. If the count reaches FFFFh in Mode 1
or the Max stretch time in Mode 2 and some edges were detected, but less than the
programmed number of edges, the maximum tach count value is determined by the Slow
Interrupt Enable bit located in the TACHx Options registers at offsets 90h to 93h. If slow
interrupt detection is enabled the count will be forced to FFFEh, else the count will be
forced to FFFFh.
6.1.4.8
Examples of Minimum RPMs Supported
The following tables show minimum RPMs that can be supported with the different parameters. The
first table uses 3 edges and the second table uses 2 edges.
Table 6.3 Minimum RPM Detectable Using 3 Edges – No PWM Stretching
PWM
PULSE WIDTH AT DUTY CYCLE
(PWM ”ON” TIME)
MINIMUM RPM AT DUTY CYCLE (Note 6.4)
(30/TTachPulse
FREQUENCY
)
100%
25%
50%
(MSEC)
(HZ)
(MSEC)
(MSEC)
(Note 6.3)
25%
50%
100%
87.7
58.6
44
2.85
4.27
5.68
7.1
5.7
8.53
11.36
14.2
11.36
17
10865
7175
5366
4279
5347
3554
2662
2126
2662
1774
1330
1063
22.64
28.3
35.2
SMSC EMC6D102
Revision 0.4 (04-05-05)
DATA4S1HEET
SMSC [ SMSC CORPORATION ]
