EMC2300_11 Datasheet, PDF (42/82 Pages) SMSC Corporation – Fan Control Device with High Frequency PWM and Temperature Monitors

7.1.4.8
Fan Control Device with High Frequency PWM and Temperature Monitors
Datasheet
Note 7.3
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
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 set to 0, the
count will be forced to FFFEh, else the count will be forced to FFFFh.
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.
As described in Assumptions (refer to Figure 7.5, "PWM and Tachometer Concept"): on page 39, the
TACH detection circuitry expects a fan to deliver 5 TACH edges per full revolution. When measuring
the fan speed using a PWM drive, the slowest fan speed that can be measured is dependent upon
how long the PWM drive is high as well as how many edges are being counted to make a valid
measurement.
The data shown in Table 7.2 is taken by measuring 3 TACH edges or 1/2 of a single rotation. If the
TACH signal is seen as a square wave with a fixed duty cycle and variable period (where period is
inversely proportional to the fan speed), then 3 edges would coincide with a single TACH pulse.
Therefore, in order to accurately measure a TACH signal using 3 edges, the PWM must be high for
at least as long as the full period of the TACH signal.
For example, if the PWM frequency is 87.7Hz (1st row), then the maximum measurement time
available is 11.36msec at 100% duty cycle. This implies, that the maximum period that can be
measured is also 11.36msec. Because period is inversely proportional to fan speed and 1 full period
is equivalent to 1/2 a single rotation, this means that the minimum fan speed that can be detected at
100% duty cycle is:
-----------¹------------
11.36ms
×
1_--
2
rotation
×
1_----0_---0_---0_----m_------s_-
s
×
-⁶---⁰----^s-
min
=
2641
The values shown in Table 7.2 and Table 7.3 include a guard time that occurs immediately after the
PWM is set high where the TACH pulse is not measured. The effect of this guard time is to reduce
the effective “on” time of the PWM with respect to measuring a TACH pulse.
Table 7.2 Minimum RPM Detectable Using 3 Edges
PWM
FREQUENCY
(HZ)
87.7
58.6
44
35.2
29.3
21.9
14.6
11
PULSE WIDTH AT DUTY CYCLE
(PWM ”ON” TIME)
25%
(MSEC)
50%
(MSEC)
100%
(MSEC)
(Note 7.4)
2.85
4.27
5.68
7.1
8.53
11.42
17.12
22.73
5.7
8.53
11.36
14.2
17.06
22.83
34.25
45.45
11.36
17
22.64
28.3
34
45.48
68.23
90.55
MINIMUM RPM AT DUTY CYCLE (Note 7.5)
(30/T_TachPulse)
25%
10865
7175
5366
4279
3554
2648
1761
1325
50%
5347
3554
2662
2126
1768
1319
878
661
100%
2662
1774
1330
1063
885
661
440
332
Revision 0.32 (06-23-08)
42
DATASHEET
SMSC EMC2300