AN17.4

AN 17.4
RPM to TACH Counts
Conversion
1 Preface
This application note provides look up tables for the calculation of RPM to TACH Counts for use with
the EMC2103, EMC2104, EMC2105, EMC2106, EMC2112, EMC2113, EMC2301, EMC2302,
EMC2303, and EMC2305.
2 Audience
This application note assumes that the reader is familiar with the basic controls and functionality of
the automatic Fan Speed Control Algorithm (FSC) used in various SMSC fan controllers. Furthermore,
it assumes that readers have configured at least one of these SMSC devices correctly to operate the
specific fan in their systems.
3 Overview
The FSC Algorithm used by the referenced devices uses a sophisticated algorithm to determine the
fan speed and maintain it at high accuracy. The relationship between the actual fan speed and the
presented data contains scaling factors that are determined directly by the fan settings. This
application note shows simple tables for each of the selectable ranges of fan speed operation so that
fast and simple data manipulation can be accomplished.
4 References
The following documents should be referenced when using this application note:
SMSC AN 17.4
„
SMSC EMC2103 Datasheet
„
SMSC EMC2104 Datasheet
„
SMSC EMC2105 Datasheet
„
SMSC EMC2106 Datasheet
„
SMSC EMC2112 Datasheet
„
SMSC EMC2113 Datasheet
„
SMSC EMC2301 Datasheet
„
SMSC EMC2302 Datasheet
„
SMSC EMC2303 Datasheet
„
SMSC EMC2305 Datasheet
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
5 RPM to TACH Count Conversion
The conversion from RPM to TACH Counts is calculated mathematically by Equation [1]. The equation
is simplified if the number of edges measured (n) is set based on the number of poles of the fan. This
simplified equation is shown in Equation [2] and was used to generate Table 7.2 through Table 7.5
based on the programmed range setting “m”.
where:
(n – 1)
1
RPM_R = ------------------ × --------------------------------- × f TACH × 60
1
( poles )
COUNT × ---m
poles = number of poles of the fan
(typically 2)
fTACH = The frequency of the clock
(default = 32.768kHz)
[1]
n = number of edges measured
(typically 5 for a 2 pole fan)
m = the multiplier defined by the
RANGE bits
3,932,160 × m
RPM_R = ----------------------------------COUNT
COUNT = TACH Reading Register
value (in decimal)
[2]
RPM_R = Reported fan speed
The COUNT value represents the decimal weighting of all bits of interest. If only 8-bit data is used,
then the resolution of COUNT will be as shown in Table 7.2 through Table 7.5. If 13-bit data is used,
then the resolution will be 1 count which may represent < 1 RPM depending on fan speed and range.
The “m” term is determined based on the RANGE[1:0] settings selected and will vary from a value of
“1” to a value of “8”. This term is independent of the actual fan operation. It affects the data
presentation as well as the range and resolution of the measurement.
The RANGE[1:0] settings determine the lowest fan speed that the device can measure. For the best
measurement range and resolution, the RANGE[1:0] settings should be set at the largest setting that
is below any expected fan speed used by the fan. For example, if a fan is expected to operate from
1800RPM to 4000RPM, then the RANGE[1:0] settings should be set at 1000RPM.
Equation [2] is only valid when the number of edges measured (n) is equal to 2*poles + 1. If this
condition is not met, then the reported fan speed and the actual fan speed will differ by a factor “y”.
This relationship is shown in Equation [3].
For example, if a 2-pole fan is being measured using 7 edges then the “y” term will be equal to 1.5.
The device will measure 7 edges to determine the fan speed instead of 5. For a 3000 RPM fan, the
reported TACH Count will read 3936d (7Bh) which coincides with 2000 RPM using Equation [2] instead
of the actual fan speed of 3000 RPM.
where:
RPM_A = RPM_R × y
[3]
RPM_A = Actual fan speed
RPM_R = Reported fan speed
calculated via Equations 1 or 2
(n – 1)
y = ---------------------------( 2 × poles )
n = number of edges measured
[4]
poles = number poles of the fan
Revision 0.40 (05-04-09)
2
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
6 Calculation Examples
Both of the examples below assume that a perfect 32.768kHz clock is available and used such that
the measurement will have no error. This is for example purposes only and may not be indicative of
actual performance. For the EMC2103 and EMC2113 devices, there is no port for an external
32.768kHz clock; therefore, the accuracy portrayed in these examples will be reduced.
6.1
Example #1
A user using the EMC2106 decides that he wishes to operate the fan as close to 6275 RPM as
possible. He knows that it is a 2-pole fan, so he set the EDGES[1:0] bits to measure 5 edges (which
is the default setting and recommended for a 2-pole fan).
The user doesn’t expect the fan to operate below 2200 RPM so sets the RANGE[1:0] bits to 2000
RPM.
The user has the following terms: n = 5; poles = 2; m = 4; fTACH = 32.768Hz.
Putting these terms into Equation [1] collapses it down to Equation [2]. In order to determine what the
necessary COUNT value should be, he will solve this equation for COUNT resulting in:
where:
RPM_R = Reported fan speed
calculated via Equations 1 or 2
3,932,160 × m
COUNT = ----------------------------------RPM_R
[5]
COUNT = TACH Target Register
settings
Using Equation [3] and Equation [4] the user calculates that the “y” term is equal to 1 and that the
reported RPM will be equal to the actual RPM so no further manipulation must be performed.
Then, substituting 6275 for RPM_R in Equation [5], the user calculates that the decimal equivalent
COUNT value will be 2,506.556 rounded up to 2,507.
6.1.1
TACH Target High Byte (8-bit TACH)
By examining Table 7.4 it is clear that the closest High Byte setting is 4Eh resulting in a target RPM
of 6302. The mathematics behind this result are explained below.
Determining the High Byte value is done sequentially starting with the MSB. Take the target count and
subtract the bit weighting of the highest bit (4096). If the result is positive (i.e. > 0) then the
corresponding bit should be set and the difference used for subsequent calculations. If the difference
is negative, then the corresponding bit should be cleared and the previous value used for future
calculations.
Bit 7 = 4096. 2507 - 4096 = -1589. This is less than 0 therefore Bit 7 should be set to ‘0’. The new
target count is equal to the previous target count = 2507.
Bit 6 = 2048. 2507 - 2048 = 459. This is greater than 0 therefore Bit 6 should be set to ‘1’. Because
the bit was set, the new target count is equal to the difference = 459.
Bit 5 = 1024. 459 - 1024 = -565. This is less than 0 therefore Bit 5 should be set to ‘0’. The new target
count is equal to the previous target count = 459.
Bit 4 = 512. 459 - 512 = -53. This is less than 0 therefore Bit 4 should be set to ‘0’. The new target
count is equal to the previous target count = 459.
Bit 3 = 256. 459 - 256 = 203. This is greater than 0 therefore Bit 3 should be set to ‘1’. Because the
bit was set, the new target count is equal to the difference = 203.
SMSC AN 17.4
3
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
Bit 2 = 128. 203 - 128 = 75. This is greater than 0 therefore Bit 2 should be set to ‘1’. Because the
bit was set, the new target is equal to the difference = 75.
Bit 1 = 64. 75 - 64 = 11. This is greater than 0 therefore Bit 1 should be set to ‘1. Because the bit was
set, the new target is equal the difference = 11.
Bit 0 = 32. 11 - 32 = -21. This is less than 0 therefore Bit 0 should be set to 0.
Capturing which bits were set gives a High Byte setting of 0100_1110 (4Eh). Summing the bit weighting
of the set bits gives a decimal Count value of 2496. From Table 7.4, this coincides with 6,302 RPM.
The programmed Count value is off of the target by 11 counts and the RPM value is off by 27 RPM.
Moreover, the resolution of an 8-bit TACH Target (in that region of operation) is approximately 80 RPM
so that the target cannot be reached or maintained but the resolution may be sufficiently close for the
application.
6.1.2
TACH Target Low Byte (13-bit TACH)
If additional accuracy or resolution is required, then the Low Byte can be programmed. This will
improve accuracy and resolution of the fan driver.
Determining the Low Byte value is done by extending the calculation used to determine the high byte.
The sum of the High Byte was 2496. The difference between the target count and the High Byte is
2507 - 2469 = 11. Bit 7 has a weighting of 16 and 11 - 16 = -5 therefore the 7th bit of the Low Byte
is ‘0’. Bit 6 has a weighting of 8 and 11 - 8 = 3 therefore the 6th bit of the Low Byte is ‘1’. Bit 5 has
a weighting of 4 and 3 - 4 = -1 therefore the 5th bit of the Low Byte is ‘0’. Bit 4 has a weighting of 2
and 3 - 2 = 1 therefore the 4th bit of the Low Byte is ‘1’. Finally Bit 3 has a weighting of 1 and 1 - 1
= 0 therefore the 3rd bit is ‘1’. The lower three bits of the Low Byte are not used and are always ‘0’.
Capturing which bits are set gives a Low Byte setting of 0101_1000b (58h) which is equivalent to
decimal 11. Summing the Low Byte and High Byte together makes the total TACH Target COUNT =
2507 which forces the fan driver to drive the tachometer reading to 6,273.9 RPM rounded up to 6,274
RPM (only 1 RPM from the target). Moreover, the resolution of the RPM is approximately 2.5 RPM
which means that, with an ideal clock, the tachometer target can be maintained to within -3.5 to +1.5
RPM.
6.2
Example #2
Using the same fan parameters as Example #1, the user wants to make sure that the fan is spinning
at the expected value of 8,080RPM.
6.2.1
TACH Reading High Byte (8-bit TACH)
First, the user reads only the high byte value and returns 0011_1100b (3Ch). Using this value, he
determines that the fan speed is 8,192 RPM with a resolution of ~135 RPM. This means that the fan
can be spinning anywhere from 8,326 RPM to 8,057 RPM without toggling the LSBit of the high byte.
Depending on the application this precision may not be sufficient.
6.2.2
TACH Reading Low Byte (13-bit TACH)
In order for the user to determine that the fan is spinning at the desired speed, he must read the full
13-bit tachometer value. Reading the TACH Reading Low Byte yields a value of to 1101_0000b (D0h).
Applying the bit weighting as defined in the EMC2106 Datasheet, the decimal equivalent COUNT is:
High Byte = 0011_1100b = 1024 + 512 + 256 + 128 = 1,920.
Low Byte = 1101_0000b = 16 + 8 + 2 = 26.
COUNT = High Byte + Low Byte = 1,946.
Revision 0.40 (05-04-09)
4
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Calculating the RPM using Equation [2] yields an RPM value of 8,082.54 RPM rounded up to 8,083
RPM with a resolution of +/- 4 RPM. Therefore, the user knows that the fan is spinning between 8,080
RPM and 8,086 RPM.
7 RPM to TACH Look Up Tables
Note that all of the following look up tables are built using only the High Byte of the TACH Reading.
The TACH Reading Low Byte is assumed to be 00h for all entries.
All data points were calculated using Equation [2] shown above with the “m” term set as shown in
Table 7.1. It is assumed that the number of poles and the number of edges match such that the “y”
term is equal to 1.
A value of FFh in the TACH Reading High Byte register is assumed to mean that the fan is not spinning
and a Stalled Fan interrupt will be generated. Writing a value of FFh into the TACH Target High Byte
register will cause the fan to be disabled and it will not flag any tachometer related interrupts.
Table 7.1 “m Factor
SMSC AN 17.4
RANGE
“m” FACTOR
500 RPM
1
1000 RPM
2
2000 RPM
4
4000 RPM
8
5
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
7.1
500 RPM Range
Table 7.2 Tachometer Count to RPM Look Up Table (Range = 500 RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
0
00
Disabled
4096
80
960
32
01
122880
4128
81
953
64
02
61440
4160
82
945
96
03
40960
4192
83
938
128
04
30720
4224
84
931
160
05
24576
4256
85
924
192
06
20480
4288
86
917
224
07
17554
4320
87
910
256
08
15360
4352
88
904
288
09
13653
4384
89
897
320
0A
12288
4416
8A
890
352
0B
11171
4448
8B
884
384
0C
10240
4480
8C
878
416
0D
9452
4512
8D
871
448
0E
8777
4544
8E
865
480
0F
8192
4576
8F
859
512
10
7680
4608
90
853
544
11
7228
4640
91
847
576
12
6827
4672
92
842
608
13
6467
4704
93
836
640
14
6144
4736
94
830
672
15
5851
4768
95
825
704
16
5585
4800
96
819
736
17
5343
4832
97
814
768
18
5120
4864
98
808
800
19
4915
4896
99
803
832
1A
4726
4928
9A
798
864
1B
4551
4960
9B
793
896
1C
4389
4992
9C
788
928
1D
4237
5024
9D
783
960
1E
4096
5056
9E
778
992
1F
3964
5088
9F
773
1024
20
3840
5120
A0
768
1056
21
3724
5152
A1
763
Revision 0.40 (05-04-09)
6
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Table 7.2 Tachometer Count to RPM Look Up Table (Range = 500 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
1088
22
3614
5184
A2
759
1120
23
3511
5216
A3
754
1152
24
3413
5248
A4
749
1184
25
3321
5280
A5
745
1216
26
3234
5312
A6
740
1248
27
3151
5344
A7
736
1280
28
3072
5376
A8
731
1312
29
2997
5408
A9
727
1344
2A
2926
5440
AA
723
1376
2B
2858
5472
AB
719
1408
2C
2793
5504
AC
714
1440
2D
2731
5536
AD
710
1472
2E
2671
5568
AE
706
1504
2F
2614
5600
AF
702
1536
30
2560
5632
B0
698
1568
31
2508
5664
B1
694
1600
32
2458
5696
B2
690
1632
33
2409
5728
B3
686
1664
34
2363
5760
B4
683
1696
35
2318
5792
B5
679
1728
36
2276
5824
B6
675
1760
37
2234
5856
B7
671
1792
38
2194
5888
B8
668
1824
39
2156
5920
B9
664
1856
3A
2119
5952
BA
661
1888
3B
2083
5984
BB
657
1920
3C
2048
6016
BC
654
1952
3D
2014
6048
BD
650
1984
3E
1982
6080
BE
647
2016
3F
1950
6112
BF
643
2048
40
1920
6144
C0
640
2080
41
1890
6176
C1
637
2112
42
1862
6208
C2
633
2144
43
1834
6240
C3
630
2176
44
1807
6272
C4
627
2208
45
1781
6304
C5
624
SMSC AN 17.4
7
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
Table 7.2 Tachometer Count to RPM Look Up Table (Range = 500 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
2240
46
1755
6336
C6
621
2272
47
1731
6368
C7
617
2304
48
1707
6400
C8
614
2336
49
1683
6432
C9
611
2368
4A
1661
6464
CA
608
2400
4B
1638
6496
CB
605
2432
4C
1617
6528
CC
602
2464
4D
1596
6560
CD
599
2496
4E
1575
6592
CE
597
2528
4F
1555
6624
CF
594
2560
50
1536
6656
D0
591
2592
51
1517
6688
D1
588
2624
52
1499
6720
D2
585
2656
53
1480
6752
D3
582
2688
54
1463
6784
D4
580
2720
55
1446
6816
D5
577
2752
56
1429
6848
D6
574
2784
57
1412
6880
D7
572
2816
58
1396
6912
D8
569
2848
59
1381
6944
D9
566
2880
5A
1365
6976
DA
564
2912
5B
1350
7008
DB
561
2944
5C
1336
7040
DC
559
2976
5D
1321
7072
DD
556
3008
5E
1307
7104
DE
554
3040
5F
1293
7136
DF
551
3072
60
1280
7168
E0
549
3104
61
1267
7200
E1
546
3136
62
1254
7232
E2
544
3168
63
1241
7264
E3
541
3200
64
1229
7296
E4
539
3232
65
1217
7328
E5
537
3264
66
1205
7360
E6
534
3296
67
1193
7392
E7
532
3328
68
1182
7424
E8
530
3360
69
1170
7456
E9
527
Revision 0.40 (05-04-09)
8
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Table 7.2 Tachometer Count to RPM Look Up Table (Range = 500 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
3392
6A
1159
7488
EA
525
3424
6B
1148
7520
EB
523
3456
6C
1138
7552
EC
521
3488
6D
1127
7584
ED
518
3520
6E
1117
7616
EE
516
3552
6F
1107
7648
EF
514
3584
70
1097
7680
F0
512
3616
71
1087
7712
F1
510
3648
72
1078
7744
F2
508
3680
73
1069
7776
F3
506
3712
74
1059
7808
F4
504
3744
75
1050
7840
F5
502
3776
76
1041
7872
F6
500
3808
77
1033
7904
F7
497
3840
78
1024
7936
F8
495
3872
79
1016
7968
F9
493
3904
7A
1007
8000
FA
492
3936
7B
999
8032
FB
490
3968
7C
991
8064
FC
488
4000
7D
983
8096
FD
486
4032
7E
975
8128
FE
484
4064
7F
968
8160
FF
off / not
spinning
SMSC AN 17.4
9
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
7.2
1k RPM Range
Table 7.3 Tachometer Count to RPM Look Up Table (Range = 1000 RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
0
00
Disabled
4096
80
1920
32
01
245760
4128
81
1905
64
02
122880
4160
82
1890
96
03
81920
4192
83
1876
128
04
61440
4224
84
1862
160
05
49152
4256
85
1848
192
06
40960
4288
86
1834
224
07
35109
4320
87
1820
256
08
30720
4352
88
1807
288
09
27307
4384
89
1794
320
0A
24576
4416
8A
1781
352
0B
22342
4448
8B
1768
384
0C
20480
4480
8C
1755
416
0D
18905
4512
8D
1743
448
0E
17554
4544
8E
1731
480
0F
16384
4576
8F
1719
512
10
15360
4608
90
1707
544
11
14456
4640
91
1695
576
12
13653
4672
92
1683
608
13
12935
4704
93
1672
640
14
12288
4736
94
1661
672
15
11703
4768
95
1649
704
16
11171
4800
96
1638
736
17
10685
4832
97
1628
768
18
10240
4864
98
1617
800
19
9830
4896
99
1606
832
1A
9452
4928
9A
1596
864
1B
9102
4960
9B
1586
896
1C
8777
4992
9C
1575
928
1D
8474
5024
9D
1565
960
1E
8192
5056
9E
1555
992
1F
7928
5088
9F
1546
1024
20
7680
5120
A0
1536
1056
21
7447
5152
A1
1526
Revision 0.40 (05-04-09)
10
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Table 7.3 Tachometer Count to RPM Look Up Table (Range = 1000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
1088
22
7228
5184
A2
1517
1120
23
7022
5216
A3
1508
1152
24
6827
5248
A4
1499
1184
25
6642
5280
A5
1489
1216
26
6467
5312
A6
1480
1248
27
6302
5344
A7
1472
1280
28
6144
5376
A8
1463
1312
29
5994
5408
A9
1454
1344
2A
5851
5440
AA
1446
1376
2B
5715
5472
AB
1437
1408
2C
5585
5504
AC
1429
1440
2D
5461
5536
AD
1421
1472
2E
5343
5568
AE
1412
1504
2F
5229
5600
AF
1404
1536
30
5120
5632
B0
1396
1568
31
5016
5664
B1
1388
1600
32
4915
5696
B2
1381
1632
33
4819
5728
B3
1373
1664
34
4726
5760
B4
1365
1696
35
4637
5792
B5
1358
1728
36
4551
5824
B6
1350
1760
37
4468
5856
B7
1343
1792
38
4389
5888
B8
1336
1824
39
4312
5920
B9
1328
1856
3A
4237
5952
BA
1321
1888
3B
4165
5984
BB
1314
1920
3C
4096
6016
BC
1307
1952
3D
4029
6048
BD
1300
1984
3E
3964
6080
BE
1293
2016
3F
3901
6112
BF
1287
2048
40
3840
6144
C0
1280
2080
41
3781
6176
C1
1273
2112
42
3724
6208
C2
1267
2144
43
3668
6240
C3
1260
2176
44
3614
6272
C4
1254
2208
45
3562
6304
C5
1248
SMSC AN 17.4
11
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
Table 7.3 Tachometer Count to RPM Look Up Table (Range = 1000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
2240
46
3511
6336
C6
1241
2272
47
3461
6368
C7
1235
2304
48
3413
6400
C8
1229
2336
49
3367
6432
C9
1223
2368
4A
3321
6464
CA
1217
2400
4B
3277
6496
CB
1211
2432
4C
3234
6528
CC
1205
2464
4D
3192
6560
CD
1199
2496
4E
3151
6592
CE
1193
2528
4F
3111
6624
CF
1187
2560
50
3072
6656
D0
1182
2592
51
3034
6688
D1
1176
2624
52
2997
6720
D2
1170
2656
53
2961
6752
D3
1165
2688
54
2926
6784
D4
1159
2720
55
2891
6816
D5
1154
2752
56
2858
6848
D6
1148
2784
57
2825
6880
D7
1143
2816
58
2793
6912
D8
1138
2848
59
2761
6944
D9
1133
2880
5A
2731
6976
DA
1127
2912
5B
2701
7008
DB
1122
2944
5C
2671
7040
DC
1117
2976
5D
2643
7072
DD
1112
3008
5E
2614
7104
DE
1107
3040
5F
2587
7136
DF
1102
3072
60
2560
7168
E0
1097
3104
61
2534
7200
E1
1092
3136
62
2508
7232
E2
1087
3168
63
2482
7264
E3
1083
3200
64
2458
7296
E4
1078
3232
65
2433
7328
E5
1073
3264
66
2409
7360
E6
1069
3296
67
2386
7392
E7
1064
3328
68
2363
7424
E8
1059
3360
69
2341
7456
E9
1055
Revision 0.40 (05-04-09)
12
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Table 7.3 Tachometer Count to RPM Look Up Table (Range = 1000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
3392
6A
2318
7488
EA
1050
3424
6B
2297
7520
EB
1046
3456
6C
2276
7552
EC
1041
3488
6D
2255
7584
ED
1037
3520
6E
2234
7616
EE
1033
3552
6F
2214
7648
EF
1028
3584
70
2194
7680
F0
1024
3616
71
2175
7712
F1
1020
3648
72
2156
7744
F2
1016
3680
73
2137
7776
F3
1011
3712
74
2119
7808
F4
1007
3744
75
2101
7840
F5
1003
3776
76
2083
7872
F6
999
3808
77
2065
7904
F7
995
3840
78
2048
7936
F8
991
3872
79
2031
7968
F9
987
3904
7A
2014
8000
FA
983
3936
7B
1998
8032
FB
979
3968
7C
1982
8064
FC
975
4000
7D
1966
8096
FD
971
4032
7E
1950
8128
FE
968
4064
7F
8160
FF
off / not
spinning
SMSC AN 17.4
1935
13
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
7.3
2k RPM Range
Table 7.4 Tachometer Count to RPM Look Up Table (Range = 2000 RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
0
00
Disabled
4096
80
3840
32
01
491520
4128
81
3810
64
02
245760
4160
82
3781
96
03
163840
4192
83
3752
128
04
122880
4224
84
3724
160
05
98304
4256
85
3696
192
06
81920
4288
86
3668
224
07
70217
4320
87
3641
256
08
61440
4352
88
3614
288
09
54613
4384
89
3588
320
0A
49152
4416
8A
3562
352
0B
44684
4448
8B
3536
384
0C
40960
4480
8C
3511
416
0D
37809
4512
8D
3486
448
0E
35109
4544
8E
3461
480
0F
32768
4576
8F
3437
512
10
30720
4608
90
3413
544
11
28913
4640
91
3390
576
12
27307
4672
92
3367
608
13
25869
4704
93
3344
640
14
24576
4736
94
3321
672
15
23406
4768
95
3299
704
16
22342
4800
96
3277
736
17
21370
4832
97
3255
768
18
20480
4864
98
3234
800
19
19661
4896
99
3213
832
1A
18905
4928
9A
3192
864
1B
18204
4960
9B
3171
896
1C
17554
4992
9C
3151
928
1D
16949
5024
9D
3131
960
1E
16384
5056
9E
3111
992
1F
15855
5088
9F
3091
1024
20
15360
5120
A0
3072
1056
21
14895
5152
A1
3053
Revision 0.40 (05-04-09)
14
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Table 7.4 Tachometer Count to RPM Look Up Table (Range = 2000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
1088
22
14456
5184
A2
3034
1120
23
14043
5216
A3
3015
1152
24
13653
5248
A4
2997
1184
25
13284
5280
A5
2979
1216
26
12935
5312
A6
2961
1248
27
12603
5344
A7
2943
1280
28
12288
5376
A8
2926
1312
29
11988
5408
A9
2908
1344
2A
11703
5440
AA
2891
1376
2B
11431
5472
AB
2874
1408
2C
11171
5504
AC
2858
1440
2D
10923
5536
AD
2841
1472
2E
10685
5568
AE
2825
1504
2F
10458
5600
AF
2809
1536
30
10240
5632
B0
2793
1568
31
10031
5664
B1
2777
1600
32
9830
5696
B2
2761
1632
33
9638
5728
B3
2746
1664
34
9452
5760
B4
2731
1696
35
9274
5792
B5
2716
1728
36
9102
5824
B6
2701
1760
37
8937
5856
B7
2686
1792
38
8777
5888
B8
2671
1824
39
8623
5920
B9
2657
1856
3A
8474
5952
BA
2643
1888
3B
8331
5984
BB
2628
1920
3C
8192
6016
BC
2614
1952
3D
8058
6048
BD
2601
1984
3E
7928
6080
BE
2587
2016
3F
7802
6112
BF
2573
2048
40
7680
6144
C0
2560
2080
41
7562
6176
C1
2547
2112
42
7447
6208
C2
2534
2144
43
7336
6240
C3
2521
2176
44
7228
6272
C4
2508
2208
45
7123
6304
C5
2495
SMSC AN 17.4
15
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
Table 7.4 Tachometer Count to RPM Look Up Table (Range = 2000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
2240
46
7022
6336
C6
2482
2272
47
6923
6368
C7
2470
2304
48
6827
6400
C8
2458
2336
49
6733
6432
C9
2445
2368
4A
6642
6464
CA
2433
2400
4B
6554
6496
CB
2421
2432
4C
6467
6528
CC
2409
2464
4D
6383
6560
CD
2398
2496
4E
6302
6592
CE
2386
2528
4F
6222
6624
CF
2374
2560
50
6144
6656
D0
2363
2592
51
6068
6688
D1
2352
2624
52
5994
6720
D2
2341
2656
53
5922
6752
D3
2329
2688
54
5851
6784
D4
2318
2720
55
5783
6816
D5
2308
2752
56
5715
6848
D6
2297
2784
57
5650
6880
D7
2286
2816
58
5585
6912
D8
2276
2848
59
5523
6944
D9
2265
2880
5A
5461
6976
DA
2255
2912
5B
5401
7008
DB
2244
2944
5C
5343
7040
DC
2234
2976
5D
5285
7072
DD
2224
3008
5E
5229
7104
DE
2214
3040
5F
5174
7136
DF
2204
3072
60
5120
7168
E0
2194
3104
61
5067
7200
E1
2185
3136
62
5016
7232
E2
2175
3168
63
4965
7264
E3
2165
3200
64
4915
7296
E4
2156
3232
65
4867
7328
E5
2146
3264
66
4819
7360
E6
2137
3296
67
4772
7392
E7
2128
3328
68
4726
7424
E8
2119
3360
69
4681
7456
E9
2110
Revision 0.40 (05-04-09)
16
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Table 7.4 Tachometer Count to RPM Look Up Table (Range = 2000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
3392
6A
4637
7488
EA
2101
3424
6B
4594
7520
EB
2092
3456
6C
4551
7552
EC
2083
3488
6D
4509
7584
ED
2074
3520
6E
4468
7616
EE
2065
3552
6F
4428
7648
EF
2057
3584
70
4389
7680
F0
2048
3616
71
4350
7712
F1
2040
3648
72
4312
7744
F2
2031
3680
73
4274
7776
F3
2023
3712
74
4237
7808
F4
2014
3744
75
4201
7840
F5
2006
3776
76
4165
7872
F6
1998
3808
77
4130
7904
F7
1990
3840
78
4096
7936
F8
1982
3872
79
4062
7968
F9
1974
3904
7A
4029
8000
FA
1966
3936
7B
3996
8032
FB
1958
3968
7C
3964
8064
FC
1950
4000
7D
3932
8096
FD
1943
4032
7E
3901
8128
FE
1935
4064
7F
8160
FF
off / not
spinning
SMSC AN 17.4
3870
17
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
7.4
4k RPM Range
Table 7.5 Tachometer Count to RPM Look Up Table (Range = 4000 RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
0
00
Disabled
4096
80
7680
32
01
983040
4128
81
7620
64
02
491520
4160
82
7562
96
03
327680
4192
83
7504
128
04
245760
4224
84
7447
160
05
196608
4256
85
7391
192
06
163840
4288
86
7336
224
07
140434
4320
87
7282
256
08
122880
4352
88
7228
288
09
109227
4384
89
7175
320
0A
98304
4416
8A
7123
352
0B
89367
4448
8B
7072
384
0C
81920
4480
8C
7022
416
0D
75618
4512
8D
6972
448
0E
70217
4544
8E
6923
480
0F
65536
4576
8F
6874
512
10
61440
4608
90
6827
544
11
57826
4640
91
6780
576
12
54613
4672
92
6733
608
13
51739
4704
93
6687
640
14
49152
4736
94
6642
672
15
46811
4768
95
6598
704
16
44684
4800
96
6554
736
17
42741
4832
97
6510
768
18
40960
4864
98
6467
800
19
39322
4896
99
6425
832
1A
37809
4928
9A
6383
864
1B
36409
4960
9B
6342
896
1C
35109
4992
9C
6302
928
1D
33898
5024
9D
6261
960
1E
32768
5056
9E
6222
992
1F
31711
5088
9F
6183
1024
20
30720
5120
A0
6144
1056
21
29789
5152
A1
6106
Revision 0.40 (05-04-09)
18
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Table 7.5 Tachometer Count to RPM Look Up Table (Range = 4000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
1088
22
28913
5184
A2
6068
1120
23
28087
5216
A3
6031
1152
24
27307
5248
A4
5994
1184
25
26569
5280
A5
5958
1216
26
25869
5312
A6
5922
1248
27
25206
5344
A7
5886
1280
28
24576
5376
A8
5851
1312
29
23977
5408
A9
5817
1344
2A
23406
5440
AA
5783
1376
2B
22861
5472
AB
5749
1408
2C
22342
5504
AC
5715
1440
2D
21845
5536
AD
5682
1472
2E
21370
5568
AE
5650
1504
2F
20916
5600
AF
5617
1536
30
20480
5632
B0
5585
1568
31
20062
5664
B1
5554
1600
32
19661
5696
B2
5523
1632
33
19275
5728
B3
5492
1664
34
18905
5760
B4
5461
1696
35
18548
5792
B5
5431
1728
36
18204
5824
B6
5401
1760
37
17873
5856
B7
5372
1792
38
17554
5888
B8
5343
1824
39
17246
5920
B9
5314
1856
3A
16949
5952
BA
5285
1888
3B
16662
5984
BB
5257
1920
3C
16384
6016
BC
5229
1952
3D
16115
6048
BD
5201
1984
3E
15855
6080
BE
5174
2016
3F
15604
6112
BF
5147
2048
40
15360
6144
C0
5120
2080
41
15124
6176
C1
5093
2112
42
14895
6208
C2
5067
2144
43
14672
6240
C3
5041
2176
44
14456
6272
C4
5016
2208
45
14247
6304
C5
4990
SMSC AN 17.4
19
APPLICATION NOTE
Revision 0.40 (05-04-09)
RPM to TACH Counts Conversion
Table 7.5 Tachometer Count to RPM Look Up Table (Range = 4000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
2240
46
14043
6336
C6
4965
2272
47
13846
6368
C7
4940
2304
48
13653
6400
C8
4915
2336
49
13466
6432
C9
4891
2368
4A
13284
6464
CA
4867
2400
4B
13107
6496
CB
4843
2432
4C
12935
6528
CC
4819
2464
4D
12767
6560
CD
4795
2496
4E
12603
6592
CE
4772
2528
4F
12444
6624
CF
4749
2560
50
12288
6656
D0
4726
2592
51
12136
6688
D1
4704
2624
52
11988
6720
D2
4681
2656
53
11844
6752
D3
4659
2688
54
11703
6784
D4
4637
2720
55
11565
6816
D5
4615
2752
56
11431
6848
D6
4594
2784
57
11299
6880
D7
4572
2816
58
11171
6912
D8
4551
2848
59
11045
6944
D9
4530
2880
5A
10923
6976
DA
4509
2912
5B
10803
7008
DB
4489
2944
5C
10685
7040
DC
4468
2976
5D
10570
7072
DD
4448
3008
5E
10458
7104
DE
4428
3040
5F
10348
7136
DF
4408
3072
60
10240
7168
E0
4389
3104
61
10134
7200
E1
4369
3136
62
10031
7232
E2
4350
3168
63
9930
7264
E3
4331
3200
64
9830
7296
E4
4312
3232
65
9733
7328
E5
4293
3264
66
9638
7360
E6
4274
3296
67
9544
7392
E7
4256
3328
68
9452
7424
E8
4237
3360
69
9362
7456
E9
4219
Revision 0.40 (05-04-09)
20
APPLICATION NOTE
SMSC AN 17.4
RPM to TACH Counts Conversion
Table 7.5 Tachometer Count to RPM Look Up Table (Range = 4000 RPM) (continued)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
TACH COUNT
(DECIMAL)
TACH HIGH
BYTE (HEX)
FAN SPEED
(RPM)
3392
6A
9274
7488
EA
4201
3424
6B
9187
7520
EB
4183
3456
6C
9102
7552
EC
4165
3488
6D
9019
7584
ED
4148
3520
6E
8937
7616
EE
4130
3552
6F
8856
7648
EF
4113
3584
70
8777
7680
F0
4096
3616
71
8699
7712
F1
4079
3648
72
8623
7744
F2
4062
3680
73
8548
7776
F3
4045
3712
74
8474
7808
F4
4029
3744
75
8402
7840
F5
4012
3776
76
8331
7872
F6
3996
3808
77
8261
7904
F7
3980
3840
78
8192
7936
F8
3964
3872
79
8124
7968
F9
3948
3904
7A
8058
8000
FA
3932
3936
7B
7992
8032
FB
3916
3968
7C
7928
8064
FC
3901
4000
7D
7864
8096
FD
3886
4032
7E
7802
8128
FE
3870
4064
7F
8160
FF
off / not
spinning
SMSC AN 17.4
7740
21
APPLICATION NOTE
Revision 0.40 (05-04-09)
8 Revision History
Table 8.1 Customer Revision History
REVISION LEVEL & DATE
Rev. 0.40 (05-04-09)
SECTION/FIGURE/ENTRY
Document
CORRECTION
Added additional devices: EMC2112, EMC2113,
EMC2301, EMC2302, EMC2303, and EMC2305.
Updated title from “Fan Control Device Look Up
Tables” to “RPM TACH Counts Conversion”.
Rev. 0.39 (10-25-07)
Initial Release
80 ARKAY DRIVE, HAUPPAUGE, NY 11788 (631) 435-6000, FAX (631) 273-3123
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Revision 0.40 (05-04-09)
22
APPLICATION NOTE
SMSC AN 17.4