
P
P
- 19 - 
 Andigilog, Inc. 2006 
www.andigilog.com
October 2006 - 70A05007 
aSC7611
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 
2Dh 
R 
Tach 3 MS Byte 
15 
14 
13 
12 
11 
10 
9 
8 
N/A 
2Eh 
2Fh 
R 
R 
Tach 4 LS Byte 
Tach 4 MS Byte 
7 
15 
6 
14 
5 
13 
4 
12 
3 
11 
2 
10 
1 
9 
0 
8 
N/A 
N/A 
The Fan Tachometer Reading registers contains the number of 11.111
μ
s periods (90 kHz) between full fan revolutions.  The 
results are based on the time interval of two tachometer pulses, since most fans produce two tachometer pulses per full 
revolution.  These registers will be updated at least once every second. Common interpretation of tachometer readings is to 
take the binary period measurement and convert it to RPM. This may be done by applying the formula:  
RPM = (90,000 x 60)/(Decimal Equivalent of binary Tach Reading) 
The value, for each fan, is represented by a 16-bit unsigned number. 
The Fan Tachometer Reading registers will always return an accurate fan tachometer measurement, even when a fan is 
disabled or non-functional, however, if PWM commands for a fan (register 30h to 32h) is zero, tach measurements are 
suspended and the last reading may remain in the register. 
FF FFh indicates that the fan is not spinning, or that the tachometer input is not connected to a valid signal. This value may 
be FF FEh or FF FCh if Measurement Duration, bits 1:0 of register 3A-3Dh are set to 01 or 00, respectively. These registers 
are read-only – a write to these registers has no effect. 
When the LSByte of the aSC7611 16-bit register is read, the other byte (MSByte) is latched at the current value until it is 
read.  At the end of the MSByte read the Fan Tachometer Reading registers are updated. During spin-up, the PWM duty 
cycle reported is 0%. 
Registers 54-5Bh: Fan Tachometer Limits 
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 
54h 
R/W 
Tach 1 Minimum 
LS Byte 
7 
6 
5 
4 
3 
2 
1 
0 
FF 
55h 
R/W 
Tach 1 Minimum 
MS Byte 
15 
14 
13 
12 
11 
10 
9 
8 
FF 
56h 
R/W 
Tach 2 Minimum 
LS Byte 
7 
6 
5 
4 
3 
2 
1 
0 
FF 
57h 
R/W 
Tach 2 Minimum 
MS Byte 
15 
14 
13 
12 
11 
10 
9 
8 
FF 
58h 
R/W 
Tach 3 Minimum 
LS Byte 
Tach 3 Minimum 
MS Byte 
7 
6 
5 
4 
3 
2 
1 
0 
FF 
59h 
R/W 
15 
14 
13 
12 
11 
10 
9 
8 
FF 
5Ah 
R/W 
Tach 4 Minimum 
LS Byte 
7 
6 
5 
4 
3 
2 
1 
0 
FF 
5Bh 
R/W 
Tach 4 Minimum 
MS Byte 
15 
14 
13 
12 
11 
10 
9 
8 
FF 
The Fan Tachometer Low Limit registers indicate the tachometer reading under which the corresponding bit will be set in the 
Interrupt Status Register 2 register.  In Auto Fan Control mode, the fan can run at low speeds, so care should be taken in 
software to ensure that the limit is high enough not to cause sporadic alerts.  The fan tachometer will not cause a bit to be 
set in Interrupt Status Register 2 if the current value in Current PWM Duty registers (30h to 32h) is 00h or if the fan is 
disabled via the Fan Configuration Register.  Interrupts will not be generated for a fan if its minimum is set to FF FFh except 
for timeout. Setting the Ready/Lock/Start/Override register Lock bit has no effect on these registers. 
Given the relative insignificance of Bit 0 and Bit 1, these bits could be programmed to designate the physical location of the 
fan generating the tachometer signal, as follows: 
Register Name 
Bit 1 
Bit 0 (LSB) 
CPU Cooler 
0 
0 
Memory Controller 
0 
1 
Chassis Front 
1 
0 
Chassis Rear 
1 
1