
- 24 - 
 Andigilog, Inc. 2006 
www.andigilog.com
August 2006 - 70A05003 
aSC7512
 Register 54-55h: Fan Tachometer Minimum 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 
55h 
R/W 
Tach Minimum LSB 
Tach Minimum MSB 
7 
15 
6 
14 
5 
13 
4 
12 
3 
11 
2 
10 
1 
9 
0 
8 
FF 
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 is 00h or if the fan 1 disabled via the Fan 
Configuration Register.  Interrupts will never be generated for a fan if its minimum is set to FF FFh. 
Given the relative insignificance of Bit 0 and Bit 1 of Tach Minimum LSB, 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 
Chassis Rear 
1 
1 
0 
1 
Table 14 Fan Location 
Setting the Ready/Lock/Start/Override register Lock bit has no effect on these registers. 
 Register 5Ch: Fan Thermal Zone Assignment and Spin-up Mode 
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 
5Ch 
R/W 
Fan Configuration 
ZON2 
ZON1 
ZON0 
INV 
RES 
SPIN2 
SPIN1 
SPIN0 
62 
X 
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.  After power up the default value is used whenever the Ready/Lock/Start/Override 
register Start bit is cleared even though modifications to this register are possible. 
Bits [7:5] Zone/Mode 
Bits [7:5] of the Fan Configuration registers associate each fan with a temperature sensor.  When in Auto Fan Mode the fan 
will be assigned to a zone, and its PWM duty cycle will be adjusted according to the temperature of that zone.  If “Hottest” 
option is selected (110), the fan will be controlled by the the hotter of zones 1, or 2.  To determine the “hotter zone”, the 
PWM level for each zone is calculated then the zone with the higher PWM value (not temperature) is selected.  When in 
manual control mode, the Current PWM duty register (30h) become Read/Write.  It is then possible to control the PWM 
outputs with software by writing to these registers.  When the fan is disabled (100) the corresponding PWM output should be 
driven low (or high, if inverted). 
Zone 1: External Diode (processor) 
Zone 2: Internal Sensor