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ADM1031
Relevant Registers for Filtered Automatic Fan
Speed Control Mode
In addition to the registers used to program the normal
automatic fan speed control mode, the following register
values written to the fan speed configuration register and
PWM duty cycle obtained.
Table 14. PWM DUTY CYCLE SELECT MODE
needs to be programmed.
Register 0 y 23 Fan Filter Register
Hex Value
00
01
PWM Duty Cycle
0%
7%
<7>
<6:5>
<4:2>
Spin-Up Disable: When this bit is set to 1,
fan spin-up is disabled (Default = 0).
Ramp Rate: These bits set the ramp rate for
filtered mode.
00 = 1 (0.416% Duty Cycle Change)
01 = 2 (0.833% Duty Cycle Change)
10 = 4 (1.66% Duty Cycle Change)
11 = 8 (3.33% Duty Cycle Change)
ADC Sample Rate
000 = 87.5 Hz
001 = 175 Hz
010 = 350 Hz
011 = 700 Hz
100 = 1.4 kHz (Default)
101 = 2.8 kHz
110 = 5.6 kHz
111 = 11.2 kHz
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
14%
20%
27%
33%
40%
47%
53%
60%
67%
73%
80%
87%
93%
100%
<1>
<0>
Fan 2 Filter Enable: When this bit is set to 1,
it enables filtering on Fan 2. Default = 0.
Fan 1 Filter Enable: When this bit is set to 1,
it enables filtering on Fan 1. Default = 0.
Bits <3:0> set the PWM duty cycle for Fan 1; Bits <7:4>
set the PWM duty cycle for Fan 2.
RPM Feedback Mode
The second method of fan speed control under software is
Programming the Filtered Automatic Fan Speed
Control Loop
1. Program a value for T MIN.
2. Program a value for the slope T RANGE.
3. T MAX = T MIN + T RANGE.
4. Program a value for fan spin-up time.
5. Program the desired automatic fan speed control
mode behavior, that is, which temperature channel
controls the fan.
6. Program a ramp rate for the filtered mode.
7. Program the ADC sample rate in the fan filter
register.
8. Set Bit 0 to enable fan filtered mode for Fan 1.
9. Set Bit 1 to enable the fan filtered mode for Fan 2.
10. Select automatic fan speed control mode by setting
Bit 7 of Configuration Register 1.
PWM Duty Cycle Select Mode
The ADM1031 can operate under software control by
clearing Bit 7 of Configuration Register 1 (Register 0 ? 00).
This allows the user to directly control PWM duty cycle for
each fan.
Clearing Bit 5 and Bit 6 of Configuration Register 1
allows fan control by varying PWM duty cycle. Values of
duty cycle between 0% and 100% can be written to the fan
speed configuration register (0 ? 22) to control the speed of
each fan. Table 14 shows the relationship between hex
RPM feedback mode. This involves programming the
desired fan RPM value to the device to set fan speed. The
advantages include a very tightly maintained fan RPM over
the fan’s life, and virtually no acoustic pollution due to fan
speed variation.
Fans typically have manufacturing tolerances of ? 20%,
meaning a wide variation in speed for a typical batch of
identical fan models. If it is required that all fans run at
exactly 5000 RPM, it can be necessary to specify fans with
a nominal fan speed of 6250 RPM. However, many of these
fans run too fast and make excess noise. A fan with nominal
speed of 6250 RPM could run as fast as 7000 RPM at 100%
PWM duty cycle. RPM mode allows all of these fans to be
programmed to run at the desired RPM value.
Clearing Bit 7 of Configuration Register 1 (Register
0 ? 00) to 0 places the ADM1031 under software control.
Once under software control, the device can be placed into
RPM feedback mode by writing to Bit 5 and Bit 6 of
Configuration Register 1. Writing a 1 to Bit 5 and Bit 6
selects RPM feedback mode for each fan.
Once RPM feedback mode has been selected, the required
fan RPM can be written to the fan tach high limit registers
(0 ? 10, 0 ? 11). The RPM feedback mode function allows a fan
RPM value to be programmed into the device, and the
ADM1031 maintains the selected RPM value by monitoring
the fan tach and speeding up the fan as necessary, should the
fan start to slow down. Conversely, should the fan start to
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