ADT7476
Rev. B | Page 36 of 72
PWM1, PWM 2, PWM3 Frequency Registers (0x5F to 0x61)
Bits [2:0] FREQ.
000 = 11.0 Hz
001 = 14.7 Hz
010 = 22.1 Hz
011 = 29.4 Hz
100 = 35.3 Hz default
101 = 44.1 Hz
110 = 58.8 Hz
111 = 88.2 Hz
High Frequency Mode PWM Drive
Setting Bit 3 of Register 0x5F, Register 0x60, and Register 0x61
enables high frequency mode for Fan 1, Fan 2, and Fan 3,
respectively.
In high frequency mode, the PWM drive frequency is always
22.5 kHz. When high frequency mode is enabled, the dc bits
are automatically asserted internally and do not need to be
changed.
Fan Speed Control
The ADT7476 controls fan speed using automatic mode and
manual mode.
In automatic fan speed control mode, fan speed is automatically
varied with temperature and without CPU intervention once
initial parameters are set up. The advantage is that, if the system
hangs, the user is guaranteed that the system is protected from
overheating.
In manual fan speed control mode, the ADT7476 allows the
duty cycle of any PWM output to be manually adjusted. This
can be useful if the user wants to change fan speed in software
or adjust PWM duty cycle output for test purposes. Bits [7:5] of
Register 0x5C to Register 0x5E (PWM Configuration) control
the behavior of each PWM output.
PWM Configuration Registers (0x5C to 0x5E)
Bits [7:5] BHVR.
111 = manual mode
Once under manual control, each PWM output can be manu-
ally updated by writing Register 0x30 to Register 0x32 (PWMx
current duty cycle registers).
Programming the PWM Current Duty Cycle Registers
The PWM current duty cycle registers are 8-bit registers that
allow the PWM duty cycle for each output to be set anywhere
from 0% to 100% in steps of 0.39%. The value to be programmed
into the PWM
MIN
register is given by
Value
(decimal) =
PWM
MIN
/0.39
Example 1: For a PWM duty cycle of 50%
Value
(decimal) = 50/0.39 = 128 (decimal)
Value
= 128 (decimal) or 0x80 (hex)
Example 2: For a PWM duty cycle of 33%
Value
(decimal) = 33/0.39 = 85 (decimal)
Value
= 85 (decimal) or 0x54 (hex)
PWM Current Duty Cycle Registers
Register 0x30, PWM1 Current Duty Cycle = 0xFF (100%
default)
Register 0x31, PWM2 Current Duty Cycle = 0xFF (100%
default)
Register 0x32, PWM3 Current Duty Cycle = 0xFF (100%
default)
By reading the PWMx current duty cycle registers, the user can
keep track of the current duty cycle on each PWM output, even
when the fans are running in automatic fan speed control mode
or acoustic enhancement mode.
PROGRAMMING T
RANGE
T
RANGE
defines the distance between T
MIN
and 100% PWM. For
the ADT7467, ADT7468, and ADT7473, T
RANGE
is effectively a
slope. For the ADT7475 and ADT7476, T
RANGE
is no longer a
slope but defines the temperature region where the PWM
output linearly ramps from PWM
MIN
to 100% PWM.
05382-047
T
MIN
PWM = 100%
PWM
MIN
PWM
MAX
PWM = 0%
T
RANGE
Figure 46. T
RANGE
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