ADT7470 Analog Devices, Inc., ADT7470 Datasheet - Page 22

• Manufacturer Part Number: ADT7470
• Description: Temperature Sensor Hub And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7470
Fan Pulses per Revolution
Different fan models can output either 1, 2, 3, or 4 tach pulses
per revolution. Once the number of fan tach pulses is deter-
mined, it can be programmed into the fan pulses per revolution
register (Register 0x43) for each fan. Alternatively, this register
can be used to determine the number or pulses/revolution
output by a given fan. By plotting fan speed measurements at
PWMMAX speed with different pulses/revolution settings, the
smoothest graph with the lowest ripple determines the correct
pulses/revolution value.
Fan Spin Up
The ADT7470 has a unique fan spin-up function. Fans are
PWMMAX on if there is no interaction with the ADT7470.
It incorporates a 2 second bus alive/dead detection feature.
If no bus activity is seen and the ADT7470 is not specifically
written to within 2 seconds, the PWM outputs autodrive
PWMMAX. This is useful where a system lock-up occurs
before software has a chance to configure the basic system
devices. This is intended as a bus communication fail-safe
feature.
Table 16. PWM1/PWM2 Configuration (Register 0x68)
Bit No.
<0>
<1>
<2>
<3>
<4>
<5>
<6>
<7>
Table 17. PWM3/PMW4 Configuration (Register 0x69)
Bit No.
<0>
<1>
<2>
<3>
<4>
<5>
<6>
<7>
Mnenonic
N/A
N/A
N/A
N/A
INV2
INV1
BHVR2
BHVR1
Mnemonic
N/A
N/A
N/A
N/A
INV4
INV3
BHVR4
BHVR3
Description
Set to 0 (default).
Set to 0 (default).
Set to 0 (default).
Set to 0 (default).
Setting this bit to 1 inverts the PWM2 out put.
Default = 0 drives a logic high for PWMMAX duty cycle.
Setting this bit to 1 inverts the PWM1 output.
Default = 0 drives a logic high for PWMMAX duty cycle.
This bit determines fan behavior for PWM2 output.
0 = Manual mode (PWM2 duty cycle controlled in software).
1 = Fastest speed calculated by all temperatures control PWM2 (automatic fan control mode).
This bit determines fan behavior for PWM1 output.
0 = Manual mode (PWM1 duty cycle controlled in software).
1 = Fastest speed calculated by all temperatures control PWM1 (automatic fan control mode).
Description
Set to 0 (default).
Set to 0 (default).
Set to 0 (default).
Set to 0 (default).
Setting this bit to 1 inverts the PWM4 output.
Default = 0 drives a logic high for PWMMAX duty cycle.
Setting this bit to 1 inverts the PWM3 output.
Default = 0 drives a logic high for PWMMAX duty cycle.
This bit determines fan behavior for PWM4 output.
0 = Manual mode (PWM4 duty cycle controlled in software).
1 = Fastest speed calculated by all temperatures control PWM4 (automatic fan control mode).
This bit determines fan behavior for PWM3 output.
0 = Manual mode (PWM3 duty cycle controlled in software).
1 = Fastest speed calculated by all temperatures control PWM3 (automatic fan control mode).
Rev. B | Page 22 of 40
Where normal communication occurs, the fans are given “grace
time” to spin up before the PWM autothrottles back to some
normal speed. For example, under normal conditions, the
ADT7470 spins the fan at PWMMAX PWM duty cycle until
2 tach pulses are detected on the tach input. Once 2 pulses are
detected, the PWM duty cycle goes to the expected running
value, for example, 33%. The advantage of this is that fans with
different spin-up characteristics that take different times to
overcome inertia still spin up without generating excess acoustic
noise. The ADT7470 runs the fans just fast enough to overcome
inertia and is quieter on spin-up than fans programmed to spin
up for a fixed spin-up time.
Fan Start-Up Timeout
To prevent false interrupts being generated as a fan spins up
(because it is below running speed), the ADT7470 includes
a fan start-up timeout function. This is the time limit allowed
for 2 tach pulses to be detected on spin-up. The fan start-up
timeout is fixed at 2 seconds, and, if no tach pulses occur within
2 seconds of the start of spin-up, a fan fault is detected and
flagged in the interrupt status registers.