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ADT7490
0xFFFF indicates that either the fan has stalled or is running
very slowly (<100 RPM).
High Limit > Comparison Performed
Because the actual fan TACH period is being measured,
falling below a fan TACH limit by 1 sets the appropriate
status bit and can be used to generate an SMBALERT.
Fan TACH Limit Registers
The fan TACH limit registers are 16-bit values consisting
of two bytes.
Table 31. FAN TACH LIMIT REGISTERS
Fan Pulses per Revolution
Different fan models can output either one, two, three, or
four TACH pulses per revolution. Once the number of fan
TACH pulses has been determined, it can be programmed
into the TACH pulses per revolution register (0x7B) for each
fan. Alternatively, this register can be used to determine the
number or pulses per revolution output by a given fan. By
plotting fan speed measurements at 100% speed with
different pulses per revolution setting, the smoothest graph
with the lowest ripple determines the correct pulses per
revolution value.
TACH Pulses per Revolution Register
Register
0x54
0x55
0x56
0x57
0x58
0x59
0x5A
0x5B
Description
TACH1 Minimum Low Byte
TACH1 Minimum High Byte
TACH2 Minimum Low Byte
TACH2 Minimum High Byte
TACH3 Minimum Low Byte
TACH3 Minimum High Byte
TACH4 Minimum Low Byte
TACH4 Minimum High Byte
Default
0xFF
0xFF
0xFF
0xFF
0xFF
0xFF
0xFF
0xFF
Bits [1:0], FAN1 default = 2 pulses per revolution
Bits [3:2], FAN2 default = 2 pulses per revolution
Bits [5:4], FAN3 default = 2 pulses per revolution
Bits [7:6], FAN4 default = 2 pulses per revolution
00 = 1 pulse per revolution
01 = 2 pulses per revolution
10 = 3 pulses per revolution
11 = 4 pulses per revolution
Fan Spin-up
Fan Speed Measurement Rate
The fan TACH readings are normally updated once
every second.
When set, the FAST bit (Bit 3) of Configuration Register 3
(0x78), updates the fan TACH readings every 250 ms.
DC Bits
If any of the fans are not being driven by a PWM channel
but are powered directly from 5.0 V or 12 V, their associated
dc bit in Configuration Register 3 should be set. This allows
TACH readings to be taken on a continuous basis for fans
connected directly to a dc source. For 4-wire fans, once high
frequency mode is enabled, the dc bits do not need to be set
because this is automatically done internally.
Calculating Fan Speed
Assuming a fan with a two pulses per revolution, and with
the ADT7490 programmed to measure two pulses per
revolution, fan speed is calculated by
Fan Speed (RPM) = (90,000 ? 60)/Fan TACH Reading
where Fan TACH Reading is the 16-bit fan tachometer
The ADT7490 has a unique fan spin-up function. It spins
the fan at 100% PWM duty cycle until two TACH pulses are
detected on the TACH input. When two TACH pulses have
been detected, the PWM duty cycle goes to the expected
running value, for example, 33%. The advantage of this is
that fans have different spin-up characteristics and take
different times to overcome inertia. The ADT7490 runs the
fans just fast enough to overcome inertia and is quieter on
spin-up than fans programmed to spin up for a given spin-up
time.
Fan Startup Timeout
To prevent the generation of false interrupts as a fan spins
up, because the fan is below running speed, the ADT7490
includes a fan startup timeout function. During this time, the
ADT7490 looks for two TACH pulses. If two TACH pulses
are not detected, an interrupt is generated.
Fan startup timeout can be disabled by setting Bit 3
(FSPDIS) of Configuration Register 7 (0x11).
Table 32. PWM1, PWM2, PWM3 CONFIGURATION
(REG. 0x5C, REG. 0x5D, REG. 0x5E)
reading.
Example
TACH1 High Byte (Register 0x29) = 0x17
TACH1 Low Byte (Register 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f ? 60)/Fan 1 TACH Reading
RPM = (90,000 ? 60)/6143
Fan Speed = 879 RPM
Bit
[2:0]
Mnemonic
SPIN
Description
These Bits Control the Startup
Timeout for PWM1 (0x5C),
PWM2 (0x5D), PWM3 (0x5E).
000 = No Startup Timeout
001 = 100 ms
010 = 250 ms (Default)
011 = 400 ms
100 = 667 ms
101 = 1 s
110 = 2 s
111 = 4 s
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