参数资料
| 型号： | ADT7468ZEVB |
| 厂商： | ON Semiconductor |
| 文件页数： | 32/81页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR ADT7468 |
| 标准包装： | 1 |
| 其它名称： | EVAL-ADT7468EB EVAL-ADT7468EB-ND |
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ADT7468
The fan tachometer reading registers report back the number of
11.11 μs period clocks (90 kHz oscillator) gated to the fan speed
counter, from the rising edge of the first fan TACH pulse to the
rising edge of the third fan TACH pulse (assuming two pulses
per revolution are being counted). Because the device is essen-
tially measuring the fan TACH period, the higher the count
value, the slower the fan is actually running. A 16-bit fan tach-
ometer reading of 0xFFFF indicates either that 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.
Reg. 0x54, TACH1 minimum low byte = 0xFF default
Reg. 0x55, TACH1 minimum high byte = 0xFF default
Reg. 0x56, TACH2 minimum low byte = 0xFF default
Reg. 0x57, TACH2 minimum high byte = 0xFF default
Reg. 0x58, TACH3 minimum low byte = 0xFF default
Reg. 0x59, TACH3 minimum high byte = 0xFF default
Reg. 0x5A, TACH4 minimum low byte = 0xFF default
Reg. 0x5B, TACH4 minimum high byte = 0xFF default
Fan Speed Measurement Rate
The fan TACH readings are normally updated once every
second.
The fast bit (Bit 3) of Configuration Register 3 (Reg. 0x78),
when set, updates the fan TACH readings every 250 ms.
If any of the fans are not being driven by a PWM channel but
are powered directly from 5 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 optimal results, the associated dc bit
should always be set when using 4-wire fans.
Calculating Fan Speed
Assuming a fan with a two pulses per revolution (and two
pulses per revolution being measured), fan speed is calculated
by
Fan Speed (RPM) = (90,000 × 60)/ Fan TACH Reading
where the Fan TACH reading is the 16-bit fan tachometer
reading.
Example:
TACH1 high byte (Reg. 0x29) = 0x17
TACH1 low byte (Reg. 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = ( f × 60)/ Fan 1 TACH Reading
RPM = (90000 × 60)/6143
Fan Speed = 879 RPM
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 has been
determined, it can be programmed into the fan pulses per
revolution register (Reg. 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 measure-
ments at 100% speed with different pulses per revolution
setting, the smoothest graph with the lowest ripple determines
the correct pulses per revolution value.
Fan Pulses per Revolution Register
<1:0> Fan 1 default = 2 pulses per revolution
<3:2> Fan 2 default = 2 pulses per revolution
<5:4> Fan 3 default = 2 pulses per revolution
<7:6> Fan 4 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
2-Wire Fan Speed Measurements
(Low Frequency Mode Only)
The ADT7468 is capable of measuring the speed of 2-wire fans,
that is, fans without TACH outputs. To do this, the fan must be
interfaced as shown in the Driving 2-Wire Fans section. In this
case, the TACH inputs should be reprogrammed as analog
inputs, AIN.
Configuration Register 2 (Reg. 0x73)
Bit 3 (AIN4) = 1, Pin 14 is reconfigured to measure the speed of
a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 2 (AIN3) = 1, Pin 9 is reconfigured to measure the speed of
a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 1 (AIN2) = 1, Pin 12 is reconfigured to measure the speed of
a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 0 (AIN1) = 1, Pin 11 is reconfigured to measure the speed of
a 2-wire fan using an external sensing resistor and coupling
capacitor.
AIN Switching Threshold
Having configured the TACH inputs as AIN inputs for 2-wire
measurements, a user can select the sensing threshold for the AIN
signal.
Rev. 3 | Page 32 of 81 | www.onsemi.com
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