ADT7476A
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30
The fan counter does not count the fan TACH output
pulses directly because the fan speed could be less than
1,000 RPM, and it takes several seconds to accumulate a
reasonably large and accurate count. Instead, the period of
the fan revolution is measured by gating an on-chip 90 kHz
oscillator into the input of a 16-bit counter for N periods of
the fan TACH output (Figure 45), so the accumulated count
is actually proportional to the fan tachometer period and
inversely proportional to the fan speed.
N, the number of pulses counted, is determined by the
settings of TACH pulses per revolution register (0x7B). This
register contains two bits for each fan, allowing one, two
(default), three, or four TACH pulses to be counted.
Figure 45. Fan Speed Measurement
1
2
3
4
CLOCK
PWM
TACH
Fan TachometerReading Registers
The fan tachometer readings are 16-bit values consisting
of a 2-byte read from the ADT7476A.
Table 35. FAN TACHOMETER READING REGISTERS
Register
Description
Default
0x28
TACH1 Low Byte
0x00
0x29
TACH1 High Byte
0x00
0x2A
TACH2 Low Byte
0x00
0x2B
TACH2 High Byte
0x00
0x2C
TACH3 Low Byte
0x00
0x2D
TACH3 High Byte
0x00
0x2E
TACH4 Low Byte
0x00
0x2F
TACH4 High Byte
0x00
Reading Fan Speed from the ADT7476A
The measurement of fan speeds involves a 2-register read
for each measurement. The low byte should be read first.
This causes the high byte to be frozen until both high and
low byte registers have been read, preventing erroneous
TACH readings. The fan tachometer reading registers report
back the number of 11.11
m
s 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 essentially measuring the fan TACH
period, the higher the count value, the slower the fan is
actually running. A 16-bit fan tachometer reading of
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.
Measuring fan TACH has the following caveat: When the
ADT7476A starts up, TACH measurements are locked. In
effect, an internal read of the low byte has been made for
each TACH input. The net result of this is that all TACH
readings are locked until the high byte is read from the
corresponding TACH registers. All TACH-related
interrupts are also ignored until the appropriate high byte is
read.
Once the corresponding high byte has been read, TACH
measurements are unlocked and interrupts are processed as
normal.
Fan TACH Limit Registers
The fan TACH limit registers are 16-bit values consisting
of two bytes.
Table 36. FAN TACH LIMIT REGISTERS
Register
Description
Default
0x54
TACH1 Minimum Low Byte
0xFF
0x55
TACH1 Minimum High Byte
0xFF
0x56
TACH2 Minimum Low Byte
0xFF
0x57
TACH2 Minimum High Byte
0xFF
0x58
TACH3 Minimum Low Byte
0xFF
0x59
TACH3 Minimum High Byte
0xFF
0x5A
TACH4 Minimum Low Byte
0xFF
0x5B
TACH4 Minimum High Byte
0xFF
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. Once high frequency
mode is enabled in 4-wire fans, the dc bits do not need to be
set because this is automatically done internally.
Calculating Fan Speed
Assuming a fan with two pulses per revolution, and with
the ADT7476A programmed to measure two pulses per
revolution, fan speed is calculated by:
Fan Speed (RPM) = (90,000
×
60)/Fan TACH Reading
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