adm1027 Analog Devices, Inc., adm1027 Datasheet - Page 24

• Manufacturer Part Number: adm1027
• Description: Dbcool? Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

ADM1027
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADM1027.
Reg. 0x28 TACH1 Low Byte = 0x00 default
Reg. 0x29 TACH1 High Byte = 0x00 default
Reg. 0x2A TACH2 Low Byte = 0x00 default
Reg. 0x2B TACH2 High Byte = 0x00 default
Reg. 0x2C TACH3 Low Byte = 0x00 default
Reg. 0x2D TACH3 High Byte = 0x00 default
Reg. 0x2E TACH4 Low Byte = 0x00 default
Reg. 0x2F TACH4 High Byte = 0x00 default
Reading Fan Speed From the ADM1027
If fan speeds are being measured, this 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 from. This prevents erroneous TACH
readings.
The fan tachometer reading registers report back the number of
11.11 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). Since 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 the fan either has
stalled or is running very slowly (< 100 RPM).
Since actual fan TACH period is being measured, exceeding a
fan TACH limit by 1 will set 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
HIGH LIMIT: > COMPARISON PERFORMED
–24–
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.
Calculating Fan Speed
Assuming a fan with 2 pulses/revolution (and 2 pulses/rev being
measured), fan speed is calculated by:
where Fan Tach Reading = 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/
revolution output by a given fan. By plotting fan speed measure-
ments at 100% speed with different pulses/rev setting, the
smoothest graph with the lowest ripple determines the correct
pulses/rev value.
Fan Pulses Per Revolution Register
<1:0> FAN1 default = 2 pulses/rev
<3:2> FAN2 default = 2 pulses/rev
<5:4> FAN3 default = 2 pulses/rev
<7:6> FAN4 default = 2 pulses/rev
Fan Speed RPM
00 = 1 pulse/rev
01 = 2 pulses/rev
10 = 3 pulses/rev
11 = 4 pulses/rev
(
)
=
(
90 000 60
,
¥
)
Fan Tach Reading
REV. A