ADM1031
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23
of a PWM high output if fan speed measurement is enabled
(Bit 2 and Bit 3 of Configuration Register 2 = 1). It then
starts counting on the rising edge of the second tach pulse
and counts for two fan tach periods, until the rising edge of
the fourth tach pulse, or until the counter overranges if the
fan tach period is too long. The measurement cycle repeats
until monitoring is disabled. The fan speed measurement is
stored in the fan speed reading register at address 008,
009. The fan speed count is given by:
(eq. 17)
Count + (f 60)ńR N
where:
f = 11.25 kHz
R = Fan Speed in RPM
N = Speed Range (either 1, 2, 4, or 8)
The frequency of the oscillator can be adjusted to suit the
expected running speed of the fan by varying N, the speed
range. The oscillator frequency is set by Bit 7 and Bit 6 of
Fan Characteristics Register 1 (020) and Fan
Characteristics Register 2 (021) as shown in Table 15.
Figure 37 shows how the fan measurements relate to the
PWM_OUT pulse trains.
Table 15. OSCILLATOR FREQUENCIES
Bit 7 Bit 6 N Oscillator Frequency (kHz)
0 0 1 11.25
0 1 2 5.625
1 0 4 2.812
1 1 8 1.406
Figure 37. Fan Speed Measurement
CLOCK
CONFIG 2
REG. BIT 2
FAN
INPUT
START OF
MONITORING
CYCLE
FAN
MEASUREMENT
PERIOD
In situations where different output drive circuits are used
for fan drive, it can be desirable to invert the PWM drive
signal. Setting Bit 3 of Configuration Register 1 (000) to 1,
inverts the PWM_OUT signal. This makes the PWM_OUT
pin high for 100% duty cycle. Bit 3 of Configuration
Register 1 should generally be set to 1 when using an n-MOS
device to drive the fan.
If using a p-MOS device, Bit 3 of Configuration
Register 1 should be cleared to 0.
FAN_FAULTs
The FAN_FAULT output (Pin 8) is an active-low,
open-drain output used to signal fan failure to the system
processor. Writing a Logic 1 to Bit 4 of Configuration
Register 1 (000) enables the FAN_FAULT
output pin. The
FAN_FAULT
output is enabled by default. The
FAN_FAULT
output asserts low only when five consecutive
interrupts are generated by the ADM1031 device due to the
fan running underspeed, or if the fan is completely stalled.
Note that the Fan Tach High Limit must be exceeded by at
least one before a FAN_FAULT
can be generated. For
example, if we are only interested in getting a FAN_FAULT
if the fan stalls, then the fan speed value is 0FF for a failed
fan. Therefore, we should make the Fan Tach High
Limit = 0FE to allow FAN_FAULT
to be asserted after five
consecutive fan tach failures.
Figure 38 shows the relationship between INT
,
FAN_FAULT
, and the PWM drive channel. The
PWM_OUT channel is driving a fan at some PWM duty
cycle, 50% for example, and the fan’s tach signal (or fan
current for a 2-wire fan) is being monitored at the
TACH/AIN pin. Tach pulses are being generated by the fan,
during the high time of the PWM duty cycle train. The tach
is pulled high during the off time of the PWM train because
the fan is connected high-side to the n-MOS device.
Suppose the fan has twice previously failed its fan speed
measurement. Looking at Figure 38, PWM_OUT is brought
high for two seconds, to restart the fan if it has stalled.
Sometime later a third tach failure occurs. This is evident by
the tach signal being low during the high time of the PWM
pulse, causing the fan speed reading register to reach its
maximum count of 255. Since the tach limit has been
exceeded, an interrupt is generated on the INT
pin. The fan
fault bit (Bit 1) of Interrupt Status Register 1
(Register 002) is also asserted. Once the processor has
acknowledged the INT
by reading the status register, the
INT
is cleared. PWM_OUT is then brought high for another
two seconds to restart the fan. Subsequent fan failures cause
INT
to be reasserted and the PWM_OUT signal is brought
high for two seconds (fan spin-up default) each time to
restart the fan. Once the fifth tach failure occurs, the failure
is deemed to be catastrophic and the FAN_FAULT
pin is
asserted low. PWM_OUT is brought high to attempt to
restart the fan. The INT
pin continues to generate interrupts
after the assertion of FAN_FAULT
since tach measurement
continues even after fan failure. Should the fan recover from
its failure condition, the FAN_FAULT
signal is negated, and
the fan returns to its normal operating speed.
Figure 39 shows a typical application circuit for the
ADM1031. Temperature monitoring can be based around a
CPU diode or discrete transistor measuring thermal
hotspots. Either 2- or 3-wire fans can be monitored by the
ADM1031, as shown.
