User guide
PWMOUT Start Duty Cycle (07h and 08h)
The PWMOUT start duty-cycle register determines the
PWM duty cycle where the fan starts spinning. Bit D2 in
the configuration byte register (MIN DUTY CYCLE)
determines the starting duty cycle. If the MIN DUTY
CYCLE bit is 1, the duty cycle is the value written to the
fan-start duty-cycle register at all temperatures below
the fan-start temperature. If the MIN DUTY CYCLE bit is
zero, the duty cycle is zero below the fan-start tempera-
ture and has this value when the fan-start temperature
is reached. A value of 240 represents 100% duty cycle.
Writing any value greater than 240 causes the fan
speed to be set to 100%. The POR state of the fan-start
duty-cycle register is 96h, 40%.
PWMOUT Max Duty Cycle (09h and 0Ah)
The PWMOUT maximum duty-cycle register sets the
maximum allowable PWMOUT duty cycle between
2/240 (0.83% duty cycle) and 240/240 (100% duty
cycle). Any values greater than 240 are recognized as
100% maximum duty cycle. The POR state of the
PWMOUT maximum duty-cycle register is F0h, 100%.
In manual control mode, this register is ignored.
PWMOUT Target Duty Cycle (0Bh and 0Ch)
In automatic fan-control mode, this register contains the
present value of the target PWM duty cycle, as deter-
mined by the measured temperature and the duty-
cycle step size. The actual duty cycle requires time
before it equals the target duty cycle if the duty-cycle
rate-of-change register is set to a value other than zero.
In manual fan-control mode, write the desired value of
the PWM duty cycle directly into this register. The POR
state of the fan-target duty-cycle register is 00h.
PWMOUT1 Instantaneous Duty Cycle,
PWMOUT2 Instantaneous Duty Cycle (0Dh, 0Eh)
These registers always contain the duty cycle of the
PWM signals presented at the PWM output.
The POR state of the PWMOUT instantaneous duty-
cycle register is 00h.
Channel 1 and Channel 2 Fan-Start Temperature
(0Fh and 10h)
These registers contain the temperatures at which fan
control begins (in automatic mode). See the Automatic
PWM Duty-Cycle Control section for details on setting
the fan-start thresholds. The POR state of the channel 1
and channel 2 fan-start temperature registers is 00h.
MAX6678
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
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Table 3. Configuration Byte Definition (02h)
BIT NAME POR STATE FUNCTION
7 Reserved; set to 0 ——
6 Reserved; set to 0 ——
5 TIMEOUT 0
Set TIMEOUT to zero to enable SMBus timeout for prevention of bus lockup. Set
to 1 to disable this function.
4 FAN1 PWM INVERT 0
Set FAN PWM INVERT to zero to force PWMOUT1 low when the duty cycle is
100%. Set to 1 to force PWMOUT1 high when the duty cycle is 100%.
3 FAN2 PWM INVERT 0
Set FAN PWM INVERT to zero to force PWMOUT2 low when the duty cycle is
100%. Set to 1 to force PWMOUT2 high when the duty cycle is 100%.
2 MIN DUTY CYCLE 0
Set MIN DUTY CYCLE to zero for a 0% duty cycle when the measured
temperature is below the fan-temperature threshold in automatic mode. When the
temperature equals the fan-temperature threshold, the duty cycle is the value in
the fan-start duty-cycle register, and it increases with increasing temperature.
Set MIN DUTY CYCLE to 1 to force the PWM duty cycle to the value in the fan-
start duty-cycle register when the measured temperature is below the fan-
temperature threshold. As the temperature increases above the temperature
threshold, the duty cycle increases as programmed.
1
TEMPERATURE
SOURCE SELECT
0
Selects either local or remote 2 as the source for temperature channel 2 register
data. When D1 = 0, the MAX6678 measures remote 2 and when D1 = 1, the
MAX6678 measures the internal die temperature.
0 SPIN-UP DISABLE 0 Set SPIN-UP DISABLE to 1 to disable spin-up. Set to zero for normal fan spin-up.