Rainbow Electronics MAX6641 - Manual (Page 10)

MAX6641
SMBus-Compatible Temperature Monitor with
Automatic PWM Fan-Speed Controller
10 ______________________________________________________________________________________
BIT NAME POR STATE FUNCTION
7 — 0 Reserved. Set to zero.
6 — 0 Reserved. Set to zero.
5 TIMEOUT 0
Set TIMEOUT to zero to enable SMBus timeout for
prevention of bus lockup. Set to 1 to disable this function.
4 FAN PWM INVERT 0
Set FAN PWM INVERT to zero to force PWMOUT low when
the duty cycle is 100%. Set to 1 to force PWMOUT high
when the duty cycle is 100%.
3 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, which 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.
2 SPIN-UP DISABLE 0
Set SPIN-UP DISABLE to 1 to disable spin-up. Set to zero
for normal fan spin-up.
1 — X Don’t care.
0 — X Don’t care.
Table 2. Configuration Byte Definition (02h)
OT
Mask (06h)
Set bit D7 to 1 in the OT mask register to prevent the
OT output from asserting on faults in the remote-diode
temperature channel. Set bit D6 to 1 to prevent the OT
output from asserting on faults in the local-diode tem-
perature channel. The POR state of the OT mask regis-
ter is 00h.
Fan-Start Duty Cycle (07h)
The fan-start duty-cycle register determines the PWM
duty cycle where the fan starts spinning. Bit D3 in the
configuration byte register (MIN DUTY CYCLE) deter-
mines 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 60h, 40%.
Fan Maximum Duty Cycle (08h)
The fan maximum duty-cycle register sets the maxi-
mum 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 fan maxi-
mum duty-cycle register is F0h, 100%. In manual con-
trol mode, this register is ignored.
Fan-Target Duty Cycle (09h)
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 needs a settling
time before it equals the target duty cycle if the duty-
cycle rate of change register is set to a value other than
zero. The actual duty cycle needs the time to settle as
defined by the value of the duty-cycle rate-of-change
register; therefore, the target duty cycle and the actual
duty cycle are often different. In manual fan-control
mode, write the desired value of the PWM duty cycle
directly into this register. The POR state of the fan-tar-
get duty-cycle register is 00h.