MAX6615/MAX6616
Duty-Cycle Step Size (13h)
Bits D7–D4 (channel 1) and bits D3–D0 (channel 2) of the
duty-cycle step-size register change the size of the duty-
cycle change for each temperature step. The POR state
of the duty-cycle step-size register is 55h (see Table 6).
PWM Frequency Select (14h)
Set bits D7, D6, and D5 (select A, B, and C) in the PWM
frequency-select register to control the PWM frequency
(see Table 7). The POR state of the PWM frequency-
select register is 40h, 33Hz. The lower frequencies are
usually used when driving the fan’s power-supply pin as
in the
Typical Application Circuit
, with 33Hz being the
most common choice. The 35kHz frequency setting is
used for controlling fans that have logic-level PWM input
pins for speed control. The minimum duty-cycle resolution
is decreased from 2/240 to 4/240 at the 35kHz frequen-
cy setting. For example, a result that would return a value
of 6/240 is truncated to 4/240.
GPIO Function Register (15h) (MAX6616)
The GPIO function register (15h) sets the GPIO states.
Write a zero to set a GPIO as an output. Write a 1 to set
a GPIO as an input.
GPIO Value Register (16h) (MAX6616)
The GPIO value register (16h) contains the state of
each GPIO input when a GPIO is configured as an
input. When configured as an output, write a 1 or zero
to set the value of the GPIO output.
Thermistor Offset Register (17h)
The thermistor offset register contains the offset for both
of the thermistors in two’s complement. Bits D7, D6, D5,
and D4 set the offset for temperature channel 1. Bits
D3, D2, D1, and D0 set the offset for temperature chan-
nel 2. The values in this register allow the thermistor
temperature readings to be shifted to help compensate
for different thermistor characteristics or different values
of R
EXT
and apply to thermistor measurements only.
The MSB is the sign bit and the LSB is 2°C. The POR
state for this register is 00h.
Tachometer Value Registers (18h and 19h)
The tachometer value registers contain the tachometer
count values for each fan. The MAX6615/MAX6616
measure the tachometer signal every 67s. It counts the
number of clock cycles between two tachometer pulses
and stores the value in the corresponding channel reg-
ister. The POR state of this register is FFh.
Tachometer Limit Registers (1Ah and 1Bh)
The tachometer limit registers contain the tachometer
limits for each fan. If the value in the tach1 value regis-
ter (18h) ever exceeds the value stored in 1Ah, a chan-
nel 1 fan failure is detected. If the value in the Tach2
value register (19h) ever exceeds the value stored in
1Bh, a channel 2 fan failure is detected. The POR state
of these registers is FFh.
Fan Configuration/Status Register (1Ch)
The fan configuration/status register contains the status
and tachometer control bits for both fans. Bits D7 and
D6 indicate whether a fan has failed the maximum
tachometer limits in registers 1Ah and 1Bh. Setting bits
D5 and D4 disables the tachometer for each fan. The
speed is not measured when these bits are set. Setting
bits D3 and D2 measure the fan speed only during
spin-up or when it reaches 100% duty cycle. Bit D1 is
the FAN_FAIL output mask. Bit D0 is the FAN_FAIL
cross drive enable. Setting this bit enables fan 2 to go
to full speed when fan 1 fails or vice versa.
Extended Temperature Registers
(1Eh and 1Fh)
The extended temperature registers contain the low-byte
results of temperature measurements. The value of the
MSB is 0.5°C and the value of D5 is 0.125°C. The POR
states of these registers are 00h.
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
16 ______________________________________________________________________________________
Table 7. PWM Frequency Select
