across the fan until the desired speed is reached. Open-
loop operation allows the µC to regulate fan speed direct-
ly. The µC reads the fan speed from the Tach-
ometer-Count Register. Based on the tachometer
count, the µC decides if the fan speed requires adjust-
ment, and changes the voltage across the fan by writ-
ing an 8-bit word to the DAC Register. Full-on mode
applies the maximum voltage across the fan, forcing it
to spin at full speed. Configuring GPIO1 (see the
General-Purpose Input/Output
section) as an active-low
input provides additional hardware control that fully
turns on the fan and overrides all software commands.
General-Purpose Input/Output
The GPIO pins connect to the drain of the internal N-
channel MOSFET and pullup resistor (Figure 5). When
the N-channel MOSFET is off (Table 5), the pullup resis-
tor provides a logic-level high output. However, with the
MOSFET off, the GPIO may serve as an input pin and
its state is read from the GPIO Status Register (Table
6). The MAX6650/MAX6651 power up with the MOSFET
off, so input signals may be safely connected to the
GPIO pins. When using the GPIO pin as a general-pur-
pose output, change the output by writing to the GPIO
Definition Register.
GPIO0 may be configured as an ALERT output that will
go low whenever a fault-condition is detected (see the
Alarm-Enable and Status Registers
section). GPIO1
may be configured as a FULL ON input to allow hard-
ware control to fully turn on the fan in case of software
or µC failure. GPIO2 (MAX6651 only) may be config-
ured as an internal clock output or as an external clock
input to allow synchronization of multiple devices.
Alarm-Enable and Status Registers
The alarms are enabled only when the appropriate bits of
the Alarm-Enable Register are set (Table 7). The maxi-
mum and minimum output level alarms function only
when the device is configured to operate in the closed-
loop mode (see the
Configuration-Byte Register
section).
The Alarm Status Register allows the system to deter-
mine which alarm caused the alert output (Table 8).
The set-alarm and alert outputs clear after reading the
Fan-Speed Regulators and Monitors
with SMBus/I
2
C-Compatible Interface
10 Maxim Integrated
MAX6650/MAX6651
Table 4. Configuration Byte Register
Figure 5. General-Purpose Input/Output Structure
