Manual

Alarm Status Register if the condition that caused the
alarm is removed.
Tachometer
The Tachometer Count Registers record the number of
pulses on the corresponding tachometer input during the
period defined by the Tachometer Count-Time Register.
The MAX6651 contains three additional tachometer
inputs, which may be used to monitor additional fans. For
accurate control of multiple fans, use identical fans.
The Tachometer Count-Time Register sets the integration
time over which the MAX6650/MAX6651 count tachome-
ter pulses. The devices can count up to 255 (FFh) pulses
during the selected count time. If more than 255 pulses
occur, the IC sets the overflow alarm and the Tachometer
Count Register reports the maximum value of 255. Set
the time register so the count register will not overflow
under worst-case conditions (maximum fan speed) while
maximizing resolution. Calculate the maximum measur-
able fan speed and minimum resolution with the following
equations:
Max Fan Speed (in RPS) = 255 / (2 x t
COUNT
)
Min Resolution (in RPS) = 1 / (2 x t
COUNT
)
where t
COUNT
is the tachometer count time; 1kHz is the
maximum allowable tachometer input frequency for the
MAX6650/MAX6651.
The first 6 bits of the Tachometer Count-Time Register
are always zero, and the last 2 bits set the count time
(Table 9). The count time may be determined from the
following equation:
t
COUNT
= 0.25s x 2
KCOUNT
where K
COUNT
is the numerical value of the two 2LSBs.
The 0.25 factor has a ±10% uncertainty.
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
with SMBus/I
2
C-Compatible Interface
______________________________________________________________________________________ 11
Table 5. GPIO Definition Register
GPIO0 outputs a logic-level high or serves as an input.11
GPIO0 outputs a logic-level low.10
GPIO0 serves as an ALERT output.
01
GPIO0 outputs a logic-level high or serves as an input.00
GPIO011
GPIO1 outputs a logic-level high or serves as an input.11
GPIO1 outputs a logic-level low.10
GPIO1 serves as a FULL ON input.
01
GPIO1 outputs a logic-level high or serves as an input.00
GPIO1113 to 2
GPIO2 outputs a logic-level high or serves as an input.11
GPIO2 outputs a logic-level low.10
GPIO2 serves as an internal clock output.01
GPIO2 serves as an external clock input.00
GPIO2
(MAX6651
only)
115 to 4
GPIO3 outputs a logic-level high or serves as an input.1
1
GPIO3 outputs a logic-level low.0
GPIO3
(MAX6651
only)
6
GPIO4 outputs a logic-level high or serves as an input.1
GPIO4
(MAX6651
only)
17 (MSB)
STATE
0
FUNCTIONPIN
GPIO4 outputs a logic-level low.
POR
(DEFAULT)
STATE
BIT
Table 6. GPIO Status Register
7 (MSB) to 5
GPIO3 (MAX6651 only)
Always 0
BIT
GPIO4 (MAX6651 only)
0
4
NAME
3 1
1
POR
(DEFAULT)
STATE
2 GPIO2 (MAX6651 only) 1
1 GPIO1 1
0 (LSB) GPIO0 1