C-compatible interface, Table 8. alarm status register bit assignments, Table 9. tachometer count-time register – Rainbow Electronics MAX6651 User Manual
Page 12: Table 7. alarm-enable register bit masks
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
with SMBus/I
2
C-Compatible Interface
12
______________________________________________________________________________________
Table 8. Alarm Status Register Bit Assignments
7 (MSB) to 5
GPIO1
—
BIT
GPIO2 (MAX6651 only)
0
4
NAME
1
MIN
3
0
0 (LSB)
0
0
POR
(DEFAULT)
STATE
MAX
0
Always 0
Minimum Output Level Alarm
GPIO1 Alarm. Set when GPIO1 is low.
GPIO2 Alarm. Set when GPIO2 is low (MAX6651 only).
FUNCTION
Maximum Output Level Alarm
Tachometer Overflow Alarm
2
TACH
0
Upon power-up, the Tachometer Count Registers reset
to 00h and the Tachometer Count-Time Register sets a
1s integration time.
Digital-to-Analog Converter
When using the open-loop mode of operation, the DAC
Register sets the voltage on the low side of the fan. An
internal operational amplifier compares the feedback
voltage (V
FB
) with the reference voltage set by the 8-bit
DAC, and adjusts the output voltage (V
OUT
) until the
two input voltages are equal. The voltage at the FB pin
may be determined by the following equation:
V
FB
= (10 x V
REF
x K
DAC
) / 256
and the voltage across the fan is:
where K
DAC
is the numerical value of the DAC Register
and V
REF
= 1.5V. The minimum feedback voltage is
limited by the voltage drop across the external MOS-
FET (R
ON
x I
FAN
), and the maximum voltage is limited
by the fan’s supply voltage (V
FAN
). For linear opera-
tion, use DAC values between 08h and TB0h (see
Typical Operating Characteristics). When using the
closed-loop mode of operation, the contents of the
DAC Register are ignored. When writing to the DAC,
wait at least 500µs before attempting to read back.
Power-on Reset (POR)
The MAX6650/MAX6651 have volatile memory. To pre-
vent ambiguous power-supply conditions from corrupt-
ing the data in the memory and causing erratic
V
k
k
K
V
FAN
DAC
REF
–
90
10
1
256
+
Table 9. Tachometer Count-Time Register
(Assumes two pulses per revolution)
0.25
MINIMUM
RESOLUTION
(Hz/COUNT)
1
0.5
2
64
2.0
0000 0011
MAXIMUM
FAN SPEED
(RPS)
256
128
0000 0010
COUNT
TIME
(s)
0000 0001
512
0.5
REGISTER
VALUE
(K
COUNT
)
0.25
1.0
0000 0000
1 = Alarm condition
Table 7. Alarm-Enable Register Bit Masks
7 (MSB) to 5
GPIO1
—
BIT
GPIO2 (MAX6651 only)
0
4
NAME
1
MIN
3
0
0 (LSB)
0
0
POR
(DEFAULT)
STATE
MAX
0
2
TACH
0
Always 0
Minimum Output Level Alarm Enable/Disable
GPIO1 Alarm Enable/Disable
GPIO2 Alarm Enable/Disable (MAX6651 only)
FUNCTION
Maximum Output Level Alarm Enable/Disable
Tachometer Overflow Alarm Enable/Disable
1 = Enabled