Rainbow Electronics LM77 - Manual (Page 8)

1.0 Functional Description
(Continued)
terrupts just after reprogramming setpoints. Typically, sys-
tem Interrupt inputs are masked prior to reprogramming trip
points. By doing a read just after resetting trip points, but
prior to unmasking, unexpected Interrupts are prevented.
Avoid programming setpoints so close that their hysteresis
values overlap. An example would be that with a T
HYST
value
of 2˚C then setting T
HIGH
and T
LOW
to within 4˚C of each
other will violate this restriction. To be more specific, with
T
HYST
set to 2˚C assume T
HIGH
set to 64˚C. If T
LOW
is set
equal to, or higher than 60˚C this restriction is violated.
1.2 DEFAULT SETTINGS
The LM77 always powers up in a known state. LM77 power
up default conditions are:
1. Comparator Interrupt Mode
2. T
LOW
set to 10˚C
3. T
HIGH
set to 64˚C
4. T_CRIT set to 80˚C
5. T
HYST
set to 2˚C
6. INT and T_CRIT_A active low
7. Pointer set to “00”; Temperature Register
The LM77 registers will always reset to these default values
when the power supply voltage is brought up from zero volts
as the supply crosses the voltage level plotted in the follow-
ing curve. The LM77 registers will reset again when the
power supply drops below the voltage plotted in this curve.
1.3 SERIAL BUS INTERFACE
The LM77 operates as a slave on the Serial Bus, so the SCL
line is an input (no clock is generated by the LM77) and the
SDA line is a bi-directional serial data line. According to
Serial Bus specifications, the LM77 has a 7-bit slave ad-
dress. The five most significant bits of the slave address are
hard wired inside the LM77 and are “10010”. The two least
significant bits of the address are assigned to pins A1–A0,
and are set by connecting these pins to ground for a low, (0);
or to +V
S
for a high, (1).
Therefore, the complete slave address is:
1 0 0 1 0 A1 A0
MSB LSB
Average Power on Reset Voltage
vs Temperature
DS100136-18
LM77
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