LM79 NSC [National Semiconductor], LM79 Datasheet - Page 20

• Manufacturer Part Number: LM79
• Description: Microprocessor System Hardware Monitor
• Manufacturer: NSC [National Semiconductor]

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Functional Description
7.0 TEMPERATURE MEASUREMENT SYSTEM
The LM79 bandgap type temperature sensor and ADC per-
form 8-bit two’s-complement conversions of the tempera-
ture. A digital comparator is also incorporated that compares
the readings to the user-programmable Overtemperature
setpoint and Hysteresis values.
*
Note: Interrupt resets occur only when interrupt Status Register 1 is read.
FIGURE 10. Temperature-to-Digital Transfer Function
(Non-Linear Scale for Clarity)
(a) Interrupt Mode
FIGURE 11. Temperature Interrupt Response Diagram
(Continued)
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Interrupt resets occur when Interrupt Status Register 1 is read but will set
again when monitoring cycle continues (as long as temperature exceeds
T
loops before the Temperature Interrupt resets.
20
OI
). When temperature descends below T
7.1 Temperature Data Format
Temperature data can be read from the Temperature, T
Point, and T
Set Point, and T
is represented by an 8-bit, two’s complement word with an
LSB (Least Significant Bit) equal to 1.0˚C:
7.2 Temperature Interrupts
The normal mode for temperature interrupts in the LM79 is
an “Interrupt” mode operating in the following way: Exceed-
ing T
until reset by reading Interrupt Status Register 1. Once an
interrupt event has occurred by crossing T
interrupt will only occur again by the temperature going
below T
being reset by reading Interrupt Status Register 1.
A “Comparator” mode for temperature interrupts can be
made available by setting the T
results in a simple “thermostat” type of function where an
interrupt will be set whenever the temperature exceeds the
T
interrupt as usual, but the interrupt will set again after the
completion of another measurement cycle. It will remain set
until the temperature goes below the T
two measurement cycles for clearing after descending below
T
OI
OI
Temperature
while in Comparator mode).
limit. Reading Interrupt Status Register 1 will clear the
OI
+125˚C
+1.0˚C
−1.0˚C
+25˚C
−25˚C
−55˚C
+0˚C
causes an interrupt that will remain active indefinitely
HYST
HYST
. Again, it will remain active indefinitely until
(b) Comparator Mode
HYST
Set Point registers; and written to the T
Set Point registers. Temperature data
0111 1101
0001 1001
0000 0001
0000 0000
1111 1111
1110 0111
1100 1001
Binary
OI
allow up to two monitoring
Digital Output
HYST
limit to 127˚C. This
OI
OI
limit (allow up to
, then reset, an
DS100036-20
Hex
7Dh
FFh
E7h
C9h
19h
01h
00h
OI
Set
OI