LM78 datasheet(20/31 Pages) NSC

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Functional Description (Continued)

7.0 TEMPERATURE MEASUREMENT SYSTEM

The LM78 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.

7.1 Temperature Data Format

Temperature data can be read from the Temperature, T

OI Set

Point, and T

HYST Set Point registers; and written to the TOI

Set Point, and T

HYST Set Point registers. Temperature data

is represented by an 8-bit, two’s complement word with an

LSB (Least Significant Bit) equal to 1.0˚C:

Temperature

Digital Output

Binary

Hex

+125˚C

0111 1101

7Dh

+25˚C

0001 1001

19h

+1.0˚C

0000 0001

01h

+0˚C

0000 0000

00h

−1.0˚C

1111 1111

FFh

−25˚C

1110 0111

E7h

−55˚C

1100 1001

C9h

7.2 Temperature Interrupts

The normal mode for temperature interrupts in the LM78 is

an “Interrupt”mode operating in the following way: Exceeding

OI causes an interrupt that will remain active indefinitely

until reset by reading Interrupt Status Register 1. Once an

interrupt event has occurred by crossing T

OI, then reset, an

interrupt will only occur again by the temperature going

below T

HYST. Again, it will remain active indefinitely until

being reset by reading Interrupt Status Register 1.

A “Comparator” mode for temperature interrupts can be

made available by setting the T

HYST limit to 127˚C. This

results in a simple “thermostat” type of function where an

interrupt will be set whenever the temperature exceeds the

OI limit. Reading Interrupt Status Register 1 will clear the

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

OI limit (allow up to

two measurement cycles for clearing after descending below

OI while in Comparator mode).

DS012873-18

FIGURE 10. Temperature-to-Digital Transfer Function

(Non-Linear Scale for Clarity)

DS012873-19

*Note: Interrupt resets occur only when interrupt Status Register 1 is read.

(a) Interrupt Mode

DS012873-20

Interrupt resets occur when Interrupt Status Register 1 is read but will set

again when monitoring cycle continues (as long as temperature exceeds

TOI). When temperature descends below TOI allow up to two monitoring

loops before the Temperature Interrupt resets.

(b) Comparator Mode

FIGURE 11. Temperature Interrupt Response Diagram

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