DS1822

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OPERATION—MEASURING TEMPERATURE

The core functionality of the DS1822 is its direct-to-digital temperature sensor. The resolution of the

temperature sensor is user-configurable to 9, 10, 11, or 12 bits, corresponding to increments of 0.5

°C,

0.25

°C, 0.125°C, and 0.0625°C, respectively. The default resolution at power-up is 12 bit. The DS1822

powers-up in a low-power idle state; to initiate a temperature measurement and A-to-D conversion, the

master must issue a Convert T [44h] command. Following the conversion, the resulting thermal data is

stored in the 2-byte temperature register in the scratchpad memory and the DS1822 returns to its idle

state. If the DS1822 is powered by an external supply, the master can issue “read-time slots” (see the 1-

WIRE BUS SYSTEM section) after the Convert T command and the DS1822 will respond by

transmitting 0 while the temperature conversion is in progress and 1 when the conversion is done. If the

DS1822 is powered with parasite power, this notification technique cannot be used since the bus must be

pulled high by a strong pullup during the entire temperature conversion. The bus requirements for parasite

power are explained in detail in the POWERING THE DS1822 section of this data sheet.

The DS1822 output temperature data is calibrated in degrees centigrade; for Fahrenheit applications, a

lookup table or conversion routine must be used. The temperature data is stored as a 16-bit sign-extended

two’s complement number in the temperature register (see Figure 2). The sign bits (S) indicate if the

temperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. If the

DS1822 is configured for 12-bit resolution, all bits in the temperature register will contain valid data. For

11-bit resolution, bit 0 is undefined. For 10-bit resolution, bits 1 and 0 are undefined, and for 9-bit

resolution bits 2, 1 and 0 are undefined. Table 2 gives examples of digital output data and the

corresponding temperature reading for 12-bit resolution conversions.

TEMPERATURE REGISTER FORMAT Figure 2

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

LS Byte

2

3

2

2

2

1

2

0

2

-1

2

-2

2

-3

2

-4

bit 15

bit 14

bit 13

bit 12

bit 11

bit 10

bit 9

bit 8

MS Byte

S

S

S

S

S

2

6

2

5

2

4

TEMPERATURE/DATA RELATIONSHIP Table 2

TEMPERATURE

DIGITAL OUTPUT

(Binary)

DIGITAL OUTPUT

(Hex)

+125°C

0000 0111 1101 0000

07D0h

+85°C*

0000 0101 0101 0000

0550h

+25.0625°C

0000 0001 1001 0001

0191h

+10.125°C

0000 0000 1010 0010

00A2h

+0.5°C

0000 0000 0000 1000

0008h

0°C

0000 0000 0000 0000

0000h

-0.5°C

1111 1111 1111 1000

FFF8h

-10.125°C

1111 1111 0101 1110

FF5Eh

-25.0625°C

1111 1110 0110 1111

FE6Fh

-55°C

1111 1100 1001 0000

FC90h

*The power on reset value of the temperature register is +85°C