Philips Semiconductors

Product data sheet

LM75A

Digital temperature sensor and thermal Watchdog

™

2004 Oct 05

10

Overtemp shut-down threshold (Tos) and hysteresis (Thyst) registers

These two registers are write/read registers, and also called set-point registers. They are used to store the user-defined temperature limits,

called overtemp shut-down threshold (Tos) and hysteresis (Thyst), for the device Watchdog operation. At the end of each conversion the Temp

data will be compared with the data stored in these two registers in order to set the state of the device OS output accordingly as described in the

“General operation” section.

Each of the set-point registers contains two 8-bit data bytes consisting of one MS data byte and one LS data byte the same as the Temp

register. However, only 9 bits of the two bytes are used to store the set-point data in 2’s complement format with the resolution of 0.5

°C. The

Tos register table (Table 8) and Thyst register table (Table 9) show the bit arrangement of the Tos data and Thyst data in the data bytes.

Notice that because only 9-bit data are used in the set-point registers, the device uses only the 9 MSB bits of the Temp data for data

comparison.

Table 8. Tos register table

Tos MS byte

Tos LS byte

MSB

LSB

MSB

LSB

B7

B6

B5

B4

B3

B2

B1

B0

B7

B6

B5

B4

B3

B2

B1

B0

Tos data (9 bits)

Not used

MSB

LSB

D8

D7

D6

D5

D4

D3

D2

D1

D0

X

X

X

X

X

X

X

Table 9. Thyst register table

Thyst MS byte

Thyst LS byte

MSB

LSB

MSB

LSB

B7

B6

B5

B4

B3

B2

B1

B0

B7

B6

B5

B4

B3

B2

B1

B0

Thyst data (9 bits)

Not used

MSB

LSB

D8

D7

D6

D5

D4

D3

D2

D1

D0

X

X

X

X

X

X

X

When a set-point register is read, all 16 bits are provided to the bus and must be collected by the controller to complete the bus operation.

However, only the 9 significant bits should be used and the 7 LSB bits of the LS byte are equal to zero and should be ignored.

The Tos and Thyst table (Table 10) shows examples of the limit data and value.

Table 10. Tos and Thyst table

Limit data

Limit temp value

11–bit Binary (2’s complement)

3-bit Hex

Decimal value

°C

0111 1101 0

0FAh

250

+125.0

°C

0001 1001 0

032h

50

+25.0

°C

0000 0000 1

001h

1

+0.5

°C

0000 0000 0

000h

0

0.0

°C

1111 1111 1

1FFh

–1

–0.5

°C

1110 0111 0

1CEh

–50

–25.0

°C

1100 1001 0

192h

–110

–55.0

°C