Philips Semiconductors

Product data sheet

LM75A

Digital temperature sensor and thermal Watchdog

™

2004 Oct 05

9

Configuration register

The Configuration register is a write/read register and contains an 8-bit non-complement data byte that is used to configure the device for

different operation conditions. The Configuration register table (Table 5) shows the bit assignments of this register.

Table 5. Configuration register table

Bit

Name

R/W

POR

Description

B7–B5

Reserved

R/W

000

Reserved for the manufacterer use.

B4–B3

OS Fault queue

R/W

00

For OS Fault Queue programming.

Programmable queue data = 0, 1 ,2, 3 for queue value = 1, 2, 4, 6 respectively. Default = 0.

B2

OS Polarity

R/W

0

For OS Polarity selection.

1 = OS active HIGH, 0 = OS active LOW (default).

B1

OS Comp/Interrupt

R/W

0

For OS operation Mode selection.

1 = OS interrupt, 0 = OS comparator (default).

B0

Shut-down

R/W

0

For Device Operation Mode selection.

1 = Shut-down, 0 = Normal (default).

Temperature register (Temp)

The Temp register holds the digital result of temperature measurement or monitor at the end each A-to-D conversion. This register is read only

and contains two 8-bit data bytes consisting of one most significant (MS) data byte and one least significant (LS) data byte. However, only 11

bits of those two bytes are used to store the Temp data in 2’s complement format with the resolution of 0.125

°C. The Temp register table

(Table 6) shows the bit arrangement of the Temp data in the data bytes.

Table 6. Temp register table

Temp MS byte

Temp LS byte

MSB

LSB

MSB

LSB

B7

B6

B5

B4

B3

B2

B1

B0

B7

B6

B5

B4

B3

B2

B1

B0

Temp data (11 bits)

Not used

MSB

LSB

D10

D9

D8

D7

D6

D5

D4

D3

D2

D1

D0

X

X

X

X

X

Notice that when the Temp register is read, all 16 bits are provided to the bus and must be all collected by the controller to complete the bus

operation. However, only the 11 significant bits should be used, and the 5 LSB bits of the LS byte are zero and should be ignored. One of the

ways to calculate the Temp value in

°C from the 11-bit Temp data is:

1. If the Temp data MSB bit D10 = 0 then the temperature is positive and

Temp value (

°C) = + (Temp data) * 0.125 °C

2. If the Temp data MSB bit D10 = 1 then the temperature is negative and

Temp value (

°C) = – (2’s complement of Temp data) * 0.125 °C

The Temp table (Table 7) shows examples of the Temp data and value.

Table 7. Temp table

Temp data

Temp value

11–bit Binary (2’s complement)

3-bit Hex

Decimal value

°C

0111 1111 000

3F8h

1016

+127.000

°C

0111 1110 111

3F7h

1015

+126.875

°C

0111 1110 001

3F1h

1009

+126.125

°C

0111 1101 000

3E8h

1000

+125.000

°C

0001 1001 000

0C8h

200

+25.000

°C

0000 0000 001

001h

1

+0.125

°C

0000 0000 000

000h

0

0.000

°C

1111 1111 111

7FFh

–1

–0.125

°C

1110 0111 000

738h

–200

–25.000

°C

11001001 001

649h

–439

–54.875

°C

1100 1001 000

648h

–440

–55.000

°C

Obviously, for 9-bit Temp data application in replacing the industry standard LM75, just use only 9 MSB bits of the two bytes and disregard

7 LSB bits of the LS byte. The 9-bit temp data with 0.5

°C resolution of the LM75A is defined exactly in the same way as for the standard LM75

and it is here similar to the Tos and Thyst that is described next.