Datasheet

Product data sheet Rev. 04 — 10 July 2007 8 of 24

NXP Semiconductors

LM75A

Digital temperature sensor and thermal watchdog

When reading register Temp, all 16 bits of the two data bytes (MSByte and LSByte) are

provided to the bus and must be all collected by the controller to complete the bus

operation. However, only the 11 most signiﬁcant bits should be used, and the 5 least

signiﬁcant bits of the LSByte 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 MSByte bit D10 = 0, then the temperature is positive and Temp value

(°C) = +(Temp data) × 0.125 °C.

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

Temp value (°C) = −(2’s complement of Temp data) × 0.125 °C.

Examples of the Temp data and value are shown in Table 10.

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 of the LSByte. 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 registers.

7.4.4 Overtemperature shutdown 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 overtemperature shutdown

threshold (Tos) and hysteresis temperature (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; see Section 7.1.

Each of the set-point registers contains two 8-bit data bytes consisting of one MSByte and

one LSByte the same as register Temp. 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. Table 11

and Table 12 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 of the Temp data for data comparison.

Table 10. Temp register value

11-bit binary

(2’s complement)

Hexadecimal value Decimal value Value

011 1111 1000 3F8 1016 +127.000 °C

011 1111 0111 3F7 1015 +126.875 °C

011 1111 0001 3F1 1009 +126.125 °C

011 1110 1000 3E8 1000 +125.000 °C

000 1100 1000 0C8 200 +25.000 °C

000 0000 0001 001 1 +0.125 °C

000 0000 0000 000 0 0.000 °C

111 1111 1111 7FF −1 −0.125 °C

111 0011 1000 738 −200 −25.000 °C

110 0100 1001 649 −439 −54.875 °C

110 0100 1000 648 −440 −55.000 °C