LM75A_4 © NXP B.V. 2007. All rights reserved.
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
