STDS75 Datasheet, PDF (18/37 Pages) STMicroelectronics – Digital temperature sensor and thermal watchdog

Functional description
STDS75
3.1.3
Note:
3.1.4
Temperature register
The Temperature register is a two-byte (16-bit) “Read only” register (see Table 8 on
page 18). Digital temperatures from the ADC are stored in the Temperature Register in
two’s complement format, and the contents of this register are updated each time the A/D
conversion is finished.
The user can read data from the Temperature Register at any time. When a A/D conversion
is completed, the new data is loaded into a comparator buffer to evaluate fault conditions,
and will update the Temperature Register if a read cycle is not ongoing. The STDS75 is
continuously evaluating fault conditions regardless of READ or WRITE activity on the bus. If
a READ is ongoing, the previous temperature will be read. The readable temperature will be
updated upon the completion of the next A/D conversion that is not masked by a read cycle.
Depending on the A/D conversion resolution, the 9-, 10-, 11- or 12-bit MSBs of the register
will contain temperature data. All unused bits following the digital temperature will be zero.
The MSB (Bit 15) of the Temperature Register denotes whether the temperature data is
positive or negative. A '0' in Bit 15 is positive and a '1' is negative.
Table 8.
Bytes
Temperature register format
MS Byte
LS Byte
MSB
THSB
Bits
15 14 13 12 11 10 9 8 7
TLSB
LSB
6
5
4 321 0
STDS75
SB
TMSB TD
TD
TD
TD TD TD
9-bit
LSB
10-bit
LSB
11-bit
LSB
12-bit
LSB
0
0
0
0
Keys: SB =Two’s complement Sign Bit
TMSB =Temperature MSB
TLSB =Temperature LSB
TD =Temperature Data
These are comparable formats to the DS75 and LM75.
Over-limit temperature register (T_OS)
The T_OS Register is a two-byte (16-bit) READ/WRITE register that stores the user-
programmable upper trip-point temperature for the thermal alarm in two’s complement
format (see Table 9 on page 19). This register defaults to 80°C at power-up (i.e., 0101 0000
0000 0000).
The format of the T_OS Register is identical to that of the Temperature Register. The 4 LSBs
of the T_OS Register are hardwired to zero, so data written to these register bits will be
ignored. The MSB position contains the sign bit for the digital temperature and Bit14
contains the temperature MSB.
The resolution setting for the A/D conversion determines how many bits of the T_OS Register
are used by the thermal alarm. For example, for 9-bit conversions, the trip-point temperature
is defined by the 9 MSBs of the T_OS register, and all remaining bits are “Don’t cares.”
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