Datasheet

DS1922L/DS1922T
Temperature Logger iButton with 8KB
Data-Log Memory
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Temperature Conversion
The DS1922L’s temperature range begins at -40°C and
ends at +85°C. The temperature range for the DS1922T
begins at 0 and ends at +125°C. Temperature values
are represented as an 8-bit or 16-bit unsigned binary
number with a resolution of 0.5°C in 8-bit mode and
0.0625°C in 16-bit mode.
The higher temperature byte TRH is always valid. In
16-bit mode, only the three highest bits of the lower
byte TRL are valid. The five lower bits all read 0. TRL is
undefined if the device is in 8-bit temperature mode. An
out-of-range temperature reading is indicated as 00h or
0000h when too cold and FFh or FFE0h when too hot.
With TRH and TRL representing the decimal equivalent
of a temperature reading, the temperature value is cal-
culated as:
ϑ(°C) = TRH/2 - 41 + TRL/512 (16-bit mode,
TLFS = 1, see address 0213h)
ϑ(°C) = TRH/2 - 41 (8-bit mode, TLFS = 0,
see address 0213h)
This equation is valid for converting temperature read-
ings stored in the data-log memory as well as for data
read from the Latest Temperature Conversion Result
register. The “-41” applies to the DS1922L. For the
DS1922T, use “-1” instead of “-41.”
To specify the temperature alarm thresholds, the previ-
ous equations are resolved to:
TALM = 2 x ϑ(°C) + 82
The “+82” applies to the DS1922L. For the DS1922T, use
“+2.” Because the temperature alarm threshold is only
one byte, the resolution or temperature increment is limit-
ed to 0.5°C. The TALM value must be converted into
hexadecimal format before it can be written to one of the
Temperature Alarm Threshold registers (Low Alarm
address 0208h; High Alarm address 0209h).
Independent of the conversion mode (8-bit or 16-bit),
only the most significant byte of a temperature conver-
sion is used to determine whether an alarm is generated.
Latest Temperature Conversion Result Register Bitmap
ADDRESS BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 BYTE
020Ch T2 T1 T0 0 0 0 0 0 TRL
020Dh T10 T9 T8 T7 T6 T5 T4 T3 TRH
Table 1. Temperature Conversion Examples
TRH TRL (°C)
MODE
HEX DECIMAL HEX DECIMAL DS1922L DS1922T
8-Bit 54h 84 — — 1.0 41.0
8-Bit 17h 23 — — -29.5 10.5
16-Bit 54h 84 00h 0 1.000 41.000
16-Bit 17h 23 60h 96 -29.3125 10.6875
TALM (DS1922T) TALM (DS1922L)
(°C)
HEX DECIMAL
(°C)
HEX DECIMAL
65.5 85h 133 25.5 85h 133
30.0 3Eh 62 -10.0 3Eh 62
Table 2. Temperature Alarm Threshold Examples