DS1923 Maxim, DS1923 Datasheet - Page 19

• Manufacturer Part Number: DS1923
• Description: The DS1923 temperature/humidity logger iButton® is a rugged, self-sufficient system that measures temperature and/or humidity and records the result in a protected memory section
• Manufacturer: Maxim

During a mission, if humidity logging is enabled, the
HRH byte (H11 to H4) is always recorded. The HRL
byte is only recorded if the DS1923 is set up for 16-bit
humidity logging. The logging mode (8-bit or 16-bit) is
selected through the HLFS bit at the Mission Control
register, address 0213h.
The result is a raw humidity reading that needs to be
corrected to achieve the specified accuracy. See the
Software Correction Algorithm for Humidity section for
further details.
To specify the humidity alarm thresholds, the equation
needs to be resolved to:
Round HALM to the nearest integer.
The HALM value needs to be converted into hexadeci-
mal before it can be written to one of the Humidity Alarm
Threshold registers (Low Alarm address 020Ah; High
Alarm address 020Bh). Independent of the conversion
Table 3. Humidity Conversion Examples
Table 4. Humidity Alarm Threshold Examples
Round IVAL down to the nearest integer; this eliminates the
undefined 4 bits of HRL.
8-bit
8-bit
16-bit
16-bit
ADDRESS
HUMIDITY (%RH)
020Eh
020Fh
ADVAL = HUMIDITY(%RH) x 0.0307 + 0.958
MODE
65
25
Hygrochron Temperature/Humidity Logger
HALM = ADVAL x 256/5.02
BIT 7
H11
IVAL = (HRH x 256 + HRL)/16
H3
ADVAL = IVAL x 5.02/4096
______________________________________________________________________________________
16-BIT MODE, HLFS = 1
HEX
97h
58h
BIT 6
HEX
H10
B5h
B5h
67h
67h
H2
iButton with 8KB Data-Log Memory
HALM
HRH
BIT 5
HUMIDITY(%RH) = (ADVAL - 0.958)/0.0307
H1
H9
DECIMAL
DECIMAL
151
88
181
103
181
103
Latest Humidity Conversion Result Register Bitmap
BIT 4
H0
H8
mode (8-bit or 16-bit), only the most significant byte of a
humidity conversion is used to determine whether an
alarm is generated. The alarm thresholds are applied to
the raw humidity readings. Therefore, if software correc-
tion is used, the effect of the software correction is to be
reversed before calculating a humidity alarm threshold.
For example, let the desired alarm threshold be 60%RH.
The 60% threshold may correspond to a raw reading of
65%RH (i.e., before correction). To set a 60%RH (after
correction) threshold, the HALM value then needs to be
calculated for 65%RH.
These examples do not include the effects of software
correction.
With HRH and HRL representing the decimal equivalent
of a humidity reading, the actual humidity is calculated
according to the algorithms shown in the table below.
BIT 3
H7
X
HEX
C0h
30h
BIT 2
H6
X
HRL
ADVAL = HRH x 5.02/256
8-BIT MODE, HLFS = 0
DECIMAL
BIT 1
(N/A)
H5
12
48
X
BIT 0
H4
X
HUMIDITY (%RH)
84.41
34.59
84.89
34.70
BYTE
HRH
HRL
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