DS1923 Maxim, DS1923 Datasheet - Page 26

• 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

Hygrochron Temperature/Humidity Logger
iButton with 8KB Data-Log Memory
Once set up for a mission, the DS1923 logs the temper-
ature and/or humidity measurements at equidistant time
points entry after entry in its data-log memory. The
data-log memory can store 8192 entries in 8-bit format
or 4096 entries in 16-bit format (Figure 7a). If tempera-
ture as well as humidity are logged, both in the same
format, the memory is split into two equal sections that
can store 4096 8-bit entries or 2048 16-bit entries
(Figure 7b). If the device is set up to log data in differ-
ent formats, e. g., temperature in 8-bit and humidity in
16-bit format, the memory is split into blocks of different
size, accommodating 2560 entries for either data
source (Figure 7c). In this case, the upper 256 bytes
are not used. In 16-bit format, the higher 8 bits of an
entry are stored at the lower address. Knowing the
starting time point (Mission Timestamp) and the interval
between temperature measurements, one can recon-
struct the time and date of each measurement.
There are two alternatives to the way the DS1923
behaves after the data-log memory is filled with data.
The user can program the device to either stop any fur-
ther recording (disable rollover) or overwrite the previ-
ously recorded data (enable rollover), one entry at a
time, starting again at the beginning of the respective
memory section. The contents of the Mission Samples
Counter in conjunction with the sample rate and the
Mission Timestamp allow reconstructing the time
points of all values stored in the data-log memory. This
gives the exact history over time for the most recent
measurements taken. Earlier measurements cannot be
reconstructed.
The typical task of the DS1923 iButton is recording tem-
perature and/or humidity. Before the device can per-
form this function, it needs to be set up properly. This
procedure is called missioning.
First, the DS1923 must have its RTC set to a valid time
and date. This reference time can be the local time, or,
when used inside of a mobile unit, UTC (also called
GMT, Greenwich Mean Time), or any other time stan-
dard that was agreed upon. The RTC oscillator must be
running (EOSC = 1). The memory assigned to store the
Mission Timestamp, Mission Samples Counter, and
alarm flags must be cleared using the Clear Memory
command. To enable the device for a mission, at least
one of the enable logging bits (ETL, EHL) must be set
to 1. These are general settings that must be made in
any case, regardless of the type of object to be moni-
tored and the duration of the mission.
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Data-Log Memory Usage
Missioning
If alarm signaling is desired, the temperature alarm
and/or humidity alarm low and high thresholds must be
defined. See the Temperature Conversion section for
information on how to convert a temperature value into
the binary code to be written to the threshold registers.
See the Humidity Conversion section for information on
determining the thresholds for the humidity alarm. In
addition, the temperature alarm and/or humidity alarm
must be enabled for the low and/or high threshold. This
makes the device respond to a Conditional Search
ROM command (see the 1-Wire ROM Function
Commands section), provided that an alarming condi-
tion has been encountered.
The setting of the RO bit (rollover enable) and sample
rate depends on the duration of the mission and the
monitoring requirements. If the most recently logged
data is important, the rollover should be enabled (RO =
1). Otherwise one should estimate the duration of the
mission in minutes and divide the number by 8192 (sin-
gle channel 8-bit format) or 4096 (single channel 16-bit
format, two channels 8-bit format) or 2048 (two channels
16-bit format) or 2560 (two channels, one 8-bit and one
16-bit format) to calculate the value of the sample rate
(number of minutes between conversions). If the esti-
mated duration of a mission is 10 days (= 14400min), for
example, then the 8192-byte capacity of the data-log
memory would be sufficient to store a new 8-bit value
every 1.8min (110s). If the data-log memory of the
DS1923 is not large enough to store all readings, one
can use several devices and set the Mission Start Delay
to values that make the second device start logging as
soon as the memory of the first device is full, and so on.
The RO-bit needs to be set to 0 to disable rollover that
would otherwise overwrite the logged data.
After the RO bit and the Mission Start Delay are set, the
sample rate must be written to the Sample Rate regis-
ter. The sample rate can be any value from 1 to 16,383,
coded as an unsigned 14-bit binary number. The
fastest sample rate is one sample per second (EHSS =
1, sample rate = 0001h) and the slowest is one sample
every 273.05hr (EHSS = 0, sample rate = 3FFFh). To
get one sample every 6min, for example, the sample
rate value must be set to 6 (EHSS = 0) or 360 decimal
(equivalent to 0168h at EHSS = 1).
If there is a risk of unauthorized access to the DS1923
or manipulation of data, one should define passwords
for read access and full access. Before the passwords
become effective, their use must be enabled. See the
Security by Password section for more details.