DS1922T-F5 MAXIM [Maxim Integrated Products], DS1922T-F5 Datasheet - Page 25

• Manufacturer Part Number: DS1922T-F5
• Description: Temperature Logger iButton With 8kB Datalog Memory
• Manufacturer: MAXIM [Maxim Integrated Products]

DS1922L/DS1922T
MISSIONING
The typical task of the DS1922L/T iButton is recording temperature. Before the device can perform this function, it
needs to be set up properly. This procedure is called missioning.
First of all, DS1922L/T needs to have its real-time clock set to valid time and date. This reference time may be the
local time, or, when used inside of a mobile unit, UTC (also called GMT, Greenwich Mean Time) or any other time
standard that was agreed upon. The real-time clock oscillator must be running (EOSC = 1). The memory assigned
to store the Mission Time Stamp, Mission Samples Counter, and Alarm Flags must be cleared using the Memory
Clear command. To enable the device for a mission, the ETL-bit must be set to 1. These are general settings that
have to be made in any case, regardless of the type of object to be monitored and the duration of the mission.
If alarm signaling is desired, the temperature alarm low and high thresholds must be defined. How to convert a
temperature value into the binary code to be written to the threshold registers is described under Temperature
Conversion earlier in this document. In addition, the temperature alarm must be enabled for the low- and/or high-
threshold. This makes the device respond to a Conditional Search command (see ROM Function Commands),
provided that an alarming condition 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 (8-bit format)
or 4096 (16-bit format) to calculate the value of the sample rate (number of minutes between conversions). If the
estimated duration of a mission is 10 days (= 14400 minutes), for example, then the 8192-byte capacity of the
datalog memory would be sufficient to store a new 8-bit value every 1.8 minutes (110 seconds). If the datalog
memory of the DS1922L/T 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 needs to be written to the Sample Rate
Register. The sample rate may be any value from 1 to 16383, coded as an unsigned 14-bit binary number. A
sample rate of all zeros is not valid and must be avoided under all circumstances. This will cause the
device to enter into an unrecoverable state. The fastest sample rate is one sample per second (EHSS = 1,
Sample Rate = 0001h) and the slowest is one sample every 273.05 hours (EHSS = 0, Sample Rate = 3FFFh). To
get one sample every 6 minutes, for example, the sample rate value needs to 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 DS1922L/T or manipulation of data, one should define passwords for
read access and full access. Before the passwords become effective, their use needs to be enabled. See Security
by Password for more details.
The last step to begin a mission is to issue the Start Mission command. As soon as it has received this command,
the DS1922L/T sets the MIP flag and clear the MEMCLR flag. With the immediate/delayed start mode (SUTA = 0),
after as many minutes as specified by the Mission Start Delay are over, the device will wake up, copy the current
date and time to the mission time stamp register, and log the first entry of the mission. This increments both the
Mission Samples Counter and Device Samples Counter. All subsequent log entries will be made as specified by
the value in the Sample Rate Register and the EHSS bit. Note that for version AX devices, the start of the
temperature logging mission may be delayed based on a residual countdown value left from a previous mission.
Please see Application Note 3429, “Datalogger Start Delay Issue in DS2422, DS1923, DS1922L, and DS1922T”
for a detailed explanation and workaround.
If the Start Upon Temperature Alarm mode is chosen (SUTA = 1) and temperature logging is enabled (ETL = 1) the
DS1922L/T first waits until the start delay is over. Then the device wakes up in intervals as specified by the sample
rate and EHSS bit and measure the temperature. This will increment the Device Samples Counter only. The first
sample of the mission is logged when the temperature alarm occurred. However, the Mission Sample Counter will
not increment. One sample period later the Mission Time Stamp is set. From then on, both the Mission Samples
Counter and Device Samples Counter increment at the same time. All subsequent log entries are made as
specified by the value in the Sample Rate Register and the EHSS bit.
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