DS1921H/Z Maxim Integrated Products, DS1921H/Z Datasheet - Page 14

• Manufacturer Part Number: DS1921H/Z
• Description: High Resolution Thermochron Ibutton Range H
• Manufacturer: Maxim Integrated Products

DS1921H/Z
MISSIONING
The typical task of the DS1921H/Z is recording the temperature of a temperature-sensitive object. Before
the device can perform this function, it needs to be configured. This procedure is called missioning.
First of all, DS1921H/Z needs to have its RTC set to valid time and date. This reference time may be
UTC (also called GMT, Greenwich Mean Time) or any other time standard that was chosen for the
application. The clock must be running (EOSC = 0). Setting a RTC alarm is optional. The memory
assigned to storing alarm time stamps and durations, temperature histogram, as well as the Mission Time
Stamp, Mission Samples Counter, Mission Start Delay, and Sample Rate must be cleared using the
Memory Clear command. To enable the device for a mission, the EM flag must be set to 0. These are
general settings that have to be made regardless of the type of object to be monitored and the duration of
the mission.
Next, the low temperature and high temperature thresholds to specify the temperature tolerance band
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.
The state of the Search Condition bits in the Control Register does not affect the mission. If multiple
devices are connected to form a 1-Wire net, the setting of the search condition will enable devices to
participate in the conditional search if certain events such as timer or temperature alarm have occurred.
Details on the search conditions are found in the section ROM Function Commands later in this document
and in the Control Register description.
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 recent temperature history 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 2048 to calculate the value of the sample rate (number of minutes between temperature
conversions). If the estimated duration of a mission is 10 days (= 14400 minutes) for example, then the
2048-byte capacity of the datalog memory would be sufficient to store a new value every 7 minutes. If the
datalog memory of the DS1921H/Z is not large enough to store all temperature readings, one can use
several devices and set the Mission Start Delay to values that make the second device start recording 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 recorded temperature log.
After the RO bit and the Mission Start Delay are set, the Sample Rate Register is the last element of data
that is written. The sample rate may be any value from 1 to 255, coded as an unsigned 8-bit binary
number. As soon as the sample rate is written, the DS1921H/Z will copy the current time and date into
the Mission Time Stamp Register, set the MIP flag, and clear the MEMCLR flag. After as many minutes
as specified by the Mission Start Delay are over, the device takes the first temperature conversion of the
mission. This increments both the Mission Samples Counter and Device Samples Counter. All subsequent
temperature measurements are taken on minute boundaries specified by the value in the Sample Rate
Register. One may read the memory of the DS1921H/Z to watch the mission as it progresses at any time.
After the mission is started, one can read the complete register page and store the contents of the
Temperature Alarm Registers up to the Device Samples Counter in encrypted form as data file in the
4096-bit SRAM section of the device. This general-purpose memory operates independently of the
memory used for recording during a mission. However, do not write any of the mission-ending registers
on the register page.
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