DS1921 DALLAS [Dallas Semiconductor], DS1921 Datasheet - Page 12

• Manufacturer Part Number: DS1921
• Description: High-Resolution Thermochron iButton
• Manufacturer: DALLAS [Dallas Semiconductor]

Mission Samples Counter Register Map
DEVICE SAMPLES COUNTER
The Device Samples Counter indicates how many temperature measurements have taken place since the
device was assembled at the factory. The value is stored as an unsigned 24-bit integer number. The
maximum number that can be represented in this format is 16777215, which is higher than the expected
lifetime of the DS1921H/Z iButton. This counter cannot be reset under software control.
Device Samples Counter Register Map
TEMPERATURE LOGGING AND HISTOGRAM
Once setup for a mission, the DS1921H/Z logs the temperature measurements simultaneously byte after
byte in the datalog memory as well as in histogram form in the histogram memory. The datalog memory
is able to store 2048 temperature values measured at equidistant time points. The first temperature value
of a mission is written to address location 1000h of the datalog memory, the second value to address
location 1001h and so on. Knowing the starting time point (Mission Time Stamp), the interval between
temperature measurements, the Mission Samples Counter, and the rollover setting, one can reconstruct
the time and date of each measurement stored in the datalog.
There are two alternatives to the way the DS1921H/Z will behave after the 2048 bytes of datalog memory
is filled with data. With rollover disabled (RO = 0), the device will fill the datalog memory with the first
2048 mission samples. Additional mission samples are not logged in the datalog, but the histogram, and
temperature alarm memory continue to update. With rollover enabled (RO = 1), the datalog will wrap
around, and overwrite previous data starting at 1000h for the every 2049
the device stores the last 2048 mission samples.
For the temperature histogram, the DS1921H/Z provides 64 bins that begin at memory address 0800h.
Each bin consists of a 16-bit, non-rolling-over binary counter that is incremented each time a temperature
value acquired during a mission falls into the range of the bin. The least significant byte of each bin is
stored at the lower address. Bin 0 begins at memory address 0800h, bin 1 at 0802h, and so on up to
087Eh for bin 63, as shown in Figure 7. The number of the bin to be updated after a temperature
conversion is determined by cutting off the two least significant bits of the binary temperature value. Out
of range values are range locked and counted as 00h or FFh.
021Ah
021Bh
021Ch
021Dh
021Eh
ADDR
ADDR
021Fh
b7
b7
b6
b6
b5
b5
b4
b4
Center Byte
Center Byte
High Byte
High Byte
Low Byte
Low Byte
b3
b3
12 of 44
b2
b2
b1
b1
th
b0
b0
mission sample. In this mode
DS1921H/Z