The DS1921L Thermochron iButtons are rugged, self-sufficient systems that measure temperature and
record the result in a protected memory section. The recording is done at a user-defined rate, both as a
direct storage of temperature values as well as in the form of a histogram. Up to 2048 temperature values
taken at equidistant intervals ranging from 1 to 255 minutes can be stored. The histogram provides 63
data bins for a resolution of 2.0°C. If the temperature leaves a user-programmable range, the DS1921L
will also record when this happened, for how long the temperature stayed outside the permitted range,
and if the temperature was too high or too low. An additional 512 bytes of read/write NV memory allow
storing information pertaining to the object to which the DS1921L is associated. Data is transferred
serially via the 1-Wire protocol, which requires only a single data lead and a ground return. Every
DS1921L is factory-lasered with a guaranteed unique 64-bit registration number that allows for absolute
traceability. The durable stainless steel package is highly resistant to environmental hazards such as dirt,
moisture, and shock. Accessories permit the DS1921L to be mounted on almost any object, including
containers, pallets, and bags.
The DS1921L Thermochron iButton is an ideal device to monitor the temperature of any object it is
attached to or shipped with, such as perishable goods or containers of temperature sensitive chemicals.
Using TMEX, the read/write NV memory can store an electronic copy of shipping information, date of
manufacture and other important data written as clear as well as encrypted files.
The block diagram in Figure 1 shows the relationships between the major control and memory sections of
the DS1921L. The device has seven main data components: 1) 64-bit lasered ROM, 2) 256-bit scratch-
pad, 3) 4096-bit general-purpose SRAM, 4) 256-bit register page of timekeeping, control, and counter
registers, 5) 96 bytes of alarm time stamp and duration logging memory, 6) 126 bytes of histogram mem-
ory, and 7) 2048 bytes of data-logging memory. Except for the ROM and the scratchpad, all other mem-
ory is arranged in a single linear address space. All memory reserved for logging purposes, counter reg-
isters and several other registers are read-only for the user. The timekeeping and control registers are
write-protected while the device is programmed for a mission.
The hierarchical structure of the 1-Wire protocol is shown in Figure 2. The bus master must first provide
one of the seven ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, 4) Condi-
tional Search ROM, 5) Skip ROM, 6) Overdrive-Skip ROM or 7) Overdrive-Match ROM. Upon comple-
tion of an Overdrive ROM command byte executed at standard speed, the device will enter Overdrive
mode, where all subsequent communication occurs at a higher speed. The protocol required for these
ROM function commands is described in Figure 12. After a ROM function command is successfully exe-
cuted, the memory functions become accessible and the master may provide any one of the seven avail-
able commands. The protocol for these memory function commands is described in Figure 10. All data is
read and written least significant bit first.
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