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DS1921

Thermochron iButton

Dallas Semiconductor -> Maxim Integrated 

DS1921G Thermochron iButton

iButton DESCRIPTION

The DS1921G Thermochron iButton is a rugged, self-sufficient system that measures temperature and

records 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 with a resolution of 2.0°C. If the temperature leaves a user-programmable range, the DS1921G

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 DS1921G is associated. Data is transferred

serially via the 1-Wire protocol, which requires only a single data lead and a ground return. Every

DS1921G is factory-lasered with a guaranteed unique electrically readable 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 DS1921G to be mounted on almost any

object, including containers, pallets, and bags.

APPLICATION

The DS1921G 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.

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. Note that the initial sealing level of

DS1921G achieves IP56. Aging and use conditions can degrade the integrity of the seal over time, so for

applications with significant exposure to liquids, sprays, or other similar environments, it is recom-

mended to place the Thermochron in the DS9107 iButton capsule. The DS9107 provides a watertight

enclosure that has been rated to IP68 (see www.maxim-ic.com/AN4126).

OVERVIEW

The block diagram in Figure 1 shows the relationships between the major control and memory sections of

the DS1921G. 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 13. 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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