What are iButtons?
iButtons are small integrated circuits, embedded in a rugged stainless steel case. The name
iButton originates in its similarity to button cell batteries. iButtons are available in different types, amongst others with the function as a data logger or as a universal memory.
Using suitable accessories, iButtons can be mounted to just about every thing. Reading and writing information is also possible while mounted to an object.
iButton as a temperature logger
Authentication through iButtons
Every iButton has a unique and unchangeable 64-bit registration number. This consists of a 48-bit serial number, an 8-bit family code and an 8-bit checksum. The registration number is engraved on the iButton.
Case Engraved with Registration Number
The two digits on the right (01) are the family code of the iButton. With that the type can be identified. The two left-sided digits (90) are the checksum of the serial number and the family code. The digit sequence in the middle (00001BF2743A) is the serial number of the iButton. The table below shows the family code of some iButton models with associated part name.
|Family Code||Part Name|
|Table: Family Codes of iButtons|
The cryptic appearance of part names of iButtons often cause confusion among the user. However it is far less complicated as it seems. In the course of this article the naming of iButtons will be discussed in more detail.
Memory iButtons are available in various types. The most simple memory iButton has a 64-bit read-only memory. The memory contains the worldwide unique registration number. These serial number iButtons are used to identify persons and other objects. Examples are identification of drivers in motor vehicles or the identification of employees at a point-of-sale terminal. Other iButtons provide memory capacities of up to 32 kilobyte where, for example, process data or individual user data can be stored.
RTC iButtons feature an integrated Real-Time-Clock and are used as timer. Thereby embedded applications, based on a microcontroller, can be extended with functions like calendar, stopwatch or timer with minimal effort.
iButtons with data logging function are also called Thermochron or Hygrochron iButtons. These are temperature/humidity logger in miniature format. With these iButtons a so-called mission is established by means of a software. The Thermochron iButton records the environmental temperature at a specified interval and saves it with a timestamp on the integrated memory. Different types can store different amounts of measuring values.
The various Thermochron iButtons mainly differ in terms of measuring range, memory capacity and accuracy. The table below shows the most common Thermochron iButtons with their most important specifications.
|DS1921G||-40°C to +85°C||±1°C from -30°C bis +70°C||0.5°C||2048|
|DS1921H||+15°C to +46°C||±1°C||0.125°C||2048|
|DS1921Z||-5°C to +26°C||±1°C||0.125°C||2048|
|DS1922L||-40°C to +85°C||±0.5°C from -10°C to +65°C||0.5°C or 0.0625°C||8192 or 4096|
|DS1922T||0°C to +125°C||±0.5°C from +20°C to +75°C||0.5°C or 0.0625°C||8192 or 4096|
|DS1922E||+15°C to +140°C||±1.5°C from +110°C to +140°C||0.5°C or 0.0625°C||8192 oder 4096|
|DS1925L||-40°C to +85°C||±0.5°C||0.5°C or 0.0625°C||122K or 61K|
|Tabelle: Specifications of a variety of Thermochron iButtons|
The data exchange with iButtons is serial via 1-Wire protocol. Thus only one data line, like the pin of a microcontroller is needed. Several iButtons can be connected to the 1-Wire bus in parallel. The data transfer is checked for errors using a checksum.
Available are user friendly accessories, enabling an iButton to be connected to the USB port of a computer. The manufacturer provides free-of-charge software for communication with iButtons.
Components of a typical iButton application
The power required by the iButton is supplied either by an integrated battery or by the data line of the 1-Wire bus. Preferably a battery is used with data logger iButtons or if the iButton features a clock (RTC). Some older types of memory iButtons need a permanent power supply because of the type of integrated memory.
In general the battery of iButtons can not be changed or charged, also because an interuption of the power would reset the integrated memory. In case of Thermochron iButtons this would result in loosing the calibration data provided by the manufacturer and thus making the iButton unusable. Depending on iButton and application, the battery life can be more then ten years.
Construction of a Thermochron iButton
The space available in the iButton case is rather limited. Besides the battery and the circuit board there is no space for further electronics as it would probably be required for charging the battery.
The part names of iButtons sound very cumbersome, but follow a simple pattern. The example of a DS1990A-F5+ shall illustrate this. The part
DS1990 is the base name for serial number iButtons. The extension
A is a sub category of the DS1990 serial number iButton. The extension F5 specifies the structural shape. Two types of casings are in use, the about 5 mm high standard case (F5 Microcan) and the flatter, about 3 mm high case (F3 Microcan). The latter one is not very common, also because the thin design makes it incompatible to many accessories.
+ at the end of the part name indicates the iButton conforms to RoHS. This is an EU directive that limits the usage of hazardous materials in electrical and electronic equipment. This marking is often omitted, because meanwhile only RoHS-compatible iButtons are produced. A
# at the end of the part name however states, the product complies to an exception of the RoHS directive.
On some types the engraved part name on the iButton can differ from the official part name of the manufacturer. For example, the DS1990A-F5+ iButton has the engraved part name DS1990+F5.
Do you have further questions or need specific information about certain iButton models? Please contact us.