How does it work?
DS1990A is a serial number iButton™ device, from MaximIntegrated. This device is
programmed by the guaranteed unique 64-bit registration number that allows for
absolute traceability. The thick stainless steel casing of the iButton ensures resistance
against dirt, dust, moisture, shock and other environmental hazards. The button shape
helps to establish a proper alignment with the probe, such the one found on the iButton
click and ensures reliable communication with a speed of 16.3kbps.
The device is capable of powering itself up through the data line by employing the so-
called parasite power supply. Parasite PSU contains an internal capacitor, which provides
enough current for the proper operation, once it has been charged by the data line. To
allow proper functioning of the parasitic PSU, the idle state of the data line is HIGH,
while the data line of the DS1990A is in an open-drain configuration, pulling the data
line to a LOW logic level when asserted.
The factory programmed serial number is read through the standard 1-Wire
communication protocol. The first 8 bytes represent a 1-Wire family code, while the
following 48 bits contain the unique serial number. The last 8 bits contain the CRC of the
previous data, ensuring data integrity. The host MCU initiates the communication by
sending a reset signal, responded by the presence signal from the attached devices.
More information about the 1-Wire signaling can be found in the DS1990A datasheet.
MikroElektronika offers a library which contains functions for reading the content of this
iButton via the iButtonclick. This library is compatible with the MikroElektronika line of
compilers and supported MCUs. The serial number information stored on this iButton
can also be extracted by any iButton reader, compatible with the Maxim Integrated
iButton devices.
