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Maxim > Design Support > Technical Documents > Application Notes > 1-Wire® Devices > APP 74
Maxim > Design Support > Technical Documents > Application Notes > i
Button® > APP 74
Maxim > Design Support > Technical Documents > Application Notes > UARTs > APP 74
Keywords: 1-Wire, OneWire, iButton, iButtons, serial port, serial communication, uart, 8253, 8250, 1wire
APPLICATION NOTE 74
Reading and Writing 1-Wire® Devices Through
Serial Interfaces
By: Bernhard Linke, Principal Member Technical Staff
Jun 26, 2009
Abstract: This application note presents the electrical aspect of the 1-Wire protocol for new and legacy
devices. A special section explains how to determine appropriate timing parameters for a network
comprised of both old and new 1-Wire slaves. The 1-Wire Master Concepts section provides references
to other documents that discuss 1-Wire masters in detail and to the related software.
Introduction
A 1994 application note explained that the only serial-port interface options for 1-Wire devices were
microcontroller port pins, UARTs, and UART-based COM ports. Since that time special driver chips have
been developed for direct connection to a UART, I²C bus, or USB port. Meanwhile, the number of 1-Wire
devices also grew to a long list. (See application note 1796, "Overview of 1-Wire® Technology and Its
Use.") These various developments made it necessary to update the earlier documentation. Instead of
merging the specifics of all relevant information into a single document, this new document refers the
reader to other application notes whenever possible.
The Technological Evolution of 1-Wire Devices
The first 1-Wire devices, the DS199x series, were produced in SRAM technology. Next the nonvolatile
EPROM technology became available, and the DS198x and DS250x series devices were released.
These EPROM devices need a 12V programming pulse and are not erasable. The next leap forward
was EEPROM technology, which allows programming and erasing at 5V or less. EEPROM technology is
found in DS197x, DS243x and DS28Exx series devices. To ensure proper power, EEPROM devices may
need a master that supports "strong pullup", a feature that temporarily bypasses the 1-Wire pullup
resistor with a low-impedance path. The extra power is needed for write cycles and, in case of the
DS1977, also for reading. Besides EEPROM devices, the strong pullup also powers 1-Wire temperature
sensors and special functions such as a SHA-1 engine, which is found in secure 1-Wire devices.
Temperature logger i
Buttons® use SRAM technology and, therefore, do not have any special, external
power requirements.
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