摘要

This article provides a general overview of Maxim’s 1-Wire^® technology, its communication concept and the benefit of low pin count package options. The main section discusses 1-Wire devices by their feature set and explains the typical applications. The article concludes with practical information on how to evaluate 1-Wire devices, explains device customization options, and references resources that assist customers with integration of 1-Wire technology in their systems.

What Is 1-Wire Technology?

The basis of 1-Wire^® technology is a serial protocol using a single data line plus ground reference for communication. A 1-Wire master initiates and controls the communication with one or more 1-Wire slave devices on the 1-Wire bus (Figure 1). Each 1-Wire slave device has a unique, unalterable, factory-programmed, 64-bit identification number (ID), which serves as device address on the 1-Wire bus. The 8-bit family code, a subset of the 64-bit ID, identifies the device type and functionality. Typically, 1-Wire slave devices operate over the following four voltage ranges:

• 1.71V (min) to 1.89V (max)
• 1.71V (min) to 3.63V (max)
• 2.97V (min) to 3.63V (max)
• 2.8V (min) to 5.25V (max)

Most 1-Wire devices have no pin for a power supply; they take their energy from the 1-Wire bus (parasitic power supply).

What Is Special About 1-Wire?

1-Wire is a voltage-based digital system that works with two contacts, data and ground, for half- duplex bidirectional communication. Compared to other serial communication systems such as I²C or SPI, 1-Wire devices are designed for use in a momentary contact environment. Either disconnecting from the 1-Wire bus or a loss of contact puts the 1-Wire slaves into a defined reset state. When the voltage returns, the slaves wake up and signal their presence. With only one contact to protect, the built-in ESD protection of 1-Wire devices is extremely high. With two contacts, 1-Wire devices are the most economical way to add electronic functionality to nonelectronic objects for identification, authentication, and delivery of calibration data or manufacturing information.

How Are 1-Wire Devices Packaged?

1-Wire devices are offered in conventional transistor (TO-92) and IC (TSOC, TDFN, SOT23) packages. Designed for contact applications and easy attachment, the 2-contact SFN¹ package accommodates parasitically powered 1-Wire devices (Figure 2). The 16mm-diameter stainless steel iButton^®2 package protects 1-Wire devices from harsh environments, making them suitable for indoor and outdoor use (Figure 3). Many devices are also available as bumped die (Figure 4).

What Device Functions Are Available, and What Are the Typical Applications?

Currently, there are a variety of 1-Wire devices. Part numbers that begin with DS19 are always in iButton packages. The remaining part numbers in this section, i.e., those that begin with DS24, DS25, and DS28, are available in conventional plastic packages. If a product is also available as an SFN, then this is mentioned in the Notes column of the following part tables. To find up-to-date package information for any device, go to the Analog website and enter the part number in the Search field.

1-Wire devices can be grouped by their functions into several categories:

• Identification only
• Identification plus control
• Identification plus temperature
• Identification plus time
• Identification plus NV SRAM
• Identification plus one-time programmable (OTP) EPROM
• Identification plus EEPROM
• Identification plus SHA-256 authentication and secure EEPROM
• Identification plus SHA-256/ECDSA authentication and secure memory
• Identification plus SHA-3 authentication, ChipDNA^™-encrypted EEPROM
• Identification plus data logging

The combination of additional resources determines the typical application areas for a device. There are numerous other applications that, for practical reasons, cannot be listed here.

Identification Only

Devices in this category are notable for their low cost. Accordingly, the level of protection that they can provide is limited. If the DS2401 or DS2411 is used for the protection of intellectual property (IP), one should consider a customized version of the device. See the Is There Any Device Customization Available? section for more details.

Identification plus Control

The applications for these parts are similar to the devices above. The programmable input/output pins (PIOs), however, add control or feedback capability to the system.

Identification plus Temperature

In large electronic equipment (racks), proper cooling/air flow is critical for reliable operation. The DS28EA00 solves this problem in a unique way through its chain function,³ which allows detecting the physical sequence of the device on the 1-Wire bus. When wired for sequence detection, one of the two PIOs is available for control functions.

Identification plus Time

These devices measure time by counting seconds using a 32-bit binary counter. Knowing the 0-time reference point (i.e., January 1, 1970, 00:00:00 hours UTC), one can identify any second within 136 years. Counting seconds is particularly convenient to measure time intervals.

Identification plus NV SRAM

Data retention for the products outlined in this category is over 10 years. The size of the nonvolatile memory is shown in the Notes column.

Identification plus OTP EPROM

All devices in this category use 12V EPROM technology. Since the packages do not have openings, the bits are OTP—once changed from 1 to 0, they cannot be reversed. To mark an entire 32-byte page as invalid and to point to an alternate location for valid data, these devices have redirection bytes, one for each page. This unique feature allows changing data while maintaining the change history. Individual memory pages can be irreversibly write-protected. Write protection makes OTP EPROMs prime candidates for applications that need a small amount of data that rarely changes, such as network addresses⁴ and PCB identification.⁵

Identification plus EEPROM

Compared to the OTP EPROMs, devices in this category do not need 12V for writing. Data can be changed (rewritten), provided that the memory area or page is not write-protected. Most of these devices have an EPROM emulation mode, which allows changing bits only from 1 to 0, as with EPROMs. Due to its high capacity and security, the DS1977 is used in the gaming industry. The DS28E80 is used extensively in the medical industry due to its gamma-radiation-resistant memory and circuits.

Identification plus SHA-256 Authentication and Secure EEPROM

All the devices in this secure category use a symmetric key challenge-response authentication model.⁶ Thus, authentication of the device and writing to the devices with proper protections activated requires knowledge of the device's secret and the ability to compute a SHA-256 MAC. Except for the secret, memory data is readable without restrictions. All the devices support challenge and response authentication, EPROM emulation for user memory pages, and various protections of the secret and memory pages. See the Is There Any Device Customization Available? section for preprogramming service.

Identification plus ECDSA Authentication and Secure Memory

All the devices in this secure category use an asymmetric key secure authentication model using the ECDSA algorithm. Some of the devices can be used for either SHA-256 or ECDSA based authentication. Authenticated or non-authenticated GPIO pins for control of external components/sensors are available on the DS28E36, DS28E83, and the DS28E84. The devices also have a decrement-only counter feature that enables the user to meter the usage of disposables. Two of the devices deploy Maxim’s ChipDNA physically unclonable function (PUF) technology to secure device data. The DS28E83/84 are high radiation resistant, thus uniquely suited for medical sterilization applications. See the Is There Any Device Customization Available? section for preprogramming service.

Identification plus SHA-3 Authentication and Secure EEPROM

The devices in this secure category use the SHA-3 challenge-response symmetric key secure authentication model. The devices deploy Maxim’s ChipDNA PUF technology to secure device data. It also has robust counter measures to protect against security attacks. See the Is There Any Device Customization Available? section for preprogramming service.

Identification plus Data Logging

These devices are self-contained data loggers for temperature data (Thermochron^®). The DS1923 can also record humidity (Hygrochron^™). The principal differences among the devices are the size of the data memory and the level of data protection (password). Devices for different temperature ranges are available to meet the requirements of the various applications.⁷

How Does One Get Hands-On Experience with 1-Wire Products?

If a PC with a free USB port and Internet access is available, the only items needed are: a 1-Wire adapter, evaluation (EV) software, some simple cabling, and the 1-Wire devices to evaluate. The current product prices and availability can be found on the Analog website. The OneWireViewer software⁸ and related drivers can be downloaded for free.

A more economical approach is to buy one of the EV kits. Each kit includes a 1-Wire adapter, a selection of 1-Wire or iButton devices, and the necessary cabling and accessories. Table 1 describes the EV kits available at the time of this publication. The contents of EV kits can change as new parts become available. Devices not included in a kit need to be purchased separately. The software that run the EV kits are available for free from the Maxim website or upon request depending upon the security level of the product. The package type (plastic or iButton) determines which kit best fits an application.

What Are the Next Steps to a 1-Wire Application?

Once the 1-Wire products are selected, one needs to choose a type of master circuit (i.e., the equivalent of the 1-Wire adapter in the kits). For embedded applications, there is a multitude of choices, based on the available system resources.⁹ Applications that require significant cabling between the 1-Wire devices benefit from a specialized master circuit.¹⁰

Besides hardware, firmware is needed to communicate with the 1-Wire devices, to write or read data, and to exercise control functions. A selection of software drivers ¹¹ for various platforms is available for free download from the Maxim website. For device-specific drivers, application program interfaces (APIs), and software examples, see the 1-Wire Software Resource Guide.¹² This guide also includes API functions based on the 1-Wire File Structure (OWFS),¹³ which are particularly useful when working with more than a few hundred bytes of data.

Is There Any Device Customization Available?

Maxim offers two types of device customization. Custom ROM is quite popular with identification-only devices, but it is applicable to all devices with a 64-bit ID number. With custom ROM, a pool of 68.7 × 10⁹ numbers (equivalent to 36 bits) is committed to the sole use of a single customer. Examples of custom ROM are found in Maxim application note 178.⁵ This type of customization occurs before packaging, which causes a long lead-time. For SHA-256/ECDSA/SHA-3 enabled EEPROM/OTP/FRAM devices, Maxim provides a secure preprogramming service to install data and the secret. For details, submit a tech support request to the factory.

Summary

1-Wire technology is based on a serial communication protocol that uses a single data line plus ground reference between the master and slave. The 1-Wire slaves are available in plastic packages as bumped die or stainless-steel iButton form. The minimum function of 1-Wire slaves is a 64-bit ID number. Additional functions are PIO, temperature sensor, time counter, NV SRAM, OTP EPROM, EEPROM, SHA-256/SHA-3/ECDSA engine, SHA-256/SHA-3/ECDSA secure EEPROM, temperature logging and humidity logging. Typical applications for 1-Wire devices include identification and authentication of consumables, rack cards, PCBs, computer accessories, and the protection of IP (e.g., cloning prevention). Special uses of iButton devices are access control, asset management, guard tour systems, time and attendance, electronic cash, and temperature monitoring for food and pharmaceutical safety. Starter EV kits and software drivers are available to assist customers integrating 1-Wire technology in their systems.