APPLICATION NOTE 3925

1-Wire® Extended Network Standard


Abstract: The 1-Wire standard established in 1989 has been upgraded to accommodate noisy and long-line 1-Wire networks. This application note explains the new standard enhancements, and shows how to make a 1-Wire master that works with both standard and new devices.

Introduction

The 1-Wire bus is a simple signaling scheme that performs two-way communication over a single electrical connection. In any 1-Wire system, there is a single master and one or more slave devices sharing a common data line. Dallas Semiconductor created the 1-Wire standard in 1989 to reduce the contacts for portable data-carrying modules. The result of this was the invention of iButtons®, the 16mm battery-shaped modules that have sold more than 130 million worldwide.

The 1-Wire scheme also enabled other applications such as chip-based tagging and long-line sensor applications. The original 1-Wire front-end did not, however, anticipate the noise levels and line characteristics (e.g., line length) of some of these new applications. Satisfying these new application demands often challenged a 1-Wire implementation in the field. Therefore, to accommodate these applications a new 1-Wire front-end called the 1-Wire Extended Network Standard was developed, and incorporated into several new devices. Table 1 lists 1-Wire devices and shows which are supported by the new extended standard.

Important Features of the New Extended Standard

Noise from various sources can result in signal glitching on the 1-Wire line. The noise can come from reflections from network endpoints or branch points. (For more information, please see application note 148, "Guidelines for Reliable Long Line 1-Wire® Networks.") Noise can also come from an external source and get coupled onto the 1-Wire signal. A noise glitch during the rising edge can cause the 1-Wire device to become unsynchronized with the master. The improvements to the extended network front-end address these rising edge issues.

The new 1-Wire front-end incorporates three main components: a lowpass filter for high-frequency noise, voltage hysteresis on low-to-high switching, and a rising-edge hold-off time. Some 1-Wire devices also have slew control on the presence pulse. Figure 1 illustrates these features. The shaded pink regions show how the device ignores glitches in voltage magnitude and over a period of time during 1-Wire low-to-high transitions.

Figure 1. New <nobr>1-Wire</nobr> front-end features.
Figure 1. New 1-Wire front-end features.

Table 1. 1-Wire Devices
Device FC Description 1-Wire Extended Network Support Notes
DS1822 22 1-Wire Econo temp sensor    
DS1825 3B 1-Wire thermometer with 4-bit address    
DS18B20 28 Adjustable resolution temperature    
DS18S20 10 Temperature and alarm trips    
DS1904 24 Real-Time Clock (RTC) iButton    
DS1920 10 Temperature and alarm trips    
DS1921G
DS1921H
DS1921Z
21 Thermochron temperature logger    
DS1922E
DS1922L
DS1922T
DS1923
41 High-Capacity Thermochron and/or Hygrochron temperature and/or humidity dataloggers, respectively  
DS1961S 33 1Kb EEPROM memory with SHA-1 engine  
DS1963S 18 4Kb NVRAM memory and SHA-1 engine    
DS1971 14 256-bit EEPROM memory and 64-bit OTP register  
DS1972 2D 1Kb EEPROM memory  
DS1973 23 4Kb EEPROM memory    
DS1977 37 Password-protected 32kB (bytes) EEPROM  
DS1982 09 1Kb EPROM memory    
DS1885 0B 16Kb EPROM memory    
DS1990A
DS1990R
01 1-Wire address only    
DS1991 02 Multikey iButton, 1152-bit secure memory   Not recommended for new designs, see application note 4421, "Alternatives to the DS1991L MultiKey iButton®" for alternatives.
DS1992 08 1Kb NV RAM memory    
DS1993 06 4Kb NV RAM memory    
DS1994 04 4Kb NV RAM memory and clock, timer, alarms   Not remommended for new designs, see application note 4506, "Alternatives to the DS1994L 4Kb Plus Time Memory iButton®" for alternatives.
DS1995 0A 16Kb NV RAM memory    
DS1996 0C 64Kb NV RAM memory    
DS2401 01 1-Wire address only    
DS2404 04 4Kb NV RAM memory and clock, timer, alarms   Not recommended for new designs
DS2406 12 1Kb EPROM memory, 2-channel addressable switch    
DS2408 29 8-channel addressable switch  
DS2409 1F Dual switch, coupler   Not recommended for new designs
DS2411 01 Low-voltage, unique 64-bit serial ROM number (requires VDD connection)  
DS2413 3A Dual-channel addressable switch  
DS2417 27 RTC with interrupt    
DS2422 41 High-capacity Thermochron/Hygrochron (temperature and humidity) datalogger Not recommended for new designs
DS2430A 14 256-bit EEPROM memory and 64-bit OTP register Not recommended for new designs
DS2431 2D 1Kb EEPROM memory  
DS2432 33 1Kb EEPROM memory with SHA-1 engine    
DS2433 23 4Kb EEPROM memory    
DS2438 26 Temperature, ADC    
DS2450 20 Quad ADC    
DS2502 09 1Kb EPROM memory    
DS2505 0B 16Kb EPROM memory    
DS2703 34 Battery-packed authentication IC    
DS2740 36 1-Wire coulomb counter (high precision)    
DS2762 30 1-Wire battery monitor and protector    
DS2775
DS2776
DS2777
DS2778
DS2780
DS2784
DS2788
32 Stand-alone 1-Wire fuel gauge    
DS2781 3D Stand-alone fuel gauge IC    
DS28E04-100 1C 4Kb EEPROM memory, two-channel addressable switch, 7 address pins  
DS28EA00 42 1-Wire digital thermometer with sequence detect and PIO  
DS28EC20 43 20Kb 1-Wire EEPROM  
Note: New 1-Wire devices are constantly added to the product line. Newer parts may not be in this list. Look for an 'Improved Network Behavior' section in the device's data sheet to see if the device incorporates the new extended network front-end.

The new features in the Extended Network Standard are only fully active during standard speed communication, not in overdrive. Adding these features to the 1-Wire front-end can affect the 1-Wire timing specification. Specifically, the new standard introduces an EC table parameter, tREH, that represents the rising-edge hold-off time. This hold-off behavior increases the low time generated by the master and required in a read bit, tRL. See Table 2.

Field experience with applications using long lines to communicate with 1-Wire devices demonstrates the importance of adequate recovery between bits. As a result, all of the extended-network devices have longer recovery times, tREC. The recovery-time specification for all devices (standard and extended network) is given for one device on a 1-Wire bus. For a guide to extending this specification to multiple devices, see application note 3829, "Determining the Recovery Time for Multiple-Slave 1-Wire Networks."

Devices that incorporate slew control on the presence pulse include a parameter, tFPD, for Presence Detect Fall Time. While controlling the slew creates less reflections on long lines, it has a significant effect on the window in which a master can detect the presence pulse. Impedance matching on the 1-Wire master can be equally effective in controlling these reflections without incurring the slew-rate delay. Consequently, future devices may not incorporate the presence-pulse slew-rate feature.

Table 2. EC Table Differences
Parameter Speed Min/Max Standard Extended Network*
tREC Standard Min 1µs 5µs
Overdrive Min 1µs 2µs
tREC (before reset) Overdrive Min 1µs 5µs
tREH Standard Min Varies from 0.5µs to 0.6µs
Standard Max Varies from 2µs to 5µs
Overdrive Min Varies from 0µs to 0.6µs
Overdrive Max Varies from 0µs to 2µs
tRL Standard Min 1µs 5µs
*See the device data sheets for the actual tREH values.

Summary

A 1-Wire master can work with both standard and extended-network devices. Accommodating the extended-network devices is as simple as extending recovery time between bits and using a longer start pulse for a read bit, tRL. While the longer recovery will slow the throughput, the change in the read-bit start pulse will not affect the throughput. For networks with devices using presence-pulse slew control, tFPD, care must be taken to select the sample point for the presence pulse. For some devices and voltages the sample range may be restrictive.

Application note 126, "1-Wire Communication Through Software," describes a simple 1-Wire master with timing that is already compatible with standard and extended-network devices. The application note includes an Excel spreadsheet for customizing the parameters based on the 1-Wire slave devices and the network conditions such as rise time. Download the afore mentioned Excel sheet.


Also see: all 1-Wire devices

1-Wire is a registered trademark of Maxim Integrated Products, Inc.

iButton is a registered trademark of Maxim Integrated Products, Inc.



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© Oct 26, 2006, Maxim Integrated Products, Inc.
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APP 3925: Oct 26, 2006
APPLICATION NOTE 3925, AN3925, AN 3925, APP3925, Appnote3925, Appnote 3925