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.
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.
