DS18B20 DS1920 DS1961S DS1971 DS1972 DS1973 DS1977 DS2431 DS2432 DS2433 DS2450

Keywords: 1-wire power, EEPROM, temperature, SHA-1, Rpup, Vpup Jun 17, 2008 APPLICATION NOTE 4255 How to Power the Extended

Maxim > App Notes > 1-Wire® Devices Keywords: 1-wire power, EEPROM, temperature, SHA-1, Rpup, Vpup Jun 17, 2008 APPLICATION NOTE 4255 How to Power the Extended Features of 1-Wire® Devices Abstract: A 1-Wire bus provides both communication and power between a host and slave devices on a single line. Some 1-Wire devices offer extended features, which include EEPROM, temperature measurement, and a SHA-1 engine. Operation of these special features can require additional power, so the 1-Wire device's pull-up resistor (RPUP) must be sized accordingly. Introduction The 1-Wire bus is a simple signaling scheme that performs half-duplex bidirectional communications between a master controller and one or more slaves, all sharing a common data line. Both power delivery and data communication take place over this single line. Most 1-Wire devices use very little power, on the order of tens of microamps, to operate and communicate. Some 1-Wire devices, however, need more power during specific operations, such as an EEPROM write or a device-specific calculation or measurement. During these periods of increased power demand, it is important that the voltage on the 1-Wire bus does not fall below the device's minimum operating pullup voltage (VPUP). For most parasitic-powered 1-Wire devices, the minimum operating voltage (VPUP) is 2.8V. 1-Wire Devices that Need Extra Power Table 1 is a partial list of 1-Wire devices with special features that require extra power. Table 1. Devices that Need Extra Power Part EEPROM SHA-1 Temperature ADC DS18B20 DS18B20 DS1920 DS1920 DS1961S DS1961S DS1971 DS1971 DS1972 DS1972 DS1973 DS1973 DS1977 DS1977 DS2431 DS2431 DS2432 DS2432 DS2433 DS2433 DS2450 DS2450 DS28E01-100 DS28E01-100 DS28E04-100 DS28E04-100 DS28EA00 DS28EA00 DS28EC20 DS28EC20 Page 1 of 5 How to Identify Extra Power Requirements in the EC Table Any additional power requirements for a device are listed in the data sheet's electrical characteristics (EC) table under a variety of terms (Table 2). The pullup resistor's specification (RPUP) in the EC table is for 1-Wire communication only and does not include additional power requirements for the special operations. Table 2. EC Table Parameters Specifying Extra Power Demands Parameter Description Symbol 1-Wire Device Programming Current IPROG DS1961S DS1961S, DS1972 DS1972, DS2431 DS2431, DS28E01 DS28E01, DS28E04 DS28E04, DS28E00 DS28E00 Programming Current ILPROG DS1973 DS1973, DS2433 DS2433, DS1977 DS1977, DS2432 DS2432 Programming Current IP DS1971 DS1971 (DS2430A DS2430A) SHA Computation Current ILCSHA DS1961S DS1961S, DS28E01 DS28E01 Active Current IDD, IDQA DS1920 DS1920, DS18B20 DS18B20, DS18B20-PAR DS18B20-PAR Conversion Current ICONV DS28EA00 DS28EA00 Operating Current ICC DS2450 DS2450 Figure 1. Example of the EC table for the DS28EA00 DS28EA00. Available Power For a given VPUP and RPUP, the voltage difference between VPUP and the 1-Wire device's VPUPmin determines the current available for special functions. The available current can be calculated as IAVAIL = (VPUP - VPUPmin)/RPUP. An example calculation follows: VPUP = 5V RPUP = 2k VPUPmin = 2.8V, resulting in IAVAIL = 1.1mA So for this example, there are 1.1mA available before the 1-Wire voltage drops below the minimum VPUP. If the available current is not sufficient for the application, then a lower pullup resistor or a low-impedance bypass to the pullup resistor will be necessary. Finding the Right Pullup (RPUP) The available current can be calculated by dividing the potential voltage drop from nominal VPUP to the minimum VPUP by the pullup resistor (RPUP). Figure 2 graphs this calculation based on a VPUP of 5V with a device that has a minimum VPUP of 2.8V. A pullup resistor of 2.2k or less supports at least 1mA at 5V pullup voltage. Page 2 of 5 Figure 2. Available current for VPUP = 5V. Similarly, Figure 3 shows the available current based on a VPUP of 3.3V. With only 0.5V as the permissible voltage drop on the pullup resistor, very little current is available. Other means of providing the extra current are probably required (see LowImpedance Bypass section below). Figure 3. Available current for VPUP = 3.3V. Advanced Considerations Choosing a very-low pullup resistor value delivers the desired power to run the special function. However, this configuration raises the voltage representing logic 0 on the 1-Wire bus. If VOL levels do not meet the minimum-voltage input low (VIL) specified for the 1-Wire slave or the 1-Wire master, then reliable communication will not be possible. The most common VOL specification for 1-Wire devices is 0.4V at 4mA, maximum. This value is equivalent to an impedance of 100, maximum, when the 1-Wire device is responding with a logic 0. VIL varies from 0.3V to 0.8V, depending on the 1-Wire device. With multiple 1-Wire devices on the bus, the lowest VIL sets the limit. The pullup resistor value that meets the logic 0 requirement can be calculated as: RPUPmin = 100 × (VPUP/VILmax - 1). (Note: Instead of starting the equation with 100, one could write VOL/4mA.) Therefore, assuming a VIL maximum of 0.4V, the results are Page 3 of 5 For a VPUP = 5V: 1150 For VPUP = 3.3V: 725 Assuming a VIL maximum of 0.3V, the results are For a VPUP = 5V: 1567 For VPUP = 3.3V: 1000 The tolerances for the pullup resistor and the power supply must also be considered when selecting the proper pullup. These tolerances are not correlated, i.e., they can add up to either (positive, negative) side or cancel each other. Always check the worst combinations: voltage at upper limit with resistor at lower limit (i.e., highest VOL), and voltage lower limit with resistor at upper limit (i.e., lowest available extra current). Low-Impedance Bypass If meeting the VOL and VIL requirements requires a pullup resistor that cannot deliver the necessary current, then the extra current must be supplied by other means. There are two ways to do this: 1. Implement a discrete low impedance bypass (also called a strong pullup) that is engaged only during high current demand. 2. Utilize a 1-Wire interface device that incorporates a strong pullup. Examples of a 1-Wire master with a discrete strong pullup can be found in application note 4206, "Choosing the Right 1-Wire® Master for Embedded Applications," or application note 244, "Advanced 1-Wire Network Driver." Figure 4 shows a strong pullup controlled with an extra IO pin. Figure 4. Bidirectional port pin with optional circuit for strong pullup (dashed lines). There are three 1-Wire interface chips that incorporate a strong pullup feature (Table 3). The DS2482-100 DS2482-100 also has an external control signal that can be used to drive an additional discrete, extra-strong pullup. Table 3. 1-Wire Master Interface Devices Device Interface Features DS2480B DS2480B Serial Strong pullup, active pullup DS2482-100 DS2482-100 I²C Single 1-Wire channel with built in strong pullup, optional active pullup, control signal for extrastrong pullup DS2482-800 DS2482-800 I²C Eight 1-Wire channels with built in strong pullup, optional active pullup Page 4 of 5 Conclusion For extended features like temperature conversion, EEPROM, or a SHA-1 engine to operate properly in 1-Wire devices, those devices must be provided with sufficient current from the 1-Wire master without allowing the 1-Wire to drop below the minimum-voltage pullup (VPUP). The 1-Wire pullup resistor (RPUP) must, therefore, be sized to provide this current in accordance with application demands. If application requirements do not permit a pullup resistor of the correct size, then the current can be supplied with a discrete strong pullup circuit or a 1-Wire interface chip such as the DS2480B DS2480B or DS2482 DS2482. 1-Wire is a registered trademark of Maxim Integrated Products, Inc. Application note 4255: www.maxim-ic.com/an4255 More Information For technical support: www.maxim-ic.com/support For samples: www.maxim-ic.com/samples Other questions and comments: www.maxim-ic.com/contact Automatic Updates Would you like to be automatically notified when new application notes are published in your areas of interest? Sign up for EEMailTM. Related Parts DS18B20 DS18B20: QuickView - Full (PDF) Data Sheet - Free Samples DS1920 DS1920: QuickView - Full (PDF) Data Sheet DS1961S DS1961S: QuickView DS1971 DS1971: QuickView - Full (PDF) Data Sheet DS1972 DS1972: QuickView - Full (PDF) Data Sheet DS1973 DS1973: QuickView - Full (PDF) Data Sheet - Free Samples DS1977 DS1977: QuickView - Full (PDF) Data Sheet DS2431 DS2431: QuickView - Full (PDF) Data Sheet - Free Samples DS2432 DS2432: QuickView - Abridged Data Sheet DS2433 DS2433: QuickView - Full (PDF) Data Sheet - Free Samples DS2450 DS2450: QuickView - Full (PDF) Data Sheet - Free Samples DS28E01-100 DS28E01-100: QuickView - Abridged Data Sheet DS28E04-100 DS28E04-100: QuickView - Full (PDF) Data Sheet - Free Samples DS28EA00 DS28EA00: QuickView - Full (PDF) Data Sheet - Free Samples DS28EC20 DS28EC20: QuickView - Full (PDF) Data Sheet - Free Samples AN4255 AN4255, AN 4255, APP4255 APP4255, Appnote4255, Appnote 4255 Copyright © by Maxim Integrated Products Additional legal notices: www.maxim-ic.com/legal Page 5 of 5