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 (R_PUP) 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 (V_PUP). For most parasitic-powered 1-Wire devices, the minimum operating voltage (V_PUP) is 2.8V.

1-Wire Devices that Need Extra Power

Table 1. Devices that Need Extra Power

Part	EEPROM	SHA-1	Temperature	ADC
DS18B20
DS1920
DS1961S
DS1971
DS1972
DS1973
DS1977
DS2431
DS2432
DS2450
DS28E01-100
DS28E04-100
DS28EA00
DS28EC20

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 (R_PUP) in the EC table is for 1-Wire communication only and does not include additional power requirements for the special operations.

Available Power

For a given V_PUP and R_PUP, the voltage difference between V_PUP and the 1-Wire device's V_PUPmin determines the current available for special functions. The available current can be calculated as I_AVAIL = (V_PUP - V_PUPmin)/R_PUP. An example calculation follows:

V_PUP = 5V
R_PUP = 2kΩ
V_PUPmin = 2.8V, resulting in I_AVAIL = 1.1mA

So for this example, there are 1.1mA available before the 1-Wire voltage drops below the minimum V_PUP. 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 (R_PUP)

The available current can be calculated by dividing the potential voltage drop from nominal V_PUP to the minimum V_PUP by the pullup resistor (R_PUP). Figure 2 graphs this calculation based on a V_PUP of 5V with a device that has a minimum V_PUP of 2.8V. A pullup resistor of 2.2kΩ or less supports at least 1mA at 5V pullup voltage.

Similarly, Figure 3 shows the available current based on a V_PUP 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 Low-Impedance Bypass section below).

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 V_OL levels do not meet the minimum-voltage input low (V_IL) specified for the 1-Wire slave or the 1-Wire master, then reliable communication will not be possible. The most common V_OL specification for 1-Wire devices is 0.4V at 4mA, maximum. This value is equivalent to an impedance of 100Ω, maximum, when the 1-Wire device is responding with a logic 0. V_IL varies from 0.3V to 0.8V, depending on the 1-Wire device. With multiple 1-Wire devices on the bus, the lowest V_IL sets the limit. The pullup resistor value that meets the logic 0 requirement can be calculated as: R_PUPmin = 100Ω × (V_PUP/V_ILmax - 1).

(Note: Instead of starting the equation with 100Ω, one could write V_OL/4mA.)

Therefore, assuming a V_IL maximum of 0.4V, the results are

For a V_PUP = 5V: 1150Ω
For V_PUP = 3.3V: 725Ω

Assuming a V_IL maximum of 0.3V, the results are

For a V_PUP = 5V: 1567Ω
For V_PUP = 3.3V: 1000Ω

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 V_OL), and voltage lower limit with resistor at upper limit (i.e., lowest available extra current).

Low-Impedance Bypass

If meeting the V_OL and V_IL 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.

There are three 1-Wire interface chips that incorporate a strong pullup feature (Table 3). The DS2482-100 also has an external control signal that can be used to drive an additional discrete, extra-strong pullup.

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 (V_PUP). The 1-Wire pullup resistor (R_PUP) 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 or DS2482.