*To read Part 1 of this two-part article, click here.*

1-Wire devices power-up parasitically by charging an internal reservoir from the 1-Wire communication line. V_{PUP}, the pull-up voltage on the 1-Wire network, depends on V_{OUT} and R_{PUP}. The maximum current that can be supplied to the 1-Wire network also depends on these two parameters.

When powering a 1-Wire network from a high-impedance source like the MAX66242 voltage regulator at V_{OUT}, enough time should pass until the devices attached to the 1-Wire network are charged and ready to communicate before sending 1-Wire function commands. A 1-Wire device is charged and ready to communicate when the initial capacitance C_{IO} at its I/O pin is charged.

Most 1-Wire devices specify typical and maximum C_{IO} values that exist on their 1-Wire I/O port. The maximum C_{IO} exists when V_{PUP} is first applied to the 1-Wire network. After the 1-Wire network is fully charged, only the typical C_{IO} affects 1-Wire communication. Therefore, the C_{IO-MAX} should be charged to the minimum pull-up voltage V_{MIN-PUP} required by the 1-Wire device. Equation 1 defines the minimum pull-up voltage V_{MIN-PUP} across the total maximum capacitance C_{TOTAL}_{-MAX} of the 1-Wire network:

where:

C_{TOTAL}_{-MAX} = Σ^{N}_{i=0} (C_{MAX-IO, i} + C_{LAYOUT})

R_{S+PUP} = R_{S} + R_{PUP}

V_{MIN-PUP} = minimum pull-up voltage required on the 1-Wire network

V_{S} = open-circuit voltage at V_{OUT}.

The capacitance C_{LAYOUT} represents the capacitance introduced to the 1-Wire network because of junctions on the 1-Wire node *(Fig. 1)*.

*1. The 1-Wire network is modeled as a series of I/O capacitances C _{IO} and parasitic layout capacitance C_{LAYOUT} due to junctions on the 1-Wire node.*

V_{MIN-PUP} is the largest minimum-pull-up voltage that exists in the 1-Wire network. For example, if device number one has a minimum pull-up voltage of 2.8 V and the minimum for device number is 3.0 V, then V_{MIN-PUP} should equal to 3.0 V for the 1-Wire network.

Equation 2 determines the time t_{CHARGE} necessary to charge the total maximum capacitance C_{TOTAL}_{-MAX }to the minimum pull-up voltage V_{MIN-PUP} of a 1-Wire network:

**Parasitic Capacitance During 1-Wire Communication**

The total typical capacitance C_{TOTAL}_{-TYP} on the 1-Wire network after powering up is defined as the sum of all typical capacitances C_{TYP-IO }plus the parasitic capacitance of the layout C_{LAYOUT}.

This is represented in schematic form in *Figure 7 (see below)* by replacing C_{MAX-IO,N} with C_{TYP-IO,N} where:

The typical capacitance C_{TYP-IO} refers to the parasitic capacitance at the I/O that originates from each of the device’s internal 1-Wire receivers/transmitters. The typical capacitance C_{TYP-IO}, the pull-up voltage V_{PUP}, and the pull-up resistance R_{PUP} are responsible for the following four fundamental timing parameters used in every 1-Wire communication sequence:

- ε = Time taken to pull up from 0 V to the 1-Wire network’s threshold-high voltage V
_{TH}. - δ = Time taken to pull up from 0 V to the 1-Wire host input-high voltage V
_{IH-HOST}. - t
_{REC}= Time taken to pull up from V_{TH}to V_{PUP}. t_{REC }defines the maximum time available for the 1-Wire network to recharge during communication. - t
_{f}= Time taken to pull down from V_{PUP}to the 1-Wire network’s threshold-low voltage V_{TL.}

Time constants ε, δ, t_{REC}, and t_{f} help specify a maximum total typical capacitance C_{TOTAL}_{-TYP }that can allow proper 1-Wire communication for a given R_{PUP} and V_{PUP}. If C_{TOTAL}_{-TYP} is exceeded, then timing constraints aren’t met, rendering 1-Wire communication improbable. Refer to the datasheet for the respective 1-Wire device to find the value of the four time constants.

**Pull-up Fundamental Timing Parameters ε, δ, and t**_{REC}

_{REC}

*2. Time ε to charge the total typical capacitance C _{TYP-TOTAL} from 0 V to V_{TH}.*

*Figure 2* illustrates time ε needed to charge up the 1-Wire total typical capacitance C_{TOTAL}_{-TYP }from 0 V to V_{TH}. *Figure 3* presents this concept in schematic form.

*3. Here, the 1-Wire network is modeled as an equivalent total typical capacitance C _{TOTAL-TYP }that includes the parasitic layout capacitance C_{LAYOUT}.*

Equation 3 defines ε = Time required to charge C_{TOTAL-TYP }from 0 V to V_{TH }via R_{S+PUP}:

*Figure 4* illustrates time δ needed to charge up the 1-Wire total typical capacitance C_{TOTAL}_{-TYP }from 0 V to V_{IH-HOST}.

*4. Time δ to charge total typical capacitance C _{TYP-TOTAL} from 0 V to V_{IH-HOST}.*

Equation 4 defines ε = Time required to charge C_{TOTAL-TYP }from 0V to V_{TH }via R_{S+PUP}:

*Figure 5* illustrates the shortest time t_{REC} needs to recharge the 1-Wire total typical capacitance C_{TOTAL}_{-TYP }from V_{TH} to V_{PUP-MIN}.

*5. Shortest time (t _{REC}) that’s possible to charge total typical capacitance C_{TYP-TOTAL} from V_{TH} to V_{PUP-MIN.}*

The following three-step procedure shows how to calculate t_{REC}.

1. Calculate the time required to charge from 0 V to V_{TH}—this is equivalent to ε in Equation 3.

2. Calculate the time required to charge from 0 V to V_{PUP-MIN}:

t’ = -R_{S+PUP}C_{TOTAL-TYP}ln(1 – V_{PUP-MIN}/V_{S})

3. Use the quotient rule to find t_{REC} = t’ – ε:

**Pull-****Down T****iming ****P****arameter** **t**_{f}_{}

_{f}

_{}

Unlike ε, δ, and t_{REC}, time t_{f} doesn’t depend on R_{S} and R_{PUP}. This is because time t_{f} defines the time required for the 1-Wire host or device to pull down the 1-Wire network. Therefore, the pull-down resistance R_{PDOWN} of the 1-Wire host or device defines the time t_{f} necessary to discharge C_{TOTAL}_{-TYP} from V_{PUP} to V_{TL} as illustrated in *Figure 6*.

*6. Time t _{f} to discharge total typical capacitance C_{TYP-TOTAL} from V_{PUP-MIN} to V_{TL}.*

The pull-down resistance R_{PDOWN }for the 1-Wire host and device is derived from the maximum output low voltage V_{OL} and the corresponding output low current I_{OL }given in the electrical characteristics table of the respective datasheet. *Figure 7* illustrates the pull-down resistance R_{PDOWN} and the pull-down current I_{OL}.

*7. This simplified RC circuit models the pull-down resistance R _{PDOWN} from either the 1-Wire host or device.I_{OL} is the pull-down current.*

For example, the DS2484 I^{2}C-to-1-Wire bridge has a maximum V_{OL} of 0.4 V at 4 mA. This means that the maximum pull-down resistance of R_{PDOWN} is 100 Ω.

Equation 6 defines the discharge time t_{f}:

If the four fundamental parameters ε, δ, t_{REC}, and t_{f} are met for all devices on the network, then 1-Wire communication and power delivery are possible. By knowing the total allowable capacitance to meet all edge timings specified by the 1-Wire protocol, the maximum number of devices and bus length achievable in the NFC-powered system can be determined.

**Compatible 1-Wire Devices**

The *table* lists 1-Wire devices with their respective input/output capacitance C_{IO}, pull-up voltage V_{PUP}, pull-up resistance R_{PUP}, voltage threshold-low V_{TL}, and voltage threshold-high V_{TH} specifications. V_{TL} is the voltage below which, during a falling-edge on the 1-Wire network, a logic low is detected. V_{TH} is the voltage above which, during a rising-edge on the 1-Wire network, a logic high is detected. Both V_{TL} and V_{TH} are a function of V_{PUP} and 1-Wire recovery times.

Modeling an NFC transponder connected to a 1-Wire network as an RC circuit allows us to verify whether harvested power delivery and communication are feasible. A smartphone or any device equipped with an NFC transceiver under ISO15693 and FIPS180-4 can authenticate; identify; access memory from; conduct data acquisition on; and control a 1-Wire network. An NFC system can wirelessly power a 1-Wire network and allow secure asset and information management for a node of closed mobile systems and Internet of Things (IoT) devices.