How to Wirelessly Power and Access a 1-Wire Network (Part 2)

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:

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Eq1

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).

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Fig1

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_-MAXto the minimum pull-up voltage V_MIN-PUP of a 1-Wire network:

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Eq2

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-IOplus 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:

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Eq A

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_RECdefines 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_-TYPthat 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

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Fig2

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_-TYPfrom 0 V to V_TH. Figure 3 presents this concept in schematic form.

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Fig3

3. Here, the 1-Wire network is modeled as an equivalent total typical capacitance C_TOTAL-TYPthat includes the parasitic layout capacitance C_LAYOUT.

Equation 3 defines ε = Time required to charge C_TOTAL-TYPfrom 0 V to V_THvia R_S+PUP:

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Eq3

Figure 4 illustrates time δ needed to charge up the 1-Wire total typical capacitance C_TOTAL_-TYPfrom 0 V to V_IH-HOST.

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Fig4

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-TYPfrom 0V to V_THvia R_S+PUP:

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Eq4

Figure 5 illustrates the shortest time t_REC needs to recharge the 1-Wire total typical capacitance C_TOTAL_-TYPfrom V_TH to V_PUP-MIN.

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Fig5

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+PUPC_TOTAL-TYPln(1 – V_PUP-MIN/V_S)

3. Use the quotient rule to find t_REC = t’ – ε:

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Eq5

Pull-Down Timing Parameter t_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.

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Fig6

6. Time t_f to discharge total typical capacitance C_TYP-TOTAL from V_PUP-MIN to V_TL.

The pull-down resistance R_PDOWNfor the 1-Wire host and device is derived from the maximum output low voltage V_OL and the corresponding output low current I_OLgiven 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.

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Fig7

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 I²C-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:

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Eq6

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.

Www Electronicdesign Com Sites Electronicdesign com Files Link 1 Wire Pt2 Table

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.