Wirelessly Powering and Accessing a 1-Wire Network

要約

If the timing aligns with 1-Wire^® protocol specifications, a regulated voltage source can power and maintain communication on a 1-Wire network. Simplify the analysis process by modeling the voltage source resistance, the 1-Wire pull-up resistance, and the 1-Wire network as a capacitive load. This way, you can calculate first-order equations related to initially charging up the 1-Wire network and pulling the 1-Wire parasitic capacitance to the supply rail and ground. Time constants e, d, tREC, and tf help specify a maximum total typical capacitance that can allow proper 1-Wire communication for a given pull-up resistance and pull-up voltage.A similar version of this application note originally appeared on Power Electronics on May 30, 2018.

Initial Charging of a 1-Wire Network

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, is dependent upon 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, before sending 1-Wire function commands, take care to ensure that enough time passes before the devices attached to the 1-Wire network are charged and ready to communicate. This occurs 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.

V_MIN-PUP = V_S(1 - e^{t_CHARGE/ -R_S+PUPC_TOTAL-MAX} ) [Eq. 1]

where C_TOTAL-MAX = S^N_i=0 (C_{MAX-IO, i} + C_LAYOUT), R_S+PUP = R_S + R_PUP, V_MIN-PUP is the minimum pull-up voltage required on the 1-Wire network, and VS is the 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 (See Figure 1 for a depiction.)

Figure 1. 1-Wire network modeled as a series of I/O capacitances C_IO and parasitic layout capacitance C_LAYOUT as a result of junctions on the 1-Wire node.

V_MIN-PUP is the largest minimum-pull-up voltage in the 1-Wire network. So, if device number one has a minimum pull-up voltage of 2.8V and device number two has a minimum pull-up voltage of 3.0V, then V_MIN-PUP should equal to 3.0V 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:

t_CHARGE = -R_S+PUPC_TOTAL-MAXln(1 – V_MIN-PUP / V_S) [Eq. 2]

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 9 by replacing C_MAX-IO,N with C_TYP-IO,N where C_TOTAL-TYP = ?ⁿ_i=1C_{TYP-IO, i} + C_LAYOUT. 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. In every 1-Wire communication sequence, 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:

1. e – the time taken to pull up from 0V to the 1-Wire network’s threshold-high voltage V_TH.
2. δ – the time taken to pull up from 0V to the 1-Wire host input-high voltage V_IH-HOST.
3. t_REC – the 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.
4. t_f – the time taken to pull down from V_PUP to the 1-Wire network’s threshold-low voltage V_TL.

Time constants e, 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 are not met, rendering 1-Wire communication improbable. For the value of the four time constraints, see the datasheet for the respective 1-Wire device.

Pull-up Fundamental Timing Parameters e, δ, and t_REC

Figure 2 illustrates time e needed to charge up the 1-Wire total typical capacitance CTOTAL-TYP from 0V to VTH. Figure 3 presents this concept in schematic form.

Figure 2. Time e to charge the total typical capacitance C_TYP-TOTAL from 0V to V_TH.

Figure 3. 1-Wire network modeled as an equivalent total typical capacitance C_TOTAL-TYP that includes the parasitic layout capacitance C_LAYOUT.

Equation (3) defines e – the time required to charge C_TOTAL-TYP from 0V to V_TH via R_S+PUP.

e = -R_S+PUPC_TOTAL-TYPln(1 – V_TH / V_S) [Eq. 3]

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

Figure 4. Time δ to charge total typical capacitance C_TYP-TOTAL from 0V to V_IH-HOST.

Equation (4) defines e – the time required to charge C_TOTAL-TYP from 0V to V_TH via R_S+PUP.

δ = -R_S+PUPC_TOTAL-TYPln(1 – V_IH-HOST / V_S) [Eq. 4]

Figure 5 illustrates the shortest time t_REC needed to recharge the 1-Wire total typical capacitance C_TOTAL-TYP from V_TH to V_PUP-MIN.

Figure 5. Shortest time t_REC possible to charge total typical capacitance C_TYP-TOTAL from V_TH to V_PUP-MIN.

Follow this three-step procedure to calculate t_REC:

1. Calculate the time required to charge from 0V to V_TH – this is equivalent to e in Equation (3).
2. Calculate the time required to charge from 0V 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’ – e.
   t_REC = R_S+PUPC_TOTAL-TYPln[(1 – V_TH/V_S) / (1 – V_PUP-MIN/V_S)] [Eq. 5]

Pull-Down Timing Parameter t_f
Unlike e, δ, and t_REC, time t_f does not depend on R_S and R_PUP 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 tf necessary to discharge C_TOTAL-TYP from V_PUP to V_TL as illustrated in Figure 6.

Figure 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 comes from the maximum output low-voltage V_OL and the corresponding output low-current I_OL provided 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.

Figure 7. Simplified RC circuit that 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 0.4V at 4mA. This means that the maximum pull-down resistance is R_PDOWN is 100?.

Equation (6) defines the discharge time t_f.

t_f = -R_PDOWNC_TOTAL-TYPln(V_TL / V_PUP-MIN) [Eq. 6]

Note that time t_f should be met for the 1-Wire host and the device. The 1-Wire host pulls down the 1-Wire node at the beginning of each basic operation, i.e., reset, write 1 bit, write 0 bit, and read a bit. The 1-Wire device pulls down the 1-Wire node during a reset operation to generate a presence pulse.

1-Wire communication and power delivery are possible when the four fundamental parameters e, δ, t_REC, and t_f are met for all devices on the network. You can determine the maximum number of devices and bus length achievable in the NFC-powered system by knowing the total allowable capacitance to meet all edge timings specified by the 1-Wire protocol.

For more information about e, δ, and t_REC, and how they affect 1-Wire communication in standard or overdrive mode, refer to Application Note 126 – 1-Wire Communication Through Software. For more information about how typical capacitance affects 1-Wire communication, refer to Application Note 148 – Guidelines for Reliable Long Line 1-Wire Networks.

Compatible 1-Wire Devices

Table 1 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.

Conclusion

By modeling an NFC transponder connected to a 1-Wire network as an RC circuit, we can 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. With an NFC system, we 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.