Using an NFC System to Wirelessly Power and Access a 1-Wire Network

Abstract

For inductive power transfer, access, control, and information exchange to a 1-Wire^® network, you can take advantage of a near-field communications (NFC) system. With harvested power from the NFC link, temperature sensing, authentication, and memory storage can be accomplished via a single node for 1-Wire communication. You can also analyze available harvested voltage, current, and 1-Wire timing constraints by modeling the transponder’s radio frequency (RF) power converter and 1-Wire network as an equivalent RC circuit.

Introduction

By using a single conductor and ground, 1-Wire devices provide functionality to mechanical assemblies and electronic systems and reduces the number of nodes needed in a system to just one. Through symmetric and asymmetric authentication, identification, memory, data acquisition, and control, 1-Wire technology also provides secure asset and information management, all in a low-power design. A variety of applications can benefit such as automotive, cloud networks, consumable pharmaceuticals and healthcare equipment, mobile sales channels, and the smart grid. An NFC transceiver, with a built-in 256-bit secure hash algorithm (SHA-256) coprocessor for symmetric authentication, paired with an NFC transponder that includes an I²C master/slave port and an energy-harvesting output provides security for wireless access to a node of closed portable devices. An NFC system that is compatible with ISO-15693 and FIPS 180-4 supports wireless power and secure access to 1-Wire devices. Wireless access to a 1-Wire network, in turn, offers new use cases and additional flexibility for electronics in applications where NFC is the preferred method of wireless power transfer and communication.

Translating Communication Protocols

To provide wireless access to a 1-Wire network, there must be some translation between the wireless communication protocol such as NFC and the 1-Wire communication protocol. Most major smartphone brands already incorporate NFC to allow users to connect to electronics and unpowered objects such as key fobs, labels, and cards. As such, an NFC-equipped smartphone or device can access a 1-Wire network wirelessly.

In an NFC system, the transceiver is the initiator and produces an RF field to deliver power, send function commands, and enable information exchange. The transponder powers itself from the RF field generated by the transceiver. It also receives function commands to execute and relays data from memory or from attached devices to the NFC reader.

Electromagnetic induction between nearby antennas allows NFC to transmit and receive power, obtain control, and transfer data. A transceiver starts communicating with a transponder by using properly routed PCB traces or wounded wire in a calculated manner.

In Figure 1, an NFC hardware setup using a transceiver such as the MAX66300 and a transponder like the MAX66242 is shown. Learn how to implement an antenna design for the MAX66300 in Application Note 5921 – Designing an Antenna for MAX66300. Get more details about NFC and the secure dual-interface of the MAX66242 in Application Note 5995 - Need NFC/RFID? Tomorrow is Today in This Constant State of Innovation.

The MAX66242 transponder offers two functional features to provide wireless access to attached ancillary circuitry:

1. An internal AC-DC converter converts power from the harvested RF field to a regulated output voltage at V_OUT. This allows the MAX66242 to power attached ancillary circuitry, the I²C interface, and the programmable input-output (PIO).
2. An integrated I²C master and slave interface allows bidirectional access and control.

In the I²C master mode, the MAX66242 relays information from connected circuitry such as sensors, a microcontroller, and other ancillary circuitry to an NFC transceiver such as the MAX66300 or a smartphone. In the I²C slave mode, the MAX66242 is an intermediary between the connected host circuitry and an NFC transceiver.

The PIO provides additional functional features. For example, it enables ancillary circuitry when the NFC link is ready for communication and also provides general-purpose control and monitoring information. Read the MAX66242 unabridged datasheet for more details on the PIO.

To access and control a 1-Wire network, the MAX66242 requires bidirectional I²C and 1-Wire protocol translation. An I²C-to-1-Wire bridge such as the DS2484 facilitates this task. Figure 2 shows how I²C and 1-Wire protocol translation is handled by the MAX66242 and DS2484.

The DS2484 complements the MAX66242 by converting between I²C and 1-Wire protocol with adjustable timing and a sleep mode. By allowing the MAX66242 to place the DS2484 in sleep mode through SLPZ, the DS2484 powers up when required by the NFC transceiver. Once the power down (PDN) bit is toggled to zero in the DS2484 configuration register, the 1-Wire network starts powering up for communication.

If your application requires symmetric key-based authentication of the 1-Wire network, the MAX66242 provides a built-in SHA-256 engine for computing a message authentication code (MAC) under FIPS 180-4. If the application instead needs an asymmetric public-key based authentication, the DS2475, an elliptic curve digital signature algorithm (ECDSA) coprocessor with I²C to 1-Wire master capability, can be used instead of the DS2484. The DS2475 generates certificates and signatures under the P-256 curve. Learn more about FIPS 180-4 and the P-256 curve from the National Institute of Standards and Technology (NIST). The DS2475 unabridged datasheet provides more detail on the part.

How Harvested Voltage V_OUT and Current I_OUT Can Power a 1-Wire Network

The MAX66300 and the MAX66242 complete an NFC system that meets the ISO 15693 and the FIPS 180-4 standards. When both are connected to individual PCB antennas tuned to 13.56MHz, the MAX66242 harvested output voltage V_OUT and output current I_OUT can be characterized. The measured results in this section were obtained using the MAX66300-X24EVKIT, which includes the MAX66300 and MAX66242.

Figure 3 shows the available voltage at V_OUT harvested by the MAX66242 concerning the distance from the MAX66300 antenna. The voltage values for V_OUT are obtained when both the transmit and receive antennas are parallel and centered to each other. The MAX66300 output driver antenna configuration in Figure 3 uses on-off keying (OOK) and drives the antenna differentially using the ANT1 and ANT2 pins. The voltage at V_OUT varies with different antenna configurations, orientations, and environments, examples include being in the presence of nearby metals or other transponders in the read zone. At 3.2cm or less, V_OUT can output 3.3V or 1.8V depending on the user configuration. At distances greater than 10.2cm, no harvested voltage regulation is present at V_OUT.

To power, access, and control a 1-Wire network wirelessly, the energy harvested from the NFC link must power the MAX66242, the DS2484, and the 1-Wire devices as shown in Figure 2. To properly design an NFC-powered 1-Wire network, it is critical to understand the amount of current available I_OUT from the harvested output voltage V_OUT. Figure 4 models I_OUT and V_OUT as an ideal rectified voltage V_S and source resistance R_S.

The ideal rectified voltage V_S equals the open-circuit voltage at V_OUT. The source resistance R_S effectively increases the pullup resistance of the 1-Wire network to R_S + R_PUP, where R_PUP is internal to the DS2484. Get more details about the resistance value of R_PUP, including the active pullup (APU) and the strong pullup (SPU), from the DS2484 data sheet. V_DD is the voltage supply for the DS2484. Note that V_OUT shares the same node as V_DD and, therefore, are equal. The 1-Wire network pullup voltage is V_PUP. Figure 5 shows the load regulation of V_OUT with the same configuration described in Figure 3 at a distance of 0cm from the antenna of the NFC transceiver.

The output voltage (V_OUT) is defined as the ideal rectified voltage (V_S) minus the voltage drop across the source resistance (R_S) due to I_OUT. The source resistance (R_S) is defined in Equation (1) as follows:

R_S = (V_S – V_OUT) / I_OUT (1)

Figure 6 shows how the source resistance (R_S) varies concerning the measured output current (I_OUT) when the output voltage (V_OUT) is set to the 3.3V mode. Source resistance (R_S) exhibits nonlinear characteristics because of the challenge in maintaining a constant harvested V_OUT with an increasing I_OUT demand.

Not only does R_S depend on I_OUT and V_OUT, but it is also initially influenced by the internal pullup resistance (R_PUP) of the DS2484. As such, if the 1-Wire network in Figure 4 is represented by an equivalent discharged capacitance (CTOTAL) as shown in Figure 7, R_S could have a value dependent on V_OUT and on an I_OUT that is initially set by R_PUP.

As C_TOTAL accumulates charge, R_S changes over time because I_OUT and V_OUT also adjust over time. Using Figure 5, V_OUT can be determined by knowing the instantaneous value of I_OUT. R_S reaches a final value once the charge on C_TOTAL reaches a steady-state value.

Given the nonlinearity between I_OUT and V_OUT, an iterative approach is needed to calculate the first R_S value. A convergent result for R_S can be determined by solving for I_OUT and by looking up the value of V_OUT on Figure 5.

Figure 8 illustrates the iterative procedure to calculate the initial value for R_S for an output current (I_OUT). A result for R_S is considered convergent if I^N+1_OUT differs by 10% or less compared to I^N_OUT. N represents the number of iterations in the flow diagram.

Note: Instead of calculating iterative versions of R_S, Figure 6 can be used in conjunction with Figure 5 to find a final value. However, a better method is to calculate R_S using the flow diagram and to use Figure 5 to look up V_OUT as shown in Figure 8. With this approach, only one plot instead of two is used for extrapolation from the measured results to reduce error.

Figure 8 can also be used to recalculate the source resistance (R_S) while the equivalent capacitance (C_TOTAL) continues to accumulate charge at time t_N. Set the boolean for “New time t_N?” to ‘YES’ to use the exponential decay function to calculate I_OUT at a later time (t_N+1).

For applications that require more output current (I_OUT), an NFC transceiver with a high-voltage output driver and high-gain receiver can increase the range and power delivery.

Conclusion

With the MAX66242 as an RC circuit, and by modeling the regulated voltage harvested from the NFC RF field, we can calculate the source resistance added to the 1-Wire pullup resistance that exists on the 1-Wire network. Calculating the source resistance (R_S) recursively allows us to find the output current (I_OUT) supplied by the harvested regulated voltage (V_OUT) of the MAX66242.