USB-C NFC Reader with Power Delivery Passthrough and STM32L476
A USB-C powered NFC reader based on the ST25R200 and STM32L476 that can operate from tablet power or from a secondary USB-C power input while also charging the tablet.
Advanced Project
— This is advanced because it combines USB-C/PD power-role negotiation, bidirectional power-path management, USB data routing, and an RF NFC front end that all have tight layout and firmware interactions.
Assumptions:
- The tablet connection is USB-C and the NFC reader only needs USB 2.0 data, not USB 3.x.
- The secondary USB-C port is intended as a power input/output port for USB-C PD negotiation, not as a second data port.
- The ST25R200 will be used as the NFC/RFID front end over SPI to the STM32L476.
- Prototype means a custom PCB or wired prototype is acceptable, but the design should use readily available dev-board-friendly parts.
Bill of Materials
Compatibility Notes
- STM32L476 logic is 3.3 V, so the ST25R200, USB switch, and any PD controller interface must be 3.3 V compatible or level-shifted as needed.
- USB-C data to the tablet should be treated as USB 2.0 unless you explicitly need SuperSpeed; that keeps routing and switching much simpler.
- If the secondary USB-C port must both accept power and source power to the tablet, you need explicit power-role management and power-path control; a simple connector plus regulator is not enough.
- The ST25R200 SPI interface should share the STM32L476's 3.3 V domain, which simplifies firmware and avoids level translation on the NFC control bus.
- If you use STM32L476RG or STM32L476RG, verify how the controller hands off policy control to the STM32L476 and whether you want autonomous or firmware-managed PD behavior.
You'll Also Need
- USB-C receptacles for both ports.
- ESD protection for CC, D+/D-, and VBUS lines.
- Inductors, capacitors, sense resistors, and any required power-path MOSFETs for the chosen PD/power architecture.
- The ST25R200 matching network, antenna coil, and RF tuning components.
- A PCB with controlled routing for USB and NFC RF sections.
- Firmware for USB-C policy, tablet charging behavior, and NFC reader stack.
- If you need a battery-backed prototype, a Li-Ion cell and protection circuitry are still needed.
Estimated BOM Cost: $30-35 (based on live distributor pricing)
Design Considerations
USB-C Power-Role Architecture
Your requirement is not just USB-C connectivity; it is dual-role power behavior with source/sink switching. That means the secondary USB-C port must be able to accept input power and also source power to the tablet when present, which usually requires a real PD controller and a power-path design, not just CC resistors. Plan the state machine carefully so the device never back-feeds a port in the wrong role.
Power Budget
The ST25R200 plus NFC antenna can draw meaningful current during reader activity, and USB-C source mode to a tablet can easily dominate the budget. If you expect to charge a tablet, even a modest 5 V at 1.5 A is 7.5 W, so your input source and thermal design must be sized accordingly. For a prototype, define the maximum source current early and make sure the controller and power path can sustain it continuously.
USB Data Integrity
Keep the tablet USB 2.0 D+/D- routing short, symmetric, and impedance-conscious, and place ESD protection close to the connector. If you use TS3USB221, route the switch close to the connector and avoid stubs, because USB 2.0 is forgiving compared with SuperSpeed but still sensitive to poor layout. Also separate the noisy power-path copper from the data pair to reduce common-mode noise injection.
NFC RF Layout
The ST25R200 antenna loop and matching network are the most layout-sensitive part of the design. Keep the antenna away from metal, large ground pours, and high-current switching nodes, and expect to tune the matching network on the bench with a VNA or at least empirical read-range testing. Small changes in enclosure or tablet proximity can shift resonance enough to matter.
Firmware State Machine
You will need a robust state machine for attach detection, power-role negotiation, NFC reader operation, and fault handling. Use watchdog recovery and explicit timeout handling for USB-C attach/detach events, because a tablet disconnect or power-supply hot-plug can leave the system in a partially negotiated state. Make the NFC reader firmware tolerant of power-role transitions so it can pause and resume cleanly.
Prototype Validation
Test the design in three separate modes: tablet-powered only, external-supply only, and hot-plug transitions between the two. Measure VBUS, inrush current, and thermal rise while the tablet is being charged, because those are the first places a prototype usually fails. For NFC, validate read range and tag detection with the final enclosure and with the tablet physically attached, since nearby metal and cable routing can change RF performance.
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