NFC exploration

Sandbox repo to explore ST25DV04KC (ISO15693 / NFC-V) tap-to-trigger flows for power control and waking circuits such as inki.

Goal

Build a small, reproducible test platform (ST25DV04KC + Raspberry Pi Pico/RP2040 + antenna + simple enclosure) to validate:

• RF-field / V_EH wake behavior in the real case
• Phone → tag writes (EEPROM / Type 5)
• MCU reads request over I²C and clears/acks
• End-to-end timing assumptions for a future “tap-to-book” workflow

Current validated behavior

• ST25DV04KC bring-up over I2C is stable on Pico (GP20/GP21), with ST25 power switched by GP18.
• End-to-end NFC-V write from phone to tag user memory is working.
• Firmware parses 16-byte INKI request payloads from the last user-memory slot (0x01F0 on ST25DV04KC).
• Firmware command opcode handling is active:
  • 0x11 -> LED1 slow blink
  • 0x12 -> LED1 fast blink (same onboard LED)
  • 0x01 -> legacy alias mapped to LED1 slow blink
• Incomplete/invalid INKI frames are ignored (to avoid partial-write races).
• Valid requests are cleared only after RF field goes OFF (reduces interference with phone-side readback).
• Power latch is asserted early by Pico on boot (GP28) and auto-released after NFC_AUTO_POWER_OFF_MS (default 10000 ms).
• Runtime logging is event-driven: startup diagnostics once, then RF field edges and request actions only.

Tuning firmware (antenna / power characterization)

A dedicated firmware (nfc_tune, source: firmware/src/tune_main.c) for real-time RF energy-harvesting characterization. It samples the ST25DV V_EH output via Pico ADC and renders a live scrolling bar graph in the terminal showing harvested power delta above baseline.

What it measures

• ADC voltage on V_EH pin (default ADC1 = GP27), averaged and EMA-smoothed
• Delta above a startup baseline capture (no-RF reference)
• RF field state derived from voltage thresholds (configurable ON/OFF hysteresis)
• ST25 EH control register state (EH_EN, EH_ON, FIELD_ON, VCC_ON) via I2C

Build and flash

make firmware-tune                    # build nfc_tune
# then either:
make firmware-tune-flash              # flash via SWD
# or drag-and-drop firmware/build/nfc_tune.uf2 in BOOTSEL mode

Tuning workflow

1. Connect Pico via USB (serial + power).
2. Open terminal: sudo tio /dev/ttyACM0
3. Wait for baseline capture (keep RF away for ~2 seconds at boot).
4. Tap phone to antenna — the live graph shows V_EH delta in mV, RF ON/OFF transitions, session peak, and EH register state.
5. Adjust antenna matching (C2/C3 on NFC_harness_V0) and re-tap to compare coupling.
6. Ctrl+C to stop.

Key CMake tuning parameters

All defaults are in firmware/CMakeLists.txt:

• NFC_TUNE_ADC_INPUT – ADC channel (default 1 = GP27)
• NFC_TUNE_FIELD_ON_MV / NFC_TUNE_FIELD_OFF_MV – RF detection hysteresis thresholds (default 9 / 4 mV)
• NFC_TUNE_GRAPH_WIDTH / NFC_TUNE_GRAPH_HEIGHT – terminal graph dimensions (default 72x20)
• NFC_TUNE_GRAPH_MAX_MV – vertical scale (default 3000 mV)
• NFC_TUNE_SAMPLE_COUNT / NFC_TUNE_REPORT_INTERVAL_MS – samples per report and update rate

Current Wake/Latch Circuitry (PCB: NFC_harness_V0)

• Q1 (TSM260P02CX, P-MOS) is the high-side power switch:
  • source -> Vbatt
  • drain -> Pico VSYS
• ST25 GPO_(OPEN_DRAIN) and the Q1 gate share the same control net (wake/power-on net).
• Gate network shaping:
  • R4=1M pull-up to Vbatt
  • C5=22n to GND (pulse shaping / hold)
• Pico latch-hold path:
  • Pico GPIO28 (NFC_POWER_HOLD_PIN) -> R5=100k -> Q2 base
  • R9=50k pulls base to GND
  • Q2 collector also sinks the same gate-control net
• On this PCB revision, Pico GPIO19 is unconnected.
• LED wiring status on this PCB revision:
  • Both blink commands (0x11 slow, 0x12 fast) use the firmware power-LED output (onboard Pico/Pico W path).
  • NFC_STATUS_LED_PIN (default GP15) is currently not routed to a dedicated onboard LED footprint here (external LED/wire may be needed).

Command Flow (Phone -> ST25 -> Pico)

1. Board is fully off (Vbatt connected, no USB, Pico unpowered).
2. Phone tap powers ST25 RF side and writes the 16-byte INKI payload (including opcode byte) over NFC-V into ST25 EEPROM, while Pico can still be fully off.
3. ST25 asserts GPO low on configured wake event (RF_WRITE), pulling Q1 gate low and powering Pico.
4. Pico boots and immediately asserts GPIO28 high to latch power through Q2.
5. Data transfer from ST25 to Pico happens after boot over I2C (not on GPO):
   • Pico powers ST25 VCC via GP18
   • firmware reads request bytes from the last ST25 user-memory slot
6. Firmware validates the payload, applies opcode behavior, queues clear for RF-OFF, and later clears the slot.
7. Firmware releases the latch after timeout (NFC_AUTO_POWER_OFF_MS, default 10000 ms; re-armed on accepted commands).

Note:

• Runtime-loop request processing still runs while RF field is ON, with clear deferred to RF field OFF.
• Additionally, at boot firmware now checks for a stored valid request. If RF field is already OFF, it applies the command immediately and clears it at boot.
• If no valid INKI request is found after wake and RF is already OFF, firmware releases the power latch immediately.

Repository layout

• docs/context.md – running notes, decisions, history, and solved problems
• firmware/ – Pico SDK firmware scaffold (RP2040) for ST25 I²C exploration
  • src/ app entry (main.c harness, tune_main.c tuning) and bus adapter
  • third_party/st25dv/ vendored ST25DV driver (git subtree)
  • scripts/ build/flash/reset helpers
  • cmake/ Pico SDK import helper
• pcb/NFC_harness_V0/ – KiCad project (schematic, layout, CAM profile)
• pcb/tools/run_pcb2gcode.sh – shared pcb2gcode runner
• pcb/components/ + pcb/project-libraries/ – KiCad symbols/footprints (includes ST25DV package lib submodule)
• datasheet/ – ST25DV04KC datasheet + app notes
• android/ – NFC‑V test apps (writer + hello) (not needed for Pico-only workflow)

Typical workflow (from repo root)

1. Build firmware: make firmware (uses PICO_SDK_PATH, default $HOME/pico/pico-sdk).
2. Flash:
   • Drag‑and‑drop: copy firmware/build/nfc_harness.uf2 to Pico in BOOTSEL mode, or
   • SWD: make firmware-flash (OpenOCD + CMSIS-DAP).
3. Iterate: edit firmware/src/main.c (and future helpers), rebuild, and re‑flash.
4. CAM / milling (optional): pcb/tools/run_pcb2gcode.sh -b NFC_harness_V0 to regenerate toolpaths.

Build & flash (Pico / RP2040)

Build

# from repo root (recommended)
make firmware

# or inside firmware/
cd firmware
./scripts/build.sh   # expects $PICO_SDK_PATH (defaults to $HOME/pico/pico-sdk)

More detail (pin overrides, UF2 path, picotool toggles): see firmware/README.md.

Notes:

• Picotool is disabled by default to avoid downloads. The build script auto-fetches uf2conv.py (Microsoft UF2 tool) on first run and converts nfc_harness.bin → nfc_harness.uf2.
• Set -DPICO_NO_PICOTOOL=OFF in cmake if you prefer the picotool UF2/signing path.

Clean build artifacts:

make clean   # removes firmware/build only

Flash options:

• USB drag‑and‑drop: copy firmware/build/nfc_harness.uf2 to the Pico mass‑storage device (BOOTSEL).
• SWD (OpenOCD + CMSIS‑DAP probe):

  cd firmware
  ./scripts/flash.sh

Root make targets (quick reference)

• make firmware – configure & build firmware into firmware/build/
• make firmware-tune – build dedicated antenna tuning firmware (nfc_tune)
• make firmware-flash – flash over SWD (OpenOCD + CMSIS‑DAP)
• make firmware-tune-flash – flash tuning firmware ELF over SWD
• make clean – remove firmware/build/ only

CAM workflow (pcb2gcode)

From repo root:

pcb/tools/run_pcb2gcode.sh -b NFC_harness_V0

The profile lives at pcb/NFC_harness_V0/cam/pcb2gcode/profiles/default.millprojects and uses repo-relative paths.

Quick RF test (phone + Pico)

1. Connect a loop antenna to J3 (AC0/AC1) on the PCB.
2. Build and flash firmware:
   • make firmware
   • copy firmware/build/nfc_harness.uf2 to Pico in BOOTSEL mode (or use SWD).
3. Open serial logs:
   • sudo tio /dev/ttyACM0
4. On phone, open the NFC-V app (inki):
   • pick command button: LED1 Slow or LED2 Fast
5. Tap phone to antenna and hold briefly.

Expected runtime log pattern:

• RF field: ON
• Request@0x01F0: ...
• INKI request: version=... opcode=... duration_min=...
• Command applied: opcode=... -> ...
• Queued clear after RF field OFF
• RF field: OFF
• Cleared request after RF OFF (...)
• Auto power-off: GP28 -> LOW

Sample serial log (successful run):

[INFO] RF field: OFF
[INFO] RF field: ON
[INFO] Request@0x01F0: 49 4E 4B 49 01 11 3C 00 ...
[OK] INKI request: version=1 opcode=0x11 duration_min=60 unix=... nonce=...
[OK] Command applied: opcode=0x11 -> LED1 slow blink
[INFO] Queued clear after RF field OFF
[INFO] RF field: OFF
[OK] Cleared request after RF OFF (16 bytes @ 0x01F0, attempts=1, 8ms)
[WARN] Auto power-off: GP28 -> LOW