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xblink/README.md
michael bae64c1e3a Initial project scaffold for xblink NFC-powered LED implant 
Set up Rust embedded project targeting SAMD21 (thumbv6m-none-eabi) with
XIAO M0 BSP for dev board prototyping. Includes LP5562 EN pin control
on D0/A0, project documentation (README, CLAUDE.md), phased status
tracker, and detailed development plan covering LP5562 driver integration,
NTAG5Link I2C driver, pattern storage format, power management, NFC
communication protocol, recovery mode, and firmware update strategy.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2026-03-03 09:32:30 -08:00

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xblink

NFC-powered LED implant using NTAG5Link, SAMD21E, and LP5562.

A battery-free subdermal LED implant that harvests energy from an NFC field. An NTAG5Link provides power and storage, a SAMD21E microcontroller reads stored LED patterns and programs the LP5562 LED driver's autonomous execution engines, then sleeps. Patterns are updated wirelessly via NFC from a phone.

Hardware Overview

                                    I2C (0x30)
  Phone/Reader )))  NFC  ((( NTAG5Link ---------> LP5562 -----> RGBW LEDs
                            |         |           |
                            | VOUT    | FD pin    | EN
                            |  (EH)   |           |
                            v         v           |
                          +--SAMD21E--+           |
                          | D0/A0 ----------------+  (GPIO enable)
                          | A4/A5 (I2C bus) ------+
                          | GPIO  <--- Hall Sensor
                          +----------+

All power comes from NTAG5Link energy harvesting — no battery.

Components

Component Part Role I2C Addr Key Specs
NFC + Power NXP NTAG5Link (NTP53x2) NFC interface, energy harvesting, pattern storage 0x54 (slave) 2048B EEPROM, 256B SRAM, ISO15693, 1.8/2.4/3.0V EH output
MCU SAMD21E18A Firmware (Rust), pattern loading, sleep/wake — 256KB flash, 32KB RAM, ARM Cortex-M0+, 32-pin QFN
LED Driver TI LP5562 RGBW LED control with autonomous patterns 0x30 4ch RGBW, 3 execution engines, 16 cmds/engine, 0-25.5mA/ch
Recovery Hall effect sensor Safe mode / recovery trigger — GPIO input with EIC wake from sleep

Dev board: Seeed XIAO M0 (SAMD21G18A) + LP5562EVM + MikroElektronika NTAG5 Link Click

Architecture

Boot Sequence

  1. NFC field detected by NTAG5Link
  2. VOUT powers SAMD21E and LP5562 (MCU drives EN pin high on D0/A0)
  3. SAMD21E boots, checks hall sensor GPIO (asserted = recovery mode)
  4. Normal boot: reads LED pattern from NTAG5Link EEPROM via I2C
  5. Programs LP5562 execution engines with pattern data
  6. Enters STANDBY sleep (~5uA)
  7. LP5562 runs patterns autonomously using internal oscillator
  8. NFC field drops → VOUT drops → clean power-off

Pattern Update Sequence

  1. Phone sends new pattern data to NTAG5Link via NFC
  2. NTAG5Link signals MCU via FD pin (SRAM write indication)
  3. SAMD21E wakes from sleep via EIC interrupt
  4. Pauses LP5562 engines (set exec to Hold)
  5. Reads new pattern from NTAG5Link SRAM mailbox via I2C
  6. Writes pattern to NTAG5Link EEPROM for persistence
  7. Reprograms LP5562 engines with new pattern
  8. Returns to STANDBY sleep

Recovery Mode

If the hall sensor is asserted at boot (magnet held near implant):

  • Loads a hardcoded default pattern (solid white, low brightness)
  • Ignores EEPROM pattern data
  • Sets recovery flag in SRAM for phone app detection
  • Remains awake for firmware update if supported

Power Budget

All power from NTAG5Link energy harvesting. Budget is constrained and TBD pending prototype measurements.

Parameter Value Notes
EH max current ~12.5mA Highest threshold setting
EH voltage 3.0V Required for LP5562 (2.7-5.5V) and SAMD21E (1.62-3.63V)
LP5562 per channel 0-25.5mA Must limit to ~2-5mA/ch for EH budget
SAMD21E active ~3-5mA During boot and pattern programming
SAMD21E standby ~5uA After LP5562 engines are running

Getting Started

Toolchain Setup

# Install Rust
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
source $HOME/.cargo/env

# Add ARM Cortex-M0+ target
rustup target add thumbv6m-none-eabi

# Install cargo-hf2 for UF2 flashing (XIAO M0)
cargo install cargo-hf2

On Linux, add udev rule for XIAO bootloader (vendor ID 0x2886):

echo 'SUBSYSTEM=="usb", ATTR{idVendor}=="2886", MODE="0666"' | sudo tee /etc/udev/rules.d/99-xiao.rules
sudo udevadm control --reload-rules

Build and Flash

cargo build --release

# Put XIAO in bootloader mode (double-tap RST), then:
cargo hf2 --release

Dev Board Wiring

Current prototype (XIAO M0 + LP5562EVM):

XIAO Pin LP5562EVM Function
A4 (SDA) SDA I2C data
A5 (SCL) SCL I2C clock
D0/A0 EN Hardware enable (GPIO)
3V3 VCC Power
GND GND Ground

Project Status

See docs/STATUS.md for current progress.

Related Projects

  • ../ntag5-samd21-lp562/ — LP5562 Rust driver prototype and SAMD21 test firmware
  • ../ntag5sensor/ — Python NTAG5Link tooling (PCSC reader, ISO15693, sensor interface)