xblink/docs/plans/2026-03-03-hall-en-design.md

Hall Sensor + LP5562 EN Wired-AND Circuit Design

Date: 2026-03-03 Status: Design in progress

Problem

The implant needs a hall sensor for three functions:

1. Pattern swap — quick magnet tap cycles to the next stored pattern
2. Runtime recovery — long hold + confirmation tap enters safe mode
3. Boot-time recovery — magnet present at power-on skips EEPROM, loads default
4. Hardware safety — LEDs forced off by magnet even if MCU firmware has crashed

These must work within extreme space and power constraints (energy-harvested implant).

Circuit: Wired-AND on LP5562 EN

Zero extra active components. Hall sensor and MCU share the EN line via open-drain outputs.

         VCC (3.0V from NTAG5Link EH)
          |
         1M   (pull-up resistor)
          |
EN ------+------- MCU D0/A0 (open-drain)
          |
          +------- Hall sensor OUT (open-drain, active-low)
          |
         LP5562 EN pin


Hall sensor OUT ---- MCU EIC pin (separate connection for interrupt/wake)

How it works

State	MCU pin	Hall sensor	EN line	LP5562
Normal operation	High-Z	High-Z (no magnet)	High (pull-up)	On
Magnet present	High-Z	Driving low	Low	Off
MCU disabling LP5562	Driving low	High-Z	Low	Off
Magnet + MCU low	Driving low	Driving low	Low	Off

Pull-up resistor: 1M

• Steady-state current: ~0 (both open-drains high-Z, no path to ground)
• Current when EN low: 3V / 1M = 3 µA (only during magnet or MCU-driven low)
• Rise time: ~10 µs (1M x ~10pF parasitic) — instant from human perspective
• LP5562 EN input is high-impedance CMOS — no DC loading

Hall Sensor: TI DRV5032

Variant	Use	Iq	Threshold	Output	Package
DRV5032FB	Prototype	1.6 µA	8.4 mT (omnipolar)	Open-drain	SOT-23
DRV5032FE	Final PCB	0.55 µA	4.5 mT (omnipolar)	Open-drain	X2SON (1x1mm)

• Omnipolar: Triggers on either magnetic pole (user doesn't need to orient magnet)
• 8.4 mT threshold (FB): Higher threshold = harder to accidentally trigger. Requires deliberate, close magnet interaction. Good for "don't accidentally enter recovery" requirement.
• Open-drain, active-low: Required for wired-AND topology
• SOT-23 for prototyping: Easy to hand-solder; X2SON for final implant PCB

Interaction Model

Pattern Swap (tap <1s)

1. Magnet near → hall pulls EN low → LEDs off instantly (hardware, zero MCU latency)
2. MCU wakes via EIC interrupt on hall sensor pin, starts timer
3. Magnet removed (hall goes high-Z, pull-up restores EN)
4. MCU detects short press (<1s) → loads next stored pattern → LEDs back on

Runtime Recovery (hold >3s + confirm)

1. Magnet near → EN low (hardware), MCU wakes via EIC
2. MCU counts hold duration while hall stays asserted
3. Magnet removed after >3s → MCU notes "long hold detected"
4. MCU re-enables LP5562 → blinks warning pattern (3x red flash)
5. MCU waits 2s for confirmation tap
6. Confirmation tap received → enters recovery mode
7. No confirmation → resumes normal operation (reloads previous pattern)

Boot-time Recovery

1. NFC field arrives, VOUT powers system
2. Hall sensor already active (magnet was present before power) → EN held low by hardware
3. MCU boots, reads hall GPIO → asserted → boot-time recovery
4. Loads hardcoded default pattern (solid white, low brightness)
5. Skips EEPROM pattern data
6. Sets recovery flag in SRAM for phone app detection
7. When magnet removed, EN goes high, default pattern runs

Hardware Safety (MCU crashed)

• Holding magnet forces EN low at circuit level regardless of MCU state
• No firmware dependency — pure hardware path
• Magnet removal restores EN via pull-up (LP5562 resumes last-programmed pattern)

Firmware State Machine (M8)

                    ┌─────────────┐
        power-on -->│  BOOT_CHECK │
                    └──────┬──────┘
                hall low?  │
               ┌───yes─────┼────no───┐
               v           │         v
        ┌──────────┐       │  ┌─────────────┐
        │ RECOVERY │       │  │   RUNNING   │<──────────────┐
        └──────────┘       │  └──────┬──────┘               │
                           │   hall EIC │                    │
                           │   interrupt│                    │
                           │         v                      │
                           │  ┌─────────────┐               │
                           │  │  HALL_ACTIVE │               │
                           │  └──────┬──────┘               │
                           │   hall released                │
                           │    │          │                │
                           │  <1s        >3s               │
                           │    │          │                │
                           │    v          v                │
                           │  ┌──────┐  ┌──────────────┐   │
                           │  │ SWAP │  │ WARN_BLINK   │   │
                           │  └──┬───┘  └──────┬───────┘   │
                           │     │        hall tap │ no tap │
                           │     │       in 2s?   │        │
                           │     │     ┌──yes  ┌──no       │
                           │     │     v       v           │
                           │     │  ┌────────┐  │          │
                           │     │  │RECOVERY│  │          │
                           │     │  └────────┘  │          │
                           │     └──────────────┴──────────┘

States:

• BOOT_CHECK: Read hall GPIO. If asserted → RECOVERY. Else → RUNNING.
• RUNNING: LP5562 engines active, MCU in STANDBY sleep. Wakes on hall EIC.
• HALL_ACTIVE: Magnet detected. Timer running. Waiting for release.
• SWAP: Load next pattern, re-enable LP5562 → RUNNING.
• WARN_BLINK: Blink 3x red. Wait 2s for confirmation tap.
• RECOVERY: Default pattern, skip EEPROM, stay awake, set SRAM flag.

Power Impact

Component	Added draw	When
Hall sensor (DRV5032FB)	1.6 µA	Always (quiescent)
Pull-up resistor (1M)	3 µA	Only when EN is low (magnet or MCU-driven)
Pull-up resistor (1M)	~0	Steady state (EN high, no current path)

Total added quiescent: ~1.6 µA — negligible vs LP5562 LED current (~12 mA).

Open Questions

Question	Notes
Debounce needed?	Hall sensors are usually clean (no mechanical bounce), but verify with DRV5032
EIC pin selection on SAMD21E	Must be EIC-capable for wake from STANDBY. Check SAMD21E pinout.
Magnet strength/distance	What N52 neodymium size triggers DRV5032FB at implant depth (~3-5mm subdermal)?
Multiple stored patterns	How many patterns to cycle through? Currently TBD (depends on EEPROM format, M5)