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ESP32‑S3 Wearable Sensor Board – Hardware Design Specification

Document Version: 1.0
Target Audience: PCB designer / hardware engineer
Status: Ready for implementation

Document index

1. Overview

This document defines the hardware design requirements for a compact wearable sensor board based on the ESP32‑S3 (preferred) or ESP32‑C3. The board targets sports/impact‑sensing applications (fist/leg strikes) and must balance:

  • Tiny size for wearable use.
  • Long‑term battery life via deep‑sleep and power‑gating.
  • High‑integrity IMU and power‑monitoring (gas‑gauge, RTC).

The designer must translate this into a schematic and double‑sided PCB layout that meets all constraints listed below.

2. MCU and wireless

  • MCU:
    • Primary: ESP32‑S3 (small‑form‑factor, integrated Wi‑Fi/BLE, USB‑UART, 45+ GPIOs).
    • Alternative (only if size is critical and Wi‑Fi/BLE robustness is secondary): ESP32‑C3.
  • Connectivity:
    • 2.4 GHz Wi‑Fi 802.11 b/g/n.
    • Bluetooth 5.0 (BLE).
  • RF layout:
    • Use either an on‑board PCB antenna or a small external antenna connector, following Espressif’s ESP32‑S3 hardware design guidelines.
    • Provide proper RF‑matching network and keep‑out zones around the antenna.

3. Power – battery, charging, and gauge

  • Battery:
    • Single‑cell Li‑Po / Li‑Ion (nominal 3.7 V).
  • Charging:
    • Include a Li‑Po charge IC (e.g., TP4056, BQ2409x‑style, or similar modern IC) that:
      • Accepts 5 V from USB‑C → charges the battery.
      • Reports charge status via GPIO‑visible signals (e.g., CHRG, DONE).
    • Charging current: 100–500 mA typical (designer’s choice based on thermal and USB limits).
  • Battery gauge (SoC monitoring):
    • Use a gas‑gauge IC (e.g., MAX170xx family, or similar) to measure:
      • Battery voltage.
      • Current.
      • State‑of‑Charge (%).
    • Interface: I²C (shared bus with IMU and RTC).
    • This is mandatory; do not rely purely on ADC‑based voltage‑only estimation.

4. USB‑C interface and firmware protection

  • USB‑C role:
    • One USB‑Type C receptacle serving:
      • Battery charging (via Li‑Po charge IC).
      • ESP32‑S3 programming and serial communication (via built‑in USB‑UART/JTAG).
  • D‑ / D+ SMD jumpers:
    • Bring D‑ and D+ from the USB‑C connector to SMD jumper pads (e.g., 0603/0402‑style pads).
    • When the pads are shorted by solder, D‑/D+ are connected to the ESP32‑S3.
    • When the shorts are desoldered, D‑/D+ are disconnected, preventing:
      • User‑induced firmware re‑flash.
      • Accidental corruption of the device in the field.
  • USB protection:
    • Include ESD protection / 5‑V clamp on USB lines.
    • Limit current via USB‑compliant resistor or IC‑based current‑limiting.

5. Power‑supply architecture

  • 3.3 V main rail:
    • Provide a 3.3 V regulator for the ESP32‑S3, IMU, OLED, and 3.3 V peripherals.
    • Prefer a buck (step‑down) converter over an LDO for efficiency and higher current capability.
    • Target: 3.3 V @ 300–500 mA continuous (or higher if justified by load).
  • Boost converter / 5 V booster:
    • A boost converter (e.g., 1.8–4.2 V input → 5 V output) must power:
      • LED strips.
      • Other 5 V peripherals.
    • The enable pin of the booster must be driven by a GPIO of the ESP32‑S3.
    • When the MCU goes into deep‑sleep, this GPIO must be set to low to disable the booster.
  • Isolation:
    • Ensure the 5 V rail is isolated from 3.3 V logic (Schottky diodes or MOSFET isolation if needed).

6. Motion sensing (IMU)

  • General requirement:
    • Use a 6‑axis or 9‑axis IMU for impact / strike detection, supporting:
      • High‑g capability (≥ 80 g) for punches/kicks.
      • Low‑noise gyro for angular motion.
    • Do not use obsolete parts like MPU‑9250 or MPU‑6050 due to supply and counterfeit risks.
  • Preferred IMU:
    • ST LSM6DSV80X (or equivalent) is recommended because:
      • 6‑axis IMU in a 2.5×3 mm package.
      • Dual accelerometer paths: 16 g low‑g and 80 g high‑g.
      • Gyroscope and embedded AI‑style FSM/MLC for impact detection and motion states.
      • Supports I²C and SPI.
  • Interface and power:
    • Run the IMU from the 3.3 V rail.
    • Connect to ESP32‑S3 via I²C (shared with battery gauge and RTC).
    • Expose all configurable pins (interrupts, reset, etc.) on solder pads so they can be jumpered or disconnected.

7. Real‑time clock (RTC)

  • Function:
    • Provide accurate timekeeping during long‑term battery‑powered operation and deep‑sleep.
  • Requirements:
    • I²C‑compatible RTC module (e.g., common 32‑kHz RTC chips).
    • Tiny coin‑cell footpring for a 3‑V rechargeable cell or supercap‑backup (e.g., CR1220‑style).
    • Alarm / interrupt output to a GPIO to wake the MCU from deep‑sleep.
  • Power:
    • Main power from 3.3 V rail.
    • Backup power from coin cell / supercap.

8. Deep‑sleep and power‑gating

  • System behavior in deep‑sleep:
    • In deep‑sleep, the ESP32‑S3 must disable all non‑critical external peripherals and isolate power rails to minimize current draw.
    • Targets for shut‑down:
      • 5 V booster (GPIO‑driven enable → low).
      • Any external modules powered by external 5 V or switchable rails.
  • Power‑gating strategy:
    • Use MOSFET‑based power‑gating or enable‑controlled DC/DC converters for:
      • 5 V booster output.
      • External I²C peripherals not needed during sleep.
    • The MCU must control enable signals for these regulators/MOSFETs via GPIOs.

9. Display and I²C peripherals

  • OLED display:
    • Provide a standard footprint / connector for an I²C OLED display (e.g., 0.96" 128×64).
    • Pins at connector:
      • VCC (3.3 V).
      • GND.
      • SDA (I²C data).
      • SCL (I²C clock).
    • Power from the 3.3 V rail.
  • General I²C:
    • Provide exposed solder pads for a secondary I²C bus or breakout of the main I²C bus for:
      • Extra sensors.
      • Expansion modules.
    • Use appropriate pull‑up resistors (e.g., 4.7 kΩ) sized for selected bus speed.

10. GPIO exposure and modularity

  • All GPIOs:
    • Every ESP32‑S3 GPIO must be accessible:
      • As through‑hole pads, 0.1" header pins, or edge‑mountable pads.
      • Labeled with: GPIO number + function (e.g., GPIO21 – I2C_SDA).
  • External‑control signals:
    • Any GPIO that controls:
      • Booster enable.
      • Other external modules / regulators.
    • Must be connected via SMD jumper pads (two pads with a short‑able bridge).
    • When the bridge is soldered, the MCU controls that function.
    • When the bridge is desoldered, that function is disconnected, allowing modular removal of that side of the board.

11. Power‑pin distribution per GPIO

To ensure easy connection to external peripherals without extra wiring:

  • For every GPIO pad / connector, place neighboring pads for:
    • GND.
    • 3V3.
    • 5V.
  • Preferred layout patterns:
    • Pin‑header style: [GPIO_X][GND][3V3][5V] repeated.
    • Solder‑pad array:
      • One row for GND.
      • One row for 3V3.
      • One row for 5V.
      • One row for GPIO_X signals.
  • Electrical rules:
    • Connect all GND pads to the main ground plane.
    • Connect 3V3 pads to the main 3.3 V buck output.
    • Connect 5V pads to the boost‑converter output, enabled only when the booster is active.
    • Size power traces and vias to handle high‑current loads (e.g., LED strips, motors).

12. Size, PCB layers, and mechanical constraints

  • Target size:
    • As small and tiny as possible to maximize space for the battery and wearable‑side components.
    • Target in the range of ~20×30 mm or smaller, depending on antenna and battery constraints.
  • PCB stack‑up:
    • Use single‑sided PCB where possible.
    • If routing density or RF layout requires it, switch to double‑sided (2‑layer) PCB.
  • Ground and RF:
    • Use a solid ground plane under RF and MCU sections.
    • Follow Espressif’s ESP32‑S3 hardware design guidelines for antenna keep‑out and RF‑matching.

13. External power input and protection

  • External power input pads:
    • Provide two dedicated solder pads for external input voltage:
      • VIN (or VEXT).
      • Adjacent GND pad.
  • Voltage protection and regulation:
    • The external input must tolerate a range above 3.3 V (e.g., roughly 4–12 V, negotiable with the designer).
    • Include:
      • A wide‑input buck regulator (or similar) that steps higher input down to:
        • A safe bus voltage for the Li‑Po charger (e.g., 4.5–5.5 V), and/or
        • Directly to 3.3 V for the MCU and peripherals.
      • Over‑voltage protection (OVP) or a TVS / ESD clamp on the VIN line.
  • Behavior:
    • The board can be powered via:
      • USB‑C.
      • Or the external VIN pads.
    • Only one power source should be active at a time.
  • Labeling:
    • Clearly mark the pads as:
      • VIN
      • GND (VIN)

14. Bill of Materials (BOM) draft

This is a functional BOM; the designer may substitute functionally equivalent parts (with your approval).

ID Function Suggested / Example Part Notes
MCU1 Main MCU ESP32‑S3‑WROOM‑32 / ESP32‑S3‑MINI Wi‑Fi/BLE, USB‑UART, 45+ GPIOs
CHG1 Li‑Po charger TP4056 or similar modern Li‑Po charge IC 5 V input, CHRG/DONE outputs
GAGE1 Battery gauge MAX1704x / MAX170xx family I²C‑based SoC estimation
IMU1 Motion sensor ST LSM6DSV80X 6‑axis, 16 g + 80 g accel, 2.5×3 mm
RTC1 RTC module Generic I²C 32‑kHz RTC with coin‑cell backup Alarm / interrupt output
REG1 3.3 V main rail 3.3 V buck converter (e.g., AP62x0x‑style) 3.3 V @ 300–500 mA+
BOOST1 5 V booster 5 V boost converter with enable pin Must be GPIO‑controllable
USB1 USB‑C receptacle Standard USB‑Type C SMD connector Dual‑role (charging + programming)
USB_PROT1 USB‑line protection USB‑ESD array / 5‑V clamp Protects D‑/D+ and VBUS
VIN_PROT1 External input protection Wide‑input buck (e.g., LM2576‑style or similar) plus TVS Steps 4–12 V down to safe rail
OLED1 OLED display connector 4‑pin header / footprint 3.3 V, GND, SDA, SCL
CRYST1 40 MHz / 32 kHz crystals 40 MHz + 32.768 kHz (if required) As per ESP32‑S3 / RTC requirements