# 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](#1-overview)  
- [2. MCU and wireless](#2-mcu-and-wireless)  
- [3. Power – battery, charging, and gauge](#3-power––battery-charging-and-gauge)  
- [4. USB‑C interface and firmware protection](#4-usb-c-interface-and-firmware-protection)  
- [5. Power‑supply architecture](#5-power-supply-architecture)  
- [6. Motion sensing (IMU)](#6-motion-sensing-imu)  
- [7. Real‑time clock (RTC)](#7-real-time-clock-rtc)  
- [8. Deep‑sleep and power‑gating](#8-deep-sleep-and-power-gating)  
- [9. Display and I²C peripherals](#9-display-and-i2c-peripherals)  
- [10. GPIO exposure and modularity](#10-gpio-exposure-and-modularity)  
- [11. Power‑pin distribution per GPIO](#11-power-pin-distribution-per-gpio)  
- [12. Size, PCB layers, and mechanical constraints](#12-size-pcb-layers-and-mechanical-constraints)  
- [13. External power input and protection](#13-external-power-input-and-protection)  
- [14. Bill of Materials (BOM) draft](#14-bill-of-materials-bom-draft)  

***

## 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 |