InkTime - Open Source Smartwatch

InkTime is a low-cost, open-source smartwatch designed for the EVT (Engineering Validation Test) phase. This project involves the full hardware design cycle, from schematic capture and PCB layout in Autodesk Fusion 360 to mechanical integration and manufacturing file generation.

Project Overview

The system architecture centers around the nRF52840 (Nordic Semiconductor) for BLE connectivity and ultra-low power consumption, paired with a high-contrast E-Paper display.

Key Features

• MCU: Nordic nRF52840 (Cortex-M4 + BLE 5.0).
• Display: 1.54" E-Paper (Ultra-low power, sunlight readable).
• Power: USB-C charging with 180mAh LiPo battery.
• Form Factor: Compact design ready for mechanical integration.

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System Architecture

Block Diagram

The core of the device integrates high-speed SPI for the display and I2C for peripheral sensors.

Hardware Functionality

• Microcontroller: The nRF52840 provides a 2.4GHz radio and ARM® Cortex®-M4 processor.
• Interfaces:
  • SPI: Dedicated for the E-Ink display to ensure high-speed data transfer.
  • I2C: Connects the IMU (Inertial Measurement Unit) and Fuel Gauge for battery monitoring.
  • GPIO: Used for tactile buttons and the haptic shaker motor (PWM controlled).
• Power System: Charged via USB-C (5V), regulated to 3.3V via a low-dropout (LDO) regulator.

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Bill of Materials (BOM)

Component	Value/Model	Package	Provider (JLC)	Datasheet
MCU	nRF52840a	QFN™73	JLC Part	Link
Display	E-Paper 1.54"	FPC	JLC Part	Link
Charger IC	MCP73832	SOT-23-5	JLC Part	Link
Battery	LiPo 180mAh	Custom	N/A	Link
LDO	3.3V Regulator	SOT-23	JLC Part	Link

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Pinout Mapping

nRF52840 Pin	Function	Description
P0.xx	SPI_SCK	Serial Clock for E-Ink
P0.yy	SPI_MOSI	Master Out Slave In for E-Ink
P0.zz	I2C_SDA	Data line for Sensors
P0.ww	PWM_VIB	Haptic motor control

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Energy Budget

• Battery Capacity: 180 mAh.
• Deep Sleep Mode: Estimated < 50 µA.
• Active Refresh: ~8-12 mA (peak during E-Ink refresh).
• Estimated Standby: Approx. 10-14 days.

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Design Log & Constraints

PCB Implementation

• Layers: 2-layer design (Top/Bottom) with dedicated ground planes.
• Trace Widths: Power (0.3mm), Signal (0.15mm) to comply with JLC manufacturing standards.
• Decoupling: 100nF capacitors (0201) placed close to MCU VCC pins.