The system architecture centers on the nRF52840 SoC for wireless connectivity, with power stabilized at 3.3V via an RT6160 buck-boost regulator. It integrates a LiPo battery with USB-C charging alongside an e-paper display, IMU, fuel gauge, and haptic feedback.
| Component | JLC Part # | Package | Description | Datasheet |
|---|---|---|---|---|
| SJ1 | N/A | Solder Jumper (Copper feature - leave open) | SMD solder JUMPER | N/A |
| R2, R3, R4 | C3920633 | 0201 | 7.68k 0201 Thin Film Surface Mount Fixed Resistor +/-0.5% 0.031W CPF0201D7K68C1 | datasheet |
| C23, C27, C34, C42 | C21012218 | - | Check availability | datasheet |
| EPD_C5 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| R1_EP_DR | C3920633 | 0201 | 7.68k 0201 Thin Film Surface Mount Fixed Resistor +/-0.5% 0.031W CPF0201D7K68C1 | datasheet |
| C15 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| C5, C7, C8, C12, C19 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| C11 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| R2_EP_DR, R9, R_PWR_EPD | C3920633 | 0201 | 7.68k 0201 Thin Film Surface Mount Fixed Resistor +/-0.5% 0.031W CPF0201D7K68C1 | datasheet |
| R5, R7, R8 | C3920633 | 0201 | 7.68k 0201 Thin Film Surface Mount Fixed Resistor +/-0.5% 0.031W CPF0201D7K68C1 | datasheet |
| C1-EP-DR | C9900156064 | 201 | Generic chip capacitor | datasheet |
| C24, C39 | C9900179830 | 402 | 0402 (1005 Metric) | N/A |
| L2 | C12669 | 402 | Generic chip inductor | datasheet |
| C1, C2, C17, C18 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| L3 | C12669 | 402 | Generic chip inductor | datasheet |
| C3, C4 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| C29, C30, C31, C32, C37, C38 | C3920633 | 0201 | 7.68k 0201 Thin Film Surface Mount Fixed Resistor +/-0.5% 0.031W CPF0201D7K68C1 | datasheet |
| EPD_C1, EPD_C2, EPD_C6, EPD_C7, EPD_C8, EPD_C9, EPD_C10, EPD_C11, EPD_C12 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| R_TYPE_SEL | C3920633 | 0201 | 7.68k 0201 Thin Film Surface Mount Fixed Resistor +/-0.5% 0.031W CPF0201D7K68C1 | datasheet |
| SW_DN, SW_ENT, SW_UP | C569760 | SMD,3.9x2.9mm | -40℃~+85℃ 1.6N 1.6mm 15V 2.9mm 20mA 3.9mm 500,000 Cycles IP67 J-Lead Rectangular Button SPST Surface Mount,Vertical White With Bracket SMD,3.9x2.9mm Tactile Switches ROHS | datasheet |
| Q1 | C2564 | TO-220AB | -55℃~+175℃ 1 P-Channel 180nC@10V 200W 20mΩ@10V 3.4nF 4V 55V 640pF 74A P-Channel TO-220AB MOSFETs ROHS | datasheet |
| C25, C33 | C9900179830 | 402 | 0402 (1005 Metric) | N/A |
| ANT1 | C2917717 | 1206 | -45℃~+125℃ 0.5dBi 1.3mm 1.6mm 100MHz 2.45GHz 2W 3.2mm 50Ω Patch Antenna 1206 Antennas ROHS | datasheet |
| L1 | C12669 | 402 | Generic chip inductor | datasheet |
| X2 | C32346 | SMD3215-2P | -40℃~+85℃ 12.5pF 32.768kHz 70kΩ Crystal Oscillator ±20ppm SMD3215-2P Crystals ROHS | datasheet |
| X1 | C9009 | SMD3225-4P | -40℃~+85℃ 12pF 32MHz Crystal Oscillator ±10ppm ±20ppm SMD3225-4P Crystals ROHS | datasheet |
| R17, R18 | C3920633 | 0201 | 7.68k 0201 Thin Film Surface Mount Fixed Resistor +/-0.5% 0.031W CPF0201D7K68C1 | datasheet |
| C43 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| C2-EP-DR | C9900156064 | 201 | Generic chip capacitor | datasheet |
| C6, C14, C20, C21 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| C16 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| J1 | C122434 | SMD,P=0.5mm,Surface Mount，Right Angle | FFC & FPC Connectors 0.5mm FPC RA SMT Dual Contact 24Ckt | datasheet |
| R1_USB, R2_USB | C3920633 | 0201 | 7.68k 0201 Thin Film Surface Mount Fixed Resistor +/-0.5% 0.031W CPF0201D7K68C1 | datasheet |
| L5 | C1329646 | SMD,4.8x4.8mm | 1.6A 1.6A 4.7uH 41.4mΩ AEC-Q200 ±30% SMD,4.8x4.8mm Power Inductors ROHS | datasheet |
| C9 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| IC3 | C189517 | LGA-12(2x2) | Accelerometers Triaxial low-g 12bit Acceleration Sensor | datasheet |
| IC1 | C3682423 | DSBGA-8(1.1x1.6) | Charger IC Lithium Ion/Polymer, Lithium Iron Phosphate 8-DSBGA (1.6x1.1) | datasheet |
| IC2 | C81079 | DSBGA-9 | Haptic Driver for ERM/LRA with Built-In Library and Smart Loop Architecture | datasheet |
| L7 | C5832368 | 1008 | 13mΩ 470nH 6.5A 7.5A ±20% 1008 Power Inductors ROHS | datasheet |
| TP (Test Pads) | N/A | N/A | Test pad | N/A |
| J4 | C709357 | SMD | -40℃~+85℃ 1 10,000 cycles 16P 30V 3A 7.81mm Female Surface Mount, Right Angle Type-C SMD USB Connectors ROHS | datasheet |
| U2 | C2682616 | DFN-8-EP(2x2) | -40℃~+85℃ 1 2.5V~4.5V 3uA I2C Lithium Battery DFN-8-EP(2x2) Battery Management ROHS | datasheet |
| D2, D4, D5 | C82046 | SOD-123 | ON SEMICONDUCTOR - MBR0530 - DIODE, SCHOTTKY, 0.5A, 30V, SOD-123 | datasheet |
| C10, C13, C22 | C9900156064 | 201 | Generic chip capacitor | datasheet |
| U1 | C3606653 | QFN-48(6x6) | nRF52840 | datasheet |
| IC9 | C7065276 | WLCSP-15B(2.3x1.4) | Buck-Boost Regulator Positive Output Step-Up/Step-Down I2C DC-DC Controller IC 15-WL-CSP (BSC) (1.4x2.3) | datasheet |
| Q3 | C469327 | SOT-323 | MOSFET N-Ch 30V 1.5A TrenchFET SC70 Vishay Si1308EDL-T1-GE3 N-channel MOSFET Transistor, 1.5 A, 30 V, 3-Pin SC-70 | datasheet |
| J2 | C90533 | P=1mm | CABLE ADAPTER 6 POS | datasheet |
| D3 | C2969755 | SOT-23-6L | Low Cap. ESD Protection Auto SOT-23-6 STMicroelectronics USBLC6-2SC6Y, Dual Uni-Directional TVS Diode Array, 6-Pin SOT-23 | datasheet |
The InkTime platform is an ultra-low-power wearable built around the Nordic nRF52840 SoC. The design focuses on maximizing battery autonomy through an E-Ink interface and advanced power management, ensuring the device remains functional for weeks on a single charge.
The nRF52840 (ARM Cortex-M4F @ 64 MHz) serves as the system's brain. It was selected for its:
- Connectivity: Native Bluetooth 5.4 LE support for smartphone syncing.
- I/O Versatility: Integrated USB, SPI, and I2C peripherals to manage all sub-systems.
- Efficiency: Advanced "Deep Sleep" modes (sub-10 µA) and a dedicated 32.768 kHz crystal for accurate low-power timing.
The power tree is designed to extract maximum energy from the 250 mAh Li-Po battery:
- Charging: A BQ25180 PMIC manages the CC/CV charging cycle via USB-C, providing overvoltage and thermal protection.
- Regulation: The RT6160 Buck-Boost converter maintains a stable 3.3V rail even as the battery voltage drops toward 3.0V, outperforming standard LDOs.
- Fuel Gauge: The MAX17048G monitor provides high-precision State of Charge (SoC) data via I2C, allowing for accurate battery percentage alerts.
- E-Paper Display (1.54"): Connected via a 24-pin FPC (SPI). It draws power only during refreshes, making it ideal for static data. A dedicated driving circuit with MOSFETs and Schottky diodes manages the necessary bias voltages.
- Motion Sensing: The BMA423 IMU (I2C) enables "wrist-tilt" wake-up and step counting, allowing the MCU to stay in sleep mode until motion is detected.
- Haptic Feedback: The DRV2605 driver handles the vibration motor (shaker), offloading PWM waveform generation from the MCU for crisp, customizable alerts.
- User Input: Three physical buttons (
SW_UP,SW_ENT,SW_DN) provide a reliable navigation interface.
With an average current draw (
In real-world scenarios, this translates to an estimated 10–14 days of continuous operation.
| Pin(s) | Peripheral | Function | Justification |
|---|---|---|---|
| P0.02 | SPI SCK | Clock | SPI clock for the E-Ink display interface. |
| P0.03 | SPI MOSI | Data | Master Out Slave In; sends pixel data to display. |
| P0.05 | EPD CS | Chip Select | Active-low signal to enable SPI display communication. |
| P0.15 | EPD DC | Control | Toggles between Data and Command modes for E-Ink. |
| P0.16 | EPD RST | Control | Provides a hardware reset path for the display. |
| P0.17 | EPD BUSY | Status Input | Hardware flag indicating active display refresh cycles. |
| P0.06 | I2C SDA | Data | Serial Data line shared by IMU, Fuel Gauge, and Haptics. |
| P0.07 | I2C SCL | Clock | Serial Clock line for the high-speed I2C bus. |
| P0.08 | IMU INT1 | Interrupt | Primary motion/event trigger from BMA421/BMA423. |
| P1.08 | IMU INT2 | Interrupt | Secondary interrupt line for advanced motion features. |
| P0.09 | Fuel Gauge ALERT | Interrupt | Real-time battery status alert from MAX17048. |
| P0.11 | PMIC INT | Interrupt | Fault or charging status update from BQ25180. |
| P0.12 | HAPTIC_EN | Control | Master enable signal for the DRV2605 haptic driver. |
| P0.13 | Button UP | GPIO Input | Digital input for the 'Up' navigation switch. |
| P0.14 | Button ENTER | GPIO Input | Digital input for the 'Enter/Select' switch. |
| P1.02 | Button DOWN | GPIO Input | Digital input for the 'Down' navigation switch. |
| P0.18 | nRESET | Reset | System hardware reset (Reset pin). |
| D+ / D- | USB Interface | USB Data | Dedicated lines for USB communication and charging. |
| SWDIO | Debug Port | Data | Serial Wire Debug bidirectional data line. |
| SWDCLK | Debug Port | Clock | Serial Wire Debug clock reference. |
- Density: Used 0201 passives and 1.0 mm PCB thickness to fit all 49 components on the TOP layer within the compact enclosure.
- RF Performance: Maintained a strict copper-free keep-out zone under the 2.4 GHz antenna and used via stitching to stabilize the ground planes.
- Routing: Power lines are 0.3 mm for stability, while data lines are 0.15 mm. Decoupling caps are placed within 0.5 mm of IC pins to minimize noise.
The following warnings were manually verified and intentionally accepted:
| Error Type | Reason for Approval |
|---|---|
| DRC: Dimensions | Button and USB-C placement is fixed by the mechanical cutouts of the 3D-printed case. |
| DRC: Overlap | Caused by via-in-pad / escape routing (0.15mm vias). Necessary for the dense fanout of the nRF52840. |
| ERC: Input Pins Only | Verified manually; all nets are functionally connected despite the CAD warning. |
| ERC: Power Pins | Confirmed that all supply rails (3V3, VBAT) are correctly routed to their regulators. |