Author: BADEA Stefan-Vasile
License: GNU GPLv3
The system architecture centers on the nRF52840 SoC, which handles wireless connectivity and peripheral management. Power delivery is optimized via an RT6160 Buck-Boost converter to ensure a stable 3.3V supply.
| 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 Smartwatch project is designed as an ultra-low-power wearable platform, designed for extended battery life and a visual interface that remains readable in any lighting conditions thanks to the E-Ink display.
The core of the device is the Nordic Semiconductor nRF52840 SoC, selected for its great balance between high performance and ultra-low power consumption:
- Processor: ARM Cortex-M4F running at 64 MHz, capable of handling complex sensor signal processing algorithms.
- Connectivity: Native support for Bluetooth 5.4 Low Energy (BLE), essential for smartphone connection.
-
Efficiency: Features an advanced power management system with "Deep Sleep"
modes drawing currents in the micro-amp range (
$\mu A$ ).
- Charging: The BQ25180 chip manages the charging cycle via USB-C using the Constant Current/Constant Voltage (CC/CV) method, providing overvoltage protection and temperature monitoring.
- Buck-Boost Regulation: The watch uses the RT6160 converter. Unlike a classic LDO regulator, this Buck-Boost converter maintains a stable 3.3V output even when the battery voltage drops below this threshold (down to 3.0V), thereby extracting the maximum energy from the battery cell.
- Monitoring: The MAX17048G (Fuel Gauge) chip reports the battery's state of charge with high precision via the I2C interface.
The device uses two primary digital interfaces to communicate with its peripherals:
- I2C Bus (SDA/SCL): This shared bus connects most of the system's smart peripherals, significantly reducing PCB complexity. It includes the BMA423 triaxial accelerometer (for step counting), the MAX17048G fuel gauge (for precise battery monitoring), and the DRV2605YZFR haptic driver.
- SPI Bus (SCK/MOSI/CS): Dedicated to the E-Paper display via the 24-pin FPC connector (J1). SPI was chosen to handle the high-bandwidth image data required for screen updates while maintaining low power consumption.
- Haptic Feedback: The DRV2605YZFR driver controls the vibration motor via PWM (Pulse Width Modulation), providing silent and customizable haptic alerts to the user.
The system is powered by a 250 mAh Polymer Lithium-Ion battery (Akyga LP502030). Thanks to the E-Ink display, which only draws power during image updates, the autonomy has been estimated as follows:
Estimated Average Current (
-
Sleep Mode (MCU + BLE):
$\approx 35\text{ µA}$ -
Sensor Polling (Active):
$\approx 120\text{ µA}$ -
Display Refresh (average/minute):
$\approx 25\text{ µA}$ -
Total Average Current:
$I_{avg} \approx 180\text{ µA} = 0.18\text{ mA}$
Theoretical Autonomy (
Real-world Autonomy: Under typical operating conditions (including BLE advertising, sensor fusion, and haptic notifications), the estimated autonomy is approximately 9 - 14 days.
| Component | Signal | nRF Pin | Direction | Technical Justification |
|---|---|---|---|---|
| E-Ink Display | SPI SCK | P0.11 | Output | High-speed clock pin for the SPI bus. |
| E-Ink Display | SPI MOSI | P0.12 | Output | Transfers image data to the display. |
| E-Ink Display | SPI CS | P0.13 | Output | Active-low Chip Select control. |
| E-Ink Display | Data/Command | P0.14 | Output | Distinguishes between command and data transmissions. |
| E-Ink Display | Reset | P0.15 | Output | Hardware reset required for display initialization. |
| E-Ink Display | Busy | P0.16 | Input | Monitors internal display status (refresh in progress). |
| I2C Sensors | SDA | P0.26 | I/O | Shared data line. |
| I2C Sensors | SCL | P0.27 | Output | Synchronous clock line for the I2C bus. |
| Button (Up) | GPIO | P1.07 | Input | Digital input on Port 1, isolated from high-speed data. |
| Button (Enter) | GPIO | P1.08 | Input | Digital input with internal pull-up enabled. |
| Button (Down) | GPIO | P1.09 | Input | UI navigation. |
| Haptic Motor | PWM | P1.02 | Output | Vibration intensity control via pulse-width modulation. |
| Power Management | DC/DC EN | P0.18 | Output | Enable pin for the RT6160 (shutdown in deep-sleep). |
| IMU Sensor | INT1 | P0.25 | Input | Hardware interrupt for the "double-tap" feature. |
The watch uses the Port 0 for peripherals requiring high data transfer rates (SPI and I2C). This decision enables the use of the nRF52840’s EasyDMA features, significantly reducing CPU load during image transfers to the display or during sensor data acquisition.
Pins P0.11 - P0.16 were physically grouped on a single side of the microcontroller. This grouping facilitates a compact SPI bus layout, reducing trace length and impedance, which ensures signal integrity even at high clock frequencies.
The standard pins P0.26 and P0.27 are used for the I2C bus, as they are internally optimized for low power consumption and feature excellent hardware support within the Zephyr RTOS and Nordic SDK. All smart peripherals — the BMA423 IMU, the MAX17048G Fuel Gauge, and the DRV2605YZFR Haptic Driver—are connected in parallel on this shared bus to conserve GPIO pins and simplify PCB routing.
By placing the user buttons on Port 1 (P1.07-P1.09), we ensured a physical separation at the layout level between the high-speed switching digital signals (SPI/I2C) and the sensitive button inputs. This design practice prevents "ghosting" or accidental triggers caused by electromagnetic noise on the PCB.
We avoided using pins P0.09 and P0.10 for buttons or sensors, despite their availability. These pins are dedicated to NFC functionality and have a different internal capacitance compared to standard GPIOs. Keeping them unassigned allows for the addition of an NFC antenna in future product iterations without requiring a complete pinout redesign.
- 0201 Package Constraint: The use of 0201 passive components was necessary to allow the routing of all 49 components on a single layer (TOP), adhering to the extremely compact dimensions of the enclosure.
- Antenna Management: The chip antenna area was kept clear of ground planes on all layers (Keep-out zone) to ensure optimal Bluetooth range, in accordance with the manufacturer's specifications.
- Power Traces: Power supply rails (VCC, 3V3, VBAT) were routed with a width of 0.3mm, while data signals were set to 0.15mm, ensuring minimal voltage drop on high-current lines.
- 100nF decoupling capacitors were placed at a maximum distance of 0.5mm from the nRF52840 power pins to effectively filter high-frequency noise.
- Via Stitching was employed to tie the Top and Bottom ground planes (GND) together, thereby reducing the reference plane impedance and improving overall RF performance.
