Open-source smartwatch hardware project based on the Nordic nRF52840 MCU, featuring a 1.54" e-paper display and optimized for ultra-low power consumption.
[ USB Type-C ] [ LiPo Battery (250mAh) ]
| 5V VBUS |
v | BAT
+---------------+ v
| BQ25180 |<--+ [MAX17048]
| (Charger IC) | | (Fuel Gauge)
+---------------+ | |
| VSYS +-----+ I2C
v v
+---------------+ +-------------+
| RT6160 | | |
| (Buck-Boost) | | nRF52840 |
+---------------+ | MCU |
| 3.3V (VDD_3V3)| (Control |
| System Rail | Hub) |
| | |
+---------+---------------+------+------+
| | | |
| v | |
| +-------------+ | | SPI
| | PFET Switch |<--GPIO-+ v
| +------+------+ | +-------------+
| | VEPD | | 1.54" E-paper|
| v | | Display |
| +-------------+ | +-------------+
| | [Display] | |
| +-------------+ | I2C Bus
| |
+---I2C==> [ BMA421 Accel ]
| |
+---I2C==> [ DRV2605L Driver ] --> [ Vibration Motor ]
|
| [ PCB Antenna ] <--> [ BLE RF ]
|
+---GPIO-> [ User Buttons ]
| Designator | Component | Package | Manufacturer | Description / Notes |
|---|---|---|---|---|
| U1 | nRF52840 | aQFN-73 | Nordic Semi | SoC Bluetooth 5.0 (Procesor principal) |
| IC1 | BQ25180Y | DSBGA-8 | Texas Inst. | Charger IC Li-Ion (Management baterie) |
| IC2 | DRV2605Y | DSBGA-9 | Texas Inst. | Haptic Driver (Control motor vibrații) |
| IC9 | RT6160AWSC | WLCSP-15 | RICHTEK | Buck-Boost Regulator 3.3V |
| IC3 | BMA423 | LGA-12 | BOSCH | Accelerometru (Detectie miscare) |
| U3 / IC6 | MAX17048 | TDFN-8 | Maxim/ADI | Monitorizare nivel baterie (Fuel Gauge) |
| ANT1 | 2450AT18B100E | 1.4mm | JOHANSON | Antena Chip 2.45GHz |
| J1 | 503480-2400 | FPC-24 | Molex | Conector ecran (0.5mm pitch) |
| J4 | KH-TYPE-C-16P | SMT | Kinghelm | Port USB Type-C |
| Q3 | SI1308EDL | SOT-323 | Vishay | MOSFET N-Ch (Comutare putere) |
| D3 | USBLC6-2 | SOT-23-6 | STMicro | Protectie ESD linii date USB |
| X1, X2 | Crystal | 2016 / 2012 | Generic | Cuart 32MHz (Sistem) / 32.768kHz (RTC) |
| L2 | 10µH | 0402 | Generic | Inductor critic pentru circuitul de boost e-Paper |
| R1_EP_DR | 0.47 Ω | 0201 | Generic | Rezistenta de sens (RESE) pentru ecran |
| R5, R7, R8 | 10k Ω | 0201 | Generic | Rezistente Pull-up (I2C / Linii control) |
| C1-EP-DR, C39 | 10µF | 0201 | Generic | Condensatori filtrare alimentare |
| C1, C2, C17, C18 | 12pF | 0201 | Generic | Condensatori pentru cuart (X1, X2) |
| C2-EP-DR | 4.7µF / 25V | 0201 | Generic | Stocare energie tensiuni mari ecran (VGH/VGL) |
To ensure maximum runtime, the system implements a dual-stage power topology. The BQ25180 manages the LiPo charging cycle and provides a system voltage (VSYS), which is then regulated to a stable 3.3V by the RT6160 Buck-Boost. The use of a Buck-Boost instead of a standard LDO allows the system to remain operational even as the battery voltage drops significantly below 3.3V.
The E-paper display is inherently energy-efficient as it only draws power during image refreshes. However, to eliminate leakage current during sleep cycles, a PFET high-side switch (SI2301) is used. The MCU physically disconnects the display's power rail (VEPD) between updates via a dedicated GPIO signal.
Unlike simple vibration circuits, the DRV2605L driver provides sophisticated haptic patterns. It controls an ERM motor via I2C commands. Motion tracking is handled by the BMA421, which monitors steps and orientation in the background, waking the nRF52840 only when necessary via hardware interrupts.
Used by the IMU (BMA421), Fuel Gauge (MAX17048), Li-Po Charger (BQ25180), Haptic Driver (DRV2605), and DC/DC Converter (RT6160).
- P0.16 — SDA (Serial Data)
- P0.14 — SCL (Serial Clock)
Connected via the 24-pin FPC connector (J1).
- P0.17 — EPD_SCK (Serial Clock)
- P0.22 — EPD_MOSI (Serial Data)
- P0.20 — EPD_CS (Chip Select)
- P0.13 — EPD_DC (Data/Command Selection)
- P0.24 — EPD_RST (Hardware Reset)
- P0.11 — EPD_BUSY (Panel Status Input)
- P0.15 — CHG_INT (Charger state/interrupt)
- P0.25 — IMU_INT1 (Primary motion interrupt)
- P0.26 — IMU_INT2 (Secondary motion interrupt)
- P0.06 — HAPTIC_INT (Haptic driver control/interrupt)
- P0.08 — FUEL_ALERT (Battery fuel gauge alert)
- P1.11 — BTN_UP (Up Navigation)
- P1.13 — BTN_ENT (Enter / Select)
- P1.15 — BTN_DN (Down Navigation)
- XC1/XC2 & XL1/XL2 — External 32MHz and 32.768kHz crystals for system and RTC stability.
- SWDIO, SWDCLK, SWO, nRESET — Routed to the Tag-Connect TC2030 debug interface for firmware flashing and real-time debugging.
- nRF52840 (aQFN-73) selected as main MCU, providing Bluetooth 5.0, native USB and sufficient GPIOs for all peripherals.
- Dual-stage power topology: BQ25180YBGR handles LiPo charging, RT6160AWSC Buck-Boost regulates the 3.3V system rail — allowing operation even as battery drops below 3.3V.
- MAX17048 fuel gauge connected on shared I2C for accurate battery level estimation.
- BMA421 triaxial accelerometer on I2C with two hardware interrupt lines to the MCU.
- 2450AT18B100E chip antenna with discrete matching network for 2.4GHz BLE.
- Dedicated e-paper drive circuit (MBR0530, SI1308EDL, boost inductor) generating PREVGH/PREVGL voltages for the 1.54" panel via a 24-pin Molex FPC connector.
- DRV2605YZFR haptic driver for ERM motor, controlled via I2C.
- USBLC6-2SC6Y ESD protection on USB lines, KH-TYPE-C-16P connector with 5.1kΩ CC resistors for power delivery negotiation.
- Three tactile buttons (UP, DOWN, ENTER) with 10kΩ pull-ups.
- TC2030-IDC SWD connector with dedicated test points for programming and debug.
- Smartwatch form factor sized for wrist-wear with a dedicated enclosure.
- Antenna and matching network placed at PCB edge with no copper pour underneath, per Nordic's nRF52840 layout guidelines.
- 100nF decoupling capacitors placed close to each nRF52840 power pin (DEC1–DEC6).
- Manufacturing files (Gerber, BOM, CPL) available in
/Manufacturing.
- Enclosure designed to house the PCB, 250mAh LiPo and 1.54" e-paper display.
- 3D assembly model available in
/Imagesfor fit and clearance verification. - USB Type-C and side buttons (UP, DOWN, ENTER) accessible without disassembly.