InkTime-Smartwatch

An open-source smartwatch powered by the nRF52840 with an e-paper display, designed for low power consumption and BLE connectivity.

Block Diagram

BOM (Bill Of Materials)

Qty	Component	Value / Part	Package	Notes
3	Buttons	EVP-AKE31A	SMD	SW_DN, SW_ENT, SW_UP
1	Solder Jumper	SJ1	SMD	Configuration jumper
3	Resistor	7.68kΩ	0201	R2, R3, R4
4	Capacitor	0.1µF	0201	Decoupling
1	Capacitor	0.1µF / 50V	0402	EPD
1	Resistor	0Ω / 0.47Ω	0201	EPD driver
1	Capacitor	1µF	0402	General
5	Capacitor	100nF	0402	Decoupling
1	Capacitor	100pF	0402	Filtering
6	Resistor	10kΩ	0201	Pull-ups / general
1	Capacitor	10µF	0201	Bulk
2	Capacitor	10µF	0402	Power
1	Inductor	10µH	0402	DC/DC
4	Capacitor	12pF	0402	Crystal load
1	Inductor	15nH	0402	RF matching
2	Capacitor	1pF	0402	RF
6	Capacitor	1µF	0201	Decoupling
9	Capacitor	1µF / 50V	0402	EPD
1	Resistor	2.2kΩ	0201	Config
1	PMOS	DMG2305UX	SOT-23	Power switching
2	Capacitor	22µF	0402	Power rail
1	Antenna	2450AT18B100E	1206	2.4GHz
1	Inductor	3.9nH	0402	RF matching
1	Crystal	32.768kHz	3215	RTC
1	Crystal	32MHz	2016	MCU clock
2	Resistor	3.3kΩ	0201	General
1	Capacitor	4.7µF	0402	Bulk
5	Capacitor	4.7µF	0402	Decoupling
1	Capacitor	47nF	0402	Filtering
1	FPC Connector	503480-2400	SMD	Display
2	Resistor	5.1kΩ	0201	USB-C CC
1	Inductor	68µH	SMD	Power
1	Capacitor	820pF	0402	Filtering
1	Accelerometer	BMA421	LGA	Sensor
1	Charger IC	BQ25180YBGR	BGA	LiPo charger
1	Haptic Driver	DRV2605YZFR	BGA	Motor driver
1	Inductor	0.47µH	2012	Power stage
14	Test Points	TP pads	PCB	Debug
1	USB-C	KH-TYPE-C-16P	SMD	Power
1	Fuel Gauge	MAX17048G+T10	TDFN-8	Battery monitor
3	Diode	MBR0530	SOD-123	Schottky
3	Capacitor	N.C.	0402	Not populated
1	MCU	nRF52840	QFN	Main MCU
1	Buck-Boost	RT6160AWSC	WLCSP	3.3V rail
1	NMOS	SI1308EDL	SOT-323	Switching
1	Debug Connector	TC2030-IDC	PCB	SWD
1	ESD Protection	USBLC6-2SC6Y	SOT-23-6	USB protection

Hardware Description

MCU — nRF52840

The nRF52840 is the central processing unit of the system.

Key features:

• ARM Cortex-M4F @ 64 MHz (with FPU)
• Bluetooth Low Energy (BLE 5.0)
• USB 2.0 Full Speed
• 1 MB Flash / 256 KB RAM

All peripherals are connected via:

• I2C
• SPI
• GPIO

---

Power Architecture

USB-C → BQ25180 (charger) → LiPo battery (AKY0106, 400 mAh) → 3.3V rail (VREG, for MCU and all peripherals)

Battery → RT6160AWSC (boost) → EPD high-voltage supply (VCOM, gate voltages)

BQ25180 (Charger IC)

• Charges the LiPo battery
• Provides system rail (VSYS)
• I2C communication with MCU
• Interrupt output for status/fault

RT6160 (Buck-Boost Regulator)

• Converts VSYS to stable 3.3V
• Powers:
  • MCU
  • sensors
  • display
  • haptic driver

MAX17048 (Fuel Gauge)

• Monitors battery voltage and state of charge
• I2C interface
• ALERT signal for battery events

---

E-Paper Display

Controlled via SPI and GPIO.

Interface

Signal	Direction	Description
SCK	MCU → Display	SPI clock
MOSI	MCU → Display	SPI data
EPD_CS	MCU → Display	Chip select
EPD_DC	MCU → Display	Data/Command
EPD_RST	MCU → Display	Reset
EPD_BUSY	Display → MCU	Status

Power Stage

Includes:

• inductors
• Schottky diodes
• MOSFETs

Generates:

• PREVGH
• PREVGL

---

IMU — BMA421

• Interface: I2C
• Features:
  • acceleration sensing
  • motion detection
  • step counting

Interrupts:

• INT1
• INT2

---

Haptic Driver — DRV2605

• Interface: I2C
• Enable pin: HAPTIC_EN
• Function: vibration motor control

---

USB Interface

• USB Type-C connector
• ESD protection: USBLC6-2SC6Y

Signals:

• VBUS
• D+
• D-

Usage:

• charging
• USB detection

---

RF & Antenna

• 2.4 GHz chip antenna (2450AT18B100E)

Design includes:

• matching network
• antenna keepout
• no copper under antenna

---

User Interface

3 buttons:

• Up

• Down

• Enter / Esc

• Connected to GPIO

• Active-low

---

Debug Interface

SWD via Tag-Connect:

• SWDIO
• SWDCLK
• RESET
• SWO
• 3.3V
• GND

---

Pin Mapping (nRF52840)

I2C (Shared Bus)

Pin	Function
P0.06	SDA
P0.07	SCL

Devices:

• BMA421
• MAX17048
• BQ25180
• RT6160
• DRV2605

---

SPI (E-Paper Display)

Pin	Function
P0.02	SCK
P0.03	MOSI
P0.05	CS
P0.15	DC
P0.16	RST
P0.17	BUSY

---

Control & Interrupts

Pin	Function
P1.01	Display power (PFET)
P0.08	BMA421 INT1
P1.08	BMA421 INT2
P0.10	MAX17048 ALERT
P0.11	Charger interrupt
P0.12	Haptic enable

---

Buttons

Pin	Function
P0.13	Up
P0.14	Down
P1.00	Enter / Esc

---

USB

Signal	Function
D+	Data
D-	Data
VBUS	Power detect

---

Clocks & Debug

• SWDIO / SWDCLK / RESET / SWO
• 32 MHz crystal (HFXO)
• 32.768 kHz crystal (LFXO)

---

Design Considerations

• Use of dedicated hardware interfaces (I2C, SPI)
• Logical grouping of signals
• Simplified PCB routing
• Interrupt-driven architecture
• Clean debug interface

---

Power Optimization

• MCU supports low-power modes
• E-paper consumes power only during refresh
• IMU is ultra low-power
• Haptic driver active only briefly

Efficiency strategies:

• enable peripherals only when needed
• regulated 3.3V rail
• independent control of display power