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Hardware Reference

Detailed hardware information for the e-paper watch project.

Bill of Materials

Component Specification Notes
MCU Seeed Xiao nRF52840 Sense Nordic nRF52840, 64MHz, 1MB Flash, 256KB RAM, BLE 5.0
Display GoodDisplay GDEH0154D67 1.54", 200×200, SPI, partial refresh capable
RTC DS3231 Module I2C, ±3.5ppm, integrated TCXO, backup battery
Battery LiPo 3.7V Recommend 300-500mAh, JST connector
Button Tactile switch (optional) For future features

Pin Connections

E-Paper Display (SPI)

MCU Pin     | Function      | EPD Pin  | Description
------------|---------------|----------|---------------------------
D10 (P1.11) | SPI MOSI      | DIN      | Data from MCU to display
D8  (P1.13) | SPI SCK       | CLK      | SPI clock signal
D1  (P0.02) | GPIO (CS)     | CS       | Chip select (active low)
D3  (P0.29) | GPIO (DC)     | DC       | Data/Command selection
D0  (P0.26) | GPIO (RST)    | RES      | Reset (active low)
D6  (P1.14) | GPIO (BUSY)   | BUSY     | Busy signal from display
            | Power         | VCC      | 3.3V from MCU
            | Ground        | GND      | Common ground

RTC (I2C)

MCU Pin     | Function      | RTC Pin  | Description
------------|---------------|----------|---------------------------
D4  (P0.04) | I2C SDA       | SDA      | I2C data line
D5  (P0.05) | I2C SCL       | SCL      | I2C clock line
D2  (P1.10) | GPIO (INT)    | SQW/INT  | Optional: interrupt/alarm
            | Power         | VCC      | 3.3V from MCU
            | Ground        | GND      | Common ground

Note: RTC module should have backup coin cell battery (CR2032) for timekeeping during power loss.

Optional Peripherals

MCU Pin     | Function      | Purpose
------------|---------------|--------------------------------
D7  (P1.15) | GPIO (Button) | User input (future features)

Power Distribution

┌──────────────┐
│   LiPo       │
│   Battery    │
│   3.7V       │
└──────┬───────┘
       │
       ├─────────► nRF52840 (onboard regulator → 3.3V)
       │                │
       │                ├─────────► E-Paper Display (3.3V)
       │                ├─────────► DS3231 RTC (3.3V)
       │                └─────────► Peripherals
       │
       └─────────► Charging Circuit (USB-C on Xiao)

Electrical Specifications

nRF52840

  • Operating Voltage: 3.3V (regulated from battery)
  • Active Current: ~5mA
  • Sleep Current: ~50µA (with RTC)
  • Flash: 1MB
  • RAM: 256KB

E-Paper Display

  • Operating Voltage: 3.3V
  • Active Current: ~5-8mA (during refresh)
  • Sleep Current: ~1µA (hibernate mode)
  • Refresh Time: Partial ~200ms, Full ~1-2s
  • Resolution: 200×200 pixels (1-bit)
  • Size: 1.54 inches

DS3231 RTC

  • Operating Voltage: 3.3V
  • Active Current: ~200µA
  • Backup Current: ~3µA (from coin cell)
  • Temperature Range: -40°C to +85°C
  • Accuracy: ±3.5ppm (±2 minutes per year)

Power Consumption

Mode Current Duty Cycle Average
nRF52840 Sleep 50µA 95% 47.5µA
nRF52840 Active 1mA 5% 50µA
Display Partial 5mA 0.4% 20µA
Display Full 8mA 0.04% 3.2µA
RTC Always On 200µA 100% 200µA
Total ~320µA

Real-world measurements: 200-500µA depending on configuration and temperature

Display Technology

E-Paper Characteristics

Advantages:

  • Ultra-low power (only consumes power during updates)
  • Readable in bright sunlight
  • Wide viewing angle (~180°)
  • Retains image without power
  • No backlight needed

Limitations:

  • Slow refresh rate (1-2 seconds)
  • Monochrome only (1-bit)
  • Ghosting with partial refresh
  • Temperature sensitive

Refresh Modes

Full Refresh:

  • Duration: ~1-2 seconds
  • Power: ~8mA
  • Effect: Complete redraw, eliminates ghosting
  • Use: Every 10 minutes, watchface changes

Partial Refresh:

  • Duration: ~200-500ms
  • Power: ~5mA
  • Effect: Update changed areas only
  • Use: Minute updates
  • Limitation: May accumulate ghosting

Assembly Notes

  1. Test components individually before assembly
  2. Check polarity of battery and power connections
  3. Verify voltage is 3.3V at all component inputs
  4. Secure connections - loose wires cause intermittent issues
  5. RTC backup battery - ensure CR2032 is installed correctly
  6. Display ribbon cable - handle carefully, easy to damage
  7. Button debouncing - add 100nF capacitor if implementing button

Troubleshooting

Display Issues

Display shows nothing:

  • Check 3.3V power to display
  • Verify all 6 SPI connections
  • Test BUSY pin (should pulse during updates)
  • Try full refresh mode

Display is garbled:

  • Check SPI connections
  • Verify ground is common
  • Lower SPI speed (library handles this)
  • Ensure correct display model (GDEH0154D67)

Ghosting:

  • Increase full refresh frequency
  • Lower temperature improves performance
  • Use full refresh more often

RTC Issues

RTC not detected:

  • Verify I2C connections (SDA, SCL)
  • Check I2C address (0x68)
  • Test with I2C scanner
  • Try different RTC module

Time drifts:

  • Check temperature (DS3231 has limits)
  • Verify backup battery
  • Replace RTC module if drift persists

Power Issues

Battery drains quickly:

  • Disable serial debug
  • Check for shorts
  • Measure current with multimeter
  • Verify display hibernates after updates

Device won't power on:

  • Check battery voltage (should be >3.0V)
  • Verify polarity
  • Test USB charging circuit
  • Check for shorts

Recommended Tools

  • Soldering iron (temperature controlled)
  • Multimeter (for voltage/current testing)
  • Logic analyzer (for debugging SPI/I2C)
  • USB-C cable (for programming and charging)
  • JST connector crimper (for battery connections)

Back to: Main README