Decabox pcb

DEV CI/BOX — Smart Storage & Monitoring Case for Cochlear Implant Batteries

A compact, intelligent battery management system for 6× Decapo batteries used with cochlear speech processors. This project monitors cell voltage, ambient light, and battery state-of-charge in real-time, with visual feedback via WS2812B RGB LEDs and a 128×64 OLED display.

Competition: element14 Project14 / Spring Clean 2026
Status: Complete firmware + hardware design
MCU: STM32U031C8T6 (56 MHz Cortex-M0+, 12KB SRAM)

Features

• 6-Cell Battery Monitoring: Per-cell voltage readout via 3× ADS1115 16-bit ADCs
• Smart LED Feedback: WS2812B RGB LEDs color-coded by battery state (green/yellow/orange/red)
• Ambient Adaptive Brightness: OPT3001 ambient light sensor scales LED brightness 15–50%
• Battery SOC Display: SSD1306 128×64 OLED shows voltage grid + state-of-charge %
• USB Power Detection: BQ24074 charger with PGOOD + CHG status pins
• Low-Power Sleep: Stop2 mode with Hall switch wake for < 15 µA idle current
• Fuel Gauge: MAX17048 LiPo fuel gauge for accurate SOC% estimate

Hardware

PCB Design

Three boards included:

1. Main PCB — STM32U031C8T6, BQ24074, MAX17048, OPT3001, SSD1306, AO3401 MOSFET, LP5907 LDO, W25Q128JVS SPI flash
2. Pin PCB ×2 — ADS1115, 3× pogo connectors for Decapo cell slots, WS2812B-4020 LEDs
3. KiCad source files — Full schematics + layouts in hardware/

MCU Pin Assignment (STM32U031C8T6, LQFP-48)

I²C Device Map

Firmware

State Machine

SLEEP (Stop2)  →  HALL_SW rises or PGOOD falls  →  WAKE
WAKE           →  Read sensors → Update LEDs + OLED → SLEEP

Sleep target: < 15 µA (Stop2 mode, OLED powered off via PA11, all timers disabled).

Architecture

firmware/
  Core/
    Inc/
      app/
        battery_map.h      — Cell voltage (mV) → LED color mapping
        display.h          — OLED framebuffer + layout
      drivers/
        max17048.h         — Fuel gauge (I²C register map)
        ads1115.h          — ADC reads, mux config
        opt3001.h          — Lux reading, auto-range
        ssd1306.h          — OLED I²C framebuffer
        ws2812b.h          — PWM/DMA bit encoding
      main.h, stm32u0xx_hal_conf.h, stm32u0xx_it.h
    Src/
      app/
        battery_map.c      — Color thresholds
        display.c          — OLED layout rendering
      drivers/
        max17048.c, ads1115.c, opt3001.c, ssd1306.c, ws2812b.c
      main.c               — FSM, init, ISRs
      stm32u0xx_hal_msp.c  — HAL clock/GPIO setup
      stm32u0xx_it.c       — Interrupt handlers
      syscalls.c, sysmem.c, system_stm32u0xx.c
  Makefile, Drivers/, startup_stm32u031xx.s, STM32U031xx_FLASH.ld

Build & Flash

From STM32CubeIDE (GUI):

• Open firmware/ as an existing project
• Build via Project → Build Project
• Flash via Run → Debug or Flash via STM32 Programmer

From CLI:

cd firmware
make -C Debug/ all
# Flash via ST-Link
st-flash write Debug/decabox.bin 0x08000000
# or via STM32_Programmer_CLI
STM32_Programmer_CLI -c port=SWD -d Debug/decabox.elf -v -rst

Debug UART (115200 baud):

minicom -D /dev/ttyACM0 -b 115200      # Linux/macOS
# or use PuTTY/Tera Term on COM port (Windows)

LED Color Mapping

Brightness: OPT3001 lux linearly mapped to 15–50% over 1–10,000 lux range.

Hardware Details

Key Components

• STM32U031C8T6: Ultra-low-power Cortex-M0+, 64KB flash, 12KB SRAM, 56 MHz max
• BQ24074: Single-cell LiPo charger, VQFN-16 (requires stencil + reflow)
• MAX17048: I²C fuel gauge for accurate battery percentage
• OPT3001: Digital light sensor with auto-range, I²C interface
• ADS1115: 16-bit I²C ADC (×3 for 6 differential channels)
• SSD1306: 128×64 OLED display, I²C interface
• WS2812B-4020: Smart RGB LEDs, 5V data line, DMA-driven
• LP5907: 3.3V LDO, always-on supply
• AO3401: P-channel MOSFET for OLED power switching
• W25Q128JVS: 128 Mbit SPI NOR flash (reserved for future use)

WS2812B Protocol

800 kHz PWM/DMA on TIM2_CH2 (PA1):

• T0H = 0.4 µs, T1H = 0.8 µs
• Bit-expanded buffer transferred via DMA at 56 MHz
• Reset sequence = 50 µs+ low

ADS1115 Configuration

Each ADS1115 reads 2 differential channels with configurable gain. Resistor divider on pogo contacts scales 0–6V to 0–2V ADC range. Calibration divider ratio defined in firmware/Core/Inc/drivers/ads1115.h.

Power Budget

• Active (sensors, OLED, LEDs): ~30–50 mA
• Stop2 idle: ~16 µA (1.5 µA MCU + 75 µA LP5907 quiescent)
• OLED powered off: ~1 µA additional savings via PA11

Critical PCB Layout

• BQ24074 VQFN-16 thermal pad → exposed copper + vias to bottom layer
• I²C pull-ups: 4.7 kΩ on SDA+SCL (main board only)
• 100 nF + 10 µF decoupling at every IC VCC
• WS2812B: 33 Ω series resistor at MCU pin, short trace
• W25Q128JVS: SOIC-8, 100 nF at VCC, matched SPI traces, 10 kΩ pull-up on CS

Bringing Up New Hardware

1. 3.3V rail: Check LP5907 output with DMM
2. Clocks: Verify main oscillator running (probe PA1 with scope after init)
3. I²C scan: Run i2c_scan() to detect all devices (should see 0x36, 0x3C, 0x44, 0x48–0x4A)
4. MAX17048: Read VCELL and SOC registers
5. ADS1115: Read differential voltages from each Decapo slot
6. OPT3001: Read lux value under ambient light
7. OLED: Initialize framebuffer, draw test pattern
8. WS2812B: Pulse RGB pattern, verify no glitches
9. USB/Hall: Simulate PGOOD and HALL_SW edges, verify wake

Testing

Firmware Tests

• RGB Color Cycle: Every 2 seconds (R→G→B), verifies WS2812B DMA
• Battery Map: Simulated cell voltages 100–2000 mV, verify color transition thresholds
• Sleep Mode: Enter Stop2, measure current with µA meter; wake on Hall/USB edge
• Display Update: Read all sensors, render OLED grid every 5 seconds

PCB Bring-Up

• Short detection: Continuity check all power/ground rails before power-on
• Thermal camera: Monitor BQ24074 during first charge cycle
• Oscilloscope: Probe WS2812B data line for 800 kHz pulse train
• UART debug: Monitor init log for device discovery and sensor reads

Datasheets

All component datasheets included in datasheets/:

• STM32U031C8T6 reference manual
• Per-IC datasheets (BQ24074, MAX17048, OPT3001, ADS1115, SSD1306, WS2812B, etc.)

Project Structure

decabox/
  README.md                    ← This file
  firmware/                    — STM32CubeIDE project
    Core/                      — Application code + drivers
    Drivers/                   — STM32 HAL (auto-generated)
    Makefile, *.ld, *.s        — Build files
  hardware/
    main/                      — Main PCB (KiCad)
    pin-pcb/                   — Pin adapter board ×2 (KiCad)
  datasheets/                  — Component datasheets (PDF)

Future Extensions

• SPI Flash (W25Q128JVS): Data logging (voltage history, charge cycles)
• Calibration: Store ADC offset/gain in EEPROM or flash
• OTA Firmware: Staging and validation in SPI flash
• Wireless: Replace UART with Bluetooth LE for mobile app integration

License

Design and firmware are open source. Refer to the LICENSE file in the repository.

Contact & Attribution

Project: DEV CI/BOX — Cochlear Implant Battery Monitor
Platform: element14 Project14
MCU: STM32U031C8T6
Build: STM32CubeIDE + GCC ARM

For questions, issues, or hardware discussions, see the GitHub repository.