Add USB CDC bug doc and team requirements

TEAM.md

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+# SaltyLab — Ideal Team
+
+## Project
+Self-balancing two-wheeled robot using a drone flight controller (STM32F722), hoverboard hub motors, and eventually a Jetson Nano for AI/SLAM.
+
+## Current Status
+- **Hardware:** Assembled — FC, motors, ESC, IMU, battery, RC all on hand
+- **Firmware:** Balance PID + hoverboard ESC protocol written, but blocked by USB CDC bug
+- **Blocker:** USB CDC TX stops working when peripheral inits (SPI/UART/GPIO) are added alongside USB OTG FS — see `USB_CDC_BUG.md`
+
+---
+
+## Roles Needed
+
+### 1. Embedded Firmware Engineer (Lead)
+**Must-have:**
+- Deep STM32 HAL experience (F7 series specifically)
+- USB OTG FS / CDC ACM debugging (TxState, endpoint management, DMA conflicts)
+- SPI + UART + USB coexistence on STM32
+- PlatformIO or bare-metal STM32 toolchain
+- DFU bootloader implementation
+
+**Nice-to-have:**
+- Betaflight/iNav/ArduPilot codebase familiarity
+- PID control loop tuning for balance robots
+- FOC motor control (hoverboard ESC protocol)
+
+**Why:** The immediate blocker is a USB peripheral conflict. Need someone who's debugged STM32 USB issues before — this is not a software logic bug, it's a hardware peripheral interaction issue.
+
+### 2. Control Systems / Robotics Engineer
+**Must-have:**
+- PID tuning for inverted pendulum / self-balancing systems
+- Complementary filter / Kalman filter for IMU sensor fusion
+- Real-time control loop design (1kHz+)
+- Safety system design (tilt cutoff, watchdog, arming sequences)
+
+**Nice-to-have:**
+- Hoverboard hub motor experience
+- ELRS/CRSF RC protocol
+- ROS2 integration
+
+**Why:** Once USB is fixed, the balance loop needs real-world tuning. PID gains, filter coefficients, motor response curves, safety margins — all need someone with hands-on balance bot experience.
+
+### 3. Perception / SLAM Engineer (Phase 2)
+**Must-have:**
+- Jetson Nano / NVIDIA Jetson platform
+- Intel RealSense D435i depth camera
+- RPLIDAR integration
+- SLAM (ORB-SLAM3, RTAB-Map, or similar)
+- ROS2
+
+**Nice-to-have:**
+- Person tracking / following
+- Obstacle avoidance
+- Nav2 stack
+
+**Why:** Phase 2 goal is autonomous navigation. Jetson Nano with RealSense + RPLIDAR for indoor mapping and person following.
+
+---
+
+## Hardware Reference
+| Component | Details |
+|-----------|---------|
+| FC | MAMBA F722S (STM32F722RET6, MPU6000) |
+| Motors | 2x 8" pneumatic hoverboard hub motors |
+| ESC | Hoverboard ESC (EFeru FOC firmware) |
+| Battery | 36V pack |
+| RC | BetaFPV ELRS 2.4GHz TX + RX |
+| AI Brain | Jetson Nano + Noctua fan |
+| Depth | Intel RealSense D435i |
+| LIDAR | RPLIDAR A1M8 |
+| Spare IMUs | BNO055, MPU6050 |
+
+## Repo
+- Gitea: https://gitea.vayrette.com/seb/saltylab-firmware
+- Design doc: `projects/saltybot/SALTYLAB.md`
+- Bug doc: `USB_CDC_BUG.md`

USB_CDC_BUG.md

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+# USB CDC TX Bug — 2026-02-28
+
+## Problem
+Balance firmware produces no USB CDC output. Minimal "hello" test firmware works fine.
+
+## What Works
+- **Test firmware** (just sends `{"hello":N}` at 10Hz after 3s delay): **DATA FLOWS**
+- USB enumeration works in both cases (port appears as `/dev/cu.usbmodemSALTY0011`)
+- DFU reboot via RTC backup register works (Betaflight-proven pattern)
+
+## What Doesn't Work
+- **Balance firmware**: port opens, no data ever arrives
+- Tried: removing init transmit, 3s boot delay, TxState recovery, DTR detection, streaming flags
+- None of it helps
+
+## Key Difference Between Working & Broken
+- **Working test**: main.c only includes USB CDC headers, HAL, string, stdio
+- **Balance firmware**: includes icm42688.h, bmp280.h, balance.h, hoverboard.h, config.h, status.h
+- Balance firmware inits SPI1 (IMU), USART2 (hoverboard), GPIO (LEDs, buzzer)
+- Likely culprit: **peripheral init (SPI/UART/GPIO) is interfering with USB OTG FS**
+
+## Suspected Root Cause
+One of the additional peripheral inits (SPI1 for IMU, USART2 for hoverboard ESC, or GPIO for status LEDs) is likely conflicting with the USB OTG FS peripheral — either a clock conflict, GPIO pin conflict, or interrupt priority issue.
+
+## Hardware
+- MAMBA F722S FC (STM32F722RET6)
+- Betaflight target: DIAT-MAMBAF722_2022B
+- IMU: MPU6000 on SPI1 (PA4/PA5/PA6/PA7)
+- USB: OTG FS (PA11/PA12)
+- Hoverboard ESC: USART2 (PA2/PA3)
+- LEDs: PC14, PC15
+- Buzzer: PB2
+
+## Files
+- PlatformIO project: `~/Projects/saltylab-firmware/` on mbpm4 (192.168.87.40)
+- Working test: was in src/main.c (replaced with balance code)
+- Balance main.c backup: src/main.c.bak
+- CDC implementation: lib/USB_CDC/src/usbd_cdc_if.c
+
+## To Debug
+1. Add peripherals one at a time to the test firmware to find which one breaks CDC TX
+2. Check for GPIO pin conflicts with USB OTG FS (PA11/PA12)
+3. Check interrupt priorities — USB OTG FS IRQ might be getting starved
+4. Check if DCache (disabled via SCB_DisableDCache) is needed for USB DMA