BOM: USB port allocation (all four USB-A taken, one shared USB 2.0 hub)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

docs/BOM.md

@@ -3,7 +3,26 @@ type: reference
 ---
 # Bill of Materials
 
-## PEAK-System PCAN-USB
+## USB port allocation (2026-10-12)
+
+All ports are taken.
+
+| Port | Device | Speed | Typical load |
+|------|--------|-------|--------------|
+| USB-A | PCAN-USB | full speed (12 Mbit/s) | ≪ 1 Mbit/s |
+| USB-A | USB camera | USB 2.0 | up to ~60 Mbit/s (1080p30 MJPEG) |
+| USB-A | SIM7600 (Waveshare HAT) | USB 2.0 | up to 150 Mbit/s down (LTE Cat 4) |
+| USB-A | IMU (WT61 on a USB-UART adapter; later NEO-M8U) | full speed | ≪ 1 Mbit/s |
+| USB-C | Tablet (Orin in device mode: NCM gadget) | USB 2.0 | ~5 MB/s paced down, ~22 MB/s up |
+
+- The four USB-A ports sit behind **one Realtek 4-port hub** (`0bda:5489` USB 2.0 side, `0bda:0489` USB 3 side). Every device above is
+  USB 2.0, so they all share **one 480 Mbit/s upstream**. Worst case ~210 Mbit/s: fits, but a second camera would not leave much room.
+- The carrier spec (SP-11324-001 Table 5-3) gives **0.5 A** for the USB-A VBUS. The modem's transmit peaks are the largest load; if the
+  LTE link drops under heavy upload, check its supply first.
+- Name devices by USB ID or serial in udev, not by port: any device can go in any port.
+- Anything more (a USB stick to offload data, a debug serial adapter) needs an external hub. Non-USB expansion is still free: Ethernet,
+  the header UART and I²C, and the M.2 Key M slots.
+
 
 | Field | Value |
 |-------|-------|