Usb passthrough

USB Passthrough — HIL Test Disconnect Device

A USB 2.0 (480 Mbit/s) passthrough board that can electrically "unplug" a USB connection under software control — used for Hardware-in-the-Loop (HIL) testing. A control port drives GPIO lines to disconnect or reconnect the D+/D− data lines, the CC lines, and VBUS independently.

USB 3.x SuperSpeed lanes are intentionally not routed — this revision targets USB 2.0 High-Speed only, which covers the vast majority of HIL test cases while keeping the BOM and routing simple. USB Power Delivery up to 20 V / 3 A (60 W) and BC 1.2 charging are supported.

⚠️ USB-C Orientation Limitation: The CC1/CC2 lines are routed straight through (CC1 → CC1, CC2 → CC2) with a single switch. The board does not include orientation detection or lane flipping, so the cable must be inserted in the same orientation at both J1 and J2. A flipped cable breaks the CC connection and PD negotiation.

System Concept

                        ┌─────────────────────────────────────────────┐
                        │               USB Passthrough              │
                        │                                             │
 USB Host ──[IN J1]────►│  D+/D− ─► TS3USB221 ─► ESD ─► [J2 OUT]────►│────► DUT
 USB Host ──[IN J1]────►│  CC1/CC2 ─► TMUX1574 ─► [J2 OUT]           │
 USB Host ──[IN J1]────►│  VBUS ─► TPS259470 eFuse ─► [J2 OUT]       │
                        │                                             │
 Host PC ──[J3 USB-A]──►│  CP2102N (USB-UART) ─► GPIO ─► switches    │
                        └─────────────────────────────────────────────┘

Each switch function is controlled by its own GPIO, so the data path, CC negotiation, and VBUS can be cut and restored independently:

All control lines default to the connected state via 10 kΩ pull-ups, so the board is fully pass-through on power-up with no host software running.

Typical use cases

• Power-cycle the DUT — drop VBUS while keeping the data path alive
• Test USB enumeration / re-enumeration — reconnect the D+/D− path without power cycling
• Simulate cable unplug — cut everything at once (USB 2.0 + CC + VBUS)
• PD fault injection — break the CC lines to kill PD negotiation while VBUS remains present

The Board

KiCad project under pcb/, revision v1.1 (v1.0 boards supported in software via -b 1).

PCB specifications

Signal path

Key active components

Power

• 3.3 V comes from the CP2102N internal LDO (fed from J3's 5 V) and supplies U2, U5, and the LED drivers — total load is under 1 mA, so no external LDO is needed.
• VBUS is switched by U4 directly (2.7–23 V), with the current limit set by R3 (820 Ω ⇒ ~4.3 A, covering all PD modes up to 20 V / 3 A; swap to 680 Ω for 5 A).

LED indicators

Revisions & errata

• v1.0 — initial hardware. Errata: wrong R3 value, U4 pin-10 fix, U5 CC swing limited to 3.3 V, green LED loading USB2_SEL.
• v1.1 — fixed R3 = 1 kΩ, U4 pin-10 corrected, U5 → TMUX1574PWR. Note: CC_EN logic is inverted on v1.1 (HIGH = CC cut), which the software handles automatically.

The Code

Located under code/. A small two-part control stack: a thin Python CLI that wraps a compiled C binary speaking raw USB control transfers to the CP2102N.

code/
├── Makefile        Build targets: all / macos / linux / clean
├── usb_ctrl.c      C binary — sends the GPIO latch write via USB control transfer
├── usb_ctrl        (built artifact)
└── usb-pass.py     Python CLI — builds if needed, lists/selects a board, calls usb_ctrl

How it works

1. usb-pass.py builds usb_ctrl on first use if necessary, then forwards the board selector (--list, a port id, or an index) to it.
2. usb_ctrl opens the device by VID/PID (10C4:EA60) directly — via IOKit on macOS or libusb-1.0 on Linux — and issues a vendor control transfer (WRITE_LATCH_CMD 0x37E1) that sets the GPIO output latch.
3. GPIO.0/1/2 are driven to the appropriate state for on / off; GPIO.3 (FLT) is never driven (input only).

Usage

With a single board connected, on/off/toggle work without specifying which board:

python3 usb-pass.py on          # connect the DUT (board v1.1, default)
python3 usb-pass.py off         # electrically unplug the DUT
python3 usb-pass.py toggle      # flip the current state
python3 usb-pass.py -b 1 on     # board v1.0 (CC_EN follows on/off)
python3 usb-pass.py --help      # all options

Multiple boards (Linux)

When more than one board is connected, pick which one to control by its USB port id or list index, or use all-on/all-off/all-toggle to control every connected board at once. status always reports every connected board and can't be combined with a port/index:

python3 usb-pass.py --list       # IDX  PORT       BUS  DEVICE
python3 usb-pass.py 0 on         # select by index (row from --list)
python3 usb-pass.py 1-9.1.2 off  # select by port id (stable across replugs of other boards)
python3 usb-pass.py all-off      # turn off every connected board
python3 usb-pass.py status       # print GPIO state (CC_EN, USB_SEL, VBUS_EN, FLT) of every board

<port> (e.g. 1-9.1.2) identifies the physical hub port the board is plugged into and stays the same regardless of what else is connected or the order boards enumerate in. <index> is just the row number from --list — convenient, but it can shift if a lower-indexed board is unplugged, so prefer <port> in scripts/automation. Board selection by port/index is currently Linux-only; on macOS, usb_ctrl always targets the first CP2102N it finds.

Building manually

# macOS (Xcode CLT only — no extra dependencies)
make macos

# Linux (requires libusb-1.0)
apt install libusb-1.0-0-dev   # Debian/Ubuntu
make linux

Equivalent direct commands: clang -framework IOKit -framework CoreFoundation -o usb_ctrl usb_ctrl.c (macOS) or gcc $(pkg-config --cflags --libs libusb-1.0) -o usb_ctrl usb_ctrl.c (Linux).

Linux udev rule

For non-root access on Linux, create /etc/udev/rules.d/99-cp2102n.rules:

SUBSYSTEM=="usb", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", MODE="0666"

Board version handling

The v1.0 and v1.1 boards differ in the CC_EN polarity. usb-pass.py/usb_ctrl default to v1.1; pass -b 1 (or -b 1.0) to target a v1.0 board.

Repository layout

├── LICENSE                     MIT license
├── README.md                   This file
├── code/                       Host control software (Python + C)
├── doc/
│   └── chip-selection/         Design rationale and chip-selection notes
└── pcb/                        KiCad project (schematic, layout, production files)
    ├── production/             BOM, positions, designators, IPC netlist
    └── symbol/                 Vendor symbol/footprint imports

License

MIT — see LICENSE.