This project is an externally powered USB 2.0 high-speed switchbox. It connects one downstream self-powered USB hub to exactly one of two USB-C host computers at a time. The primary reliability goal is stable switching for USB audio devices, especially avoiding bad re-enumeration behavior where a DAC falls back to an incorrect low-speed mode and produces distorted audio.
Intended Use
The device sits between two computers and a self-powered external USB hub. The hub carries console/peripheral devices including a keyboard, mouse, DAC, microphone, and webcam. The user presses a button on the switchbox to move the hub connection from one computer to the other.
This is a prototype/validation design, not yet production-approved.
What the Device Should Do
Switch one USB 2.0 high-speed downstream hub connection between two upstream host computers.
Support USB 2.0 high-speed operation at 480 Mb/s.
Use USB-C connectors for host and hub connections.
Use an external power supply so control logic and hub-side attach signaling remain stable during switching.
Perform a deliberate break-before-make switching sequence.
Avoid tying the two host computers’ VBUS rails together.
Keep the self-powered hub logically attached in a clean, repeatable way.
Main Features
Two USB-C host-side upstream connectors: Host A and Host B.
One USB-C hub-side downstream connector.
External 5 V power input for switchbox logic and hub-side VBUS-present signaling.
USB 2.0 high-speed differential data switch.
Pushbutton user interface.
Microcontroller-controlled switch sequencing.
ESD protection on exposed USB D+/D− lines.
3.3 V logic rail for controller and USB switch IC.
System Architecture
Diagram
Hardware Subsystems
USB Data Switching
A USB 2.0 high-speed analog mux switches D+ and D− between Host A and Host B. The mux is controlled by the microcontroller. The routing and PCB layout must treat D+ and D− as 90 Ω differential pairs with short traces, minimal stubs, and continuous ground reference.
USB-C Role Handling
Host A and Host B connectors act as USB-C device/UFP-facing ports toward the computers and use Rd pull-downs on CC1 and CC2.
Hub connector acts as a USB-C host/DFP-facing port toward the hub and uses Rp pull-ups on CC1 and CC2.
Host VBUS rails are sensed/available only for attach awareness if needed; they are not tied together and are not used to power the switchbox.
Hub-side VBUS-present is supplied from the switchbox external 5 V rail through a current-limited switch. The microcontroller controls this switch so firmware can deliberately cycle hub-side attach signaling during switching if testing shows the hub or DAC needs it.
Control Logic
A small microcontroller reads the pushbutton, controls the USB data switch, and controls the hub-side VBUS switch. Firmware implements:
Debounce pushbutton.
Disconnect current host path.
Wait a defined detach interval, initially 250 ms.
Optionally cycle hub-side VBUS-present for a clean hub attach event.
Connect the alternate host.
Optionally expose status LED outputs in a later revision.
Power
The switchbox is powered from an external 5 V supply. A 3.3 V LDO powers the microcontroller and USB switch. The downstream hub is self-powered; the switchbox only provides hub-side VBUS-present/current-limited 5 V if required for USB attach behavior.
Protection
Each externally exposed USB D+/D− pair should include low-capacitance ESD protection close to the USB-C connector. Power input should include basic input capacitance and optional fuse/TVS protection in later revisions.
Interfaces and Connections
Table
Interface
Connector
Role
Notes
Host A
USB-C receptacle
UFP/device-facing
Connects to computer A
Host B
USB-C receptacle
UFP/device-facing
Connects to computer B
Hub
USB-C receptacle
DFP/host-facing
Connects to self-powered downstream hub
Power Input
USB-C power or DC input
5 V input
Powers switchbox logic
Switch Button
Tactile pushbutton
User input
Initiates host toggle
Programming
Header/pads
MCU programming
Required for firmware loading
Power and Runtime Expectations
External supply: 5 V nominal.
Logic rail: 3.3 V.
Expected switchbox current: low, likely under 100 mA excluding any hub-side VBUS-present load.
No battery operation planned.
Device should remain powered continuously independent of selected host.
Power Tree and Power Budget
Table
Rail
Source
Loads
Estimated Current
5V_EXT
External 5 V input
3.3 V LDO, current-limited hub-side VBUS switch
TBD, design target ≥1 A available for fault margin
3V3
LDO from 5V_EXT
MCU, USB mux, pull-ups/status LEDs
<100 mA expected
VBUS_HUB
TPD3S014 from 5V_EXT
Hub-side VBUS-present
500 mA continuous target, current-limit fault testing required
HOST_A_VBUS
Host A
Sense/ESD only
No load intended
HOST_B_VBUS
Host B
Sense/ESD only
No load intended
Diagram
Manufacturing and Assembly Expectations
SMD prototype design.
Prefer common, currently available ICs and connectors.
USB 2.0 high-speed routing should use a 4-layer PCB for controlled impedance and solid ground reference.
Add test points for 5V_EXT, 3V3, GND, switch select, and MCU programming signals.
Firmware-Relevant Hardware Requirements
GPIO for USB mux select.
GPIO for USB mux enable/output disconnect if available.
GPIO for hub-side VBUS switch enable.
GPIO input for pushbutton with pull-up.
Programming/debug header or pads for the selected MCU.
Optional future GPIOs for Host A / Host B status LEDs.
Physical Design Expectations
Connector placement should support intuitive cabling: Host A and Host B on one side, hub and power on the other side if practical.
Keep USB data switch physically close to the USB-C connectors to minimize high-speed trace length and stubs.
ESD devices should be placed close to connector pins.
Board size is not fixed yet.
Important Design Decisions
Use an externally powered switchbox rather than bus-powered switching.
Use one self-powered downstream hub; the switchbox does not integrate the hub in Rev 1.
Switch only USB 2.0 D+/D− at 480 Mb/s, not USB 3.x SuperSpeed.
Use controlled break-before-make switching to improve DAC re-enumeration reliability.
Make hub-side VBUS-present MCU-controlled rather than always-on, so firmware can test whether cycling attach signaling improves DAC/hub recovery.
Avoid connecting Host A VBUS to Host B VBUS or to the switchbox supply.
Assumptions
The downstream hub has its own stable power supply.
The hub uses USB 2.0 high-speed devices or can fall back through USB 2.0 even if the physical connector is USB-C.
The webcam can operate acceptably over USB 2.0 through the external hub.
The first revision can use firmware for sequencing.
Exact enclosure and board size are TBD.
Change Notes
Initial draft created from user requirements.
Simplified architecture from multi-device switch to one USB 2.0 high-speed port/hub switch.
Added externally powered control logic and break-before-make switching requirement to address DAC transition instability.
Added MCU-controlled hub-side VBUS switch enable after review, improving attach sequencing control and reducing always-on source-port risk.