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ZeroKeyUSB connects through a USB-C receptacle but behaves as a classic USB 2.0 full-speed device. The board keeps the wiring simple so that any USB-A or USB-C host can power and communicate with the key.

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Connector wiring

No high-speed pairs or USB 3.x pins are used. Shield is connected to ground through a 1 MΩ resistor and 10 nF capacitor for ESD suppression.

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Power path

  • VBUS feeds a 3.3 V LDO regulator (TPS73533) that powers the SAMD21, OLED, touch controller, and EEPROM.
  • Total current stays under 120 mA during OLED animations, well within USB 2.0 limits.
  • Reverse current protection prevents powering the host when the device is off.
  • A PTC resettable fuse (250 mA) adds short-circuit protection.
Because ZeroKeyUSB has no battery, unplugging the cable immediately cuts power and clears volatile RAM.

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USB descriptors

Each interface has its own endpoint pair, allowing simultaneous keyboard and serial communication without re-enumeration.

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Security considerations

  • The firmware ignores vendor-specific control requests and only responds to standard USB descriptors.
  • HID reports are generated solely from user-confirmed actions; there is no host-triggered typing.
  • CDC commands require the device to be unlocked and, for destructive actions, a long-press confirmation.
  • USB suspend triggers an immediate lock after 30 seconds of inactivity.
These safeguards ensure that plugging the key into an unknown host does not expose stored secrets.

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Troubleshooting enumeration

If USB data lines are damaged, the device can still power up, but no credentials will be typed. Inspect the connector for debris or mechanical stress.