> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zerokeyusb.com/llms.txt
> Use this file to discover all available pages before exploring further.

# USB Interface

> How ZeroKeyUSB powers up, enumerates, and secures communications over USB-C.

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.

***

## Connector wiring

| Pin    | Function | Notes                                        |
| ------ | -------- | -------------------------------------------- |
| A1/B12 | GND      | Tied together for symmetrical cables         |
| A4/B9  | VBUS     | 5 V input (up to 500 mA)                     |
| A5     | CC1      | 5.1 kΩ pull-down (Rd) advertises device mode |
| B5     | CC2      | 5.1 kΩ pull-down (Rd) for flipping cables    |
| A6/B6  | D+       | Routed to SAMD21 USB pins                    |
| A7/B7  | D−       | Routed to SAMD21 USB pins                    |

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.

***

## 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.

***

## USB descriptors

| Interface   | Class               | Purpose                                                |
| ----------- | ------------------- | ------------------------------------------------------ |
| Interface 0 | HID Keyboard (0x03) | Auto-types usernames and passwords                     |
| Interface 1 | CDC ACM (0x02)      | Serial console for backups, diagnostics, and time sync |

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

***

## 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.

***

## Troubleshooting enumeration

| Symptom                        | Cause                                   | Solution                                                       |
| ------------------------------ | --------------------------------------- | -------------------------------------------------------------- |
| Device powers but not detected | CC resistors missing or incorrect       | Verify 5.1 kΩ pull-downs on CC1/CC2.                           |
| Enumerates as “Unknown Device” | Firmware not running or bootloader mode | Reflash via UF2 or check for double-tap reset.                 |
| Serial port not appearing      | CDC driver blocked                      | On Windows install the provided `.inf` from the firmware repo. |

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.