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 |
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.
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 |
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.
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. |