A fully custom USB HID keyboard dongle built on ATtiny85 — bare metal C firmware, custom PCB designed in KiCad 10, no Arduino, no HAL, no bootloader abstractions. Plug it into any USB port and it enumerates as a real HID keyboard device.
A fully custom USB HID keyboard dongle...
Plugs directly into a USB-A port and acts as an autonomous HID keyboard.
Planned automation payloads (see firmware/ for examples):
- One-click temp folder cleaner (
Win+R → %temp% → Ctrl+A → Shift+Del) - Stored credential injector (password manager dongle)
- Custom keyboard macro sequences
- Any HID keystroke automation — limited only by firmware logic
| Part | Value | Package |
|---|---|---|
| U1 | ATtiny85-20SU | SOIC-8 |
| R1, R2 | 68Ω | 1206 |
| R3 | 1.5kΩ (pull-up) | 1206 |
| R4 | 1.5kΩ (LED) | 1206 |
| C1 | 100nF | 0805 |
| D1, D2 | 3.6V Zener | SOD-123 |
| D3 | LED | 0402 |
| J1 | USB-FX | Custom PCB-edge footprint |
PCB: 31.31 × 11.8 × 0.8mm — slides directly into USB-A female socket. Custom USB-FX footprint — PCB edge pads act as USB-A plug contacts. Designed in KiCad 10. Ground pour on F.Cu.
Schematic: docs/schematic.png
PCB Layout: docs/pcb.png
3D Render: docs/3dpcb_front.png / docs/3dpcb_back.png
Gerbers + BOM: hardware/fabrication/
Pure bare-metal C. No Arduino core, no HAL, no bootloader. Uses the V-USB library (obdev, v20121206) for software USB bit-banging on PB3/PB4.
Toolchain: avr-gcc (WinAVR), avrdude, VS Code
cd firmware
make
make fuses
make flashOr use the included automation script:
69s.bat -unlock -17Aavrdude -c usbasp -p attiny85 -U lfuse:w:0xE1:m -U hfuse:w:0xDD:mlfuse 0xE1— internal 16.5MHz PLL oscillator (required for V-USB timing)hfuse 0xDD— RESET enabled, SPI programming on
| ATtiny85 Pin | Function |
|---|---|
| PB3 (pin 2) | USB D− |
| PB4 (pin 3) | USB D+ |
| PB2 (pin 7) | Status LED |
| PB0 (pin 5) | ICSP MOSI / TP2 |
| PB1 (pin 6) | ICSP MISO / TP3 |
| PB2 (pin 7) | ICSP SCK / TP4 |
| PB5 (pin 1) | RESET / TP5 |
attiny85-usb-hid/
├── docs/ # Schematic, PCB renders, design notes
│ ├── schematic.png
│ ├── pcb.png
│ ├── 3dpcb_front.png
│ ├── 3dpcb_back.png
│ └── design-notes.md
├── firmware/ # Bare metal C firmware
│ ├── main.c # Main application
│ ├── usbconfig.h # V-USB hardware configuration
│ ├── Makefile
│ ├── 69s.bat # One-shot build+flash automation
│ └── usbdrv/ # V-USB library (obdev v20121206)
├── hardware/
│ ├── fabrication/ # Gerbers, BOM, positions for JLCPCB
│ │ ├── t85-gerber.zip
│ │ ├── bom.csv
│ │ └── positions.csv
│ ├── hardware_prototype/ # Prototype build photos, all versions
│ │ ├── spider_config.jpeg # Initial 6-wire ICSP test setup
│ │ ├── vers_1_front.jpeg # Version 1 prototype
│ │ ├── vers_1_back.jpeg
│ │ ├── final_cleanup_version_front.jpg # Final build (PB4 hot-wired)
│ │ └── final_cleanup_version_back.jpg
│ ├── t85-usb.kicad_sch
│ ├── t85-usb.kicad_pcb
│ └── t85-usb.pdf
└── README.md
Why ATtiny85? 8-pin SOIC, 8KB flash, runs at 16.5MHz via internal PLL — no external crystal needed, freeing PB3/PB4 for USB D+/D−.
Why bare metal C? Full control over every cycle. No Arduino abstraction layer, no bootloader overhead. USB enumeration, HID descriptors, OSCCAL calibration — all implemented and understood at the register level.
Why V-USB? ATtiny85 has no hardware USB peripheral. V-USB bit-bangs USB low-speed (1.5 Mbit/s) in software using hand-tuned AVR assembly timed to the exact cycle. This is the same approach used by Digispark.
USB signal conditioning: ATtiny85 GPIO swings 0–5V but USB data lines must not exceed 3.3V. D1/D2 (3.6V zeners) clamp the high state. R1/R2 (68Ω) provide impedance matching for the transmission line.
OSCCAL runtime calibration: Internal RC oscillator has ±10% factory
tolerance. V-USB requires ±1% at 16.5MHz. calibrateOscillator() uses
usbMeasureFrameLength() on every power-up to tune OSCCAL to the exact
live USB SOF timing, ensuring reliable enumeration regardless of temperature
or chip-to-chip variance.
VID/PID: Uses V-USB's shared free VID/PID pool (0x16C0 / 0x27DB). Appropriate for open-source/personal projects per obdev's usage terms.
Double enumeration init: Two disconnect/reconnect cycles on startup — the first lets the host settle/timeout on an initial connection attempt, the second is the real enumeration. This mirrors the bootloader-exit → user-program-start transition pattern and significantly improves host compatibility, particularly on modern xHCI controllers.
Windows driver binding: Chose PID 0x27DB (matching Digispark's HID
keyboard PID) to ensure Windows binds the device to hidusb.sys (the
native HID class driver) rather than libusb0.sys. This is critical —
a generic libusb driver does not poll the interrupt-IN endpoint at the OS
level, so keystrokes never reach the active application regardless of
how correctly the USB descriptors enumerate.
Tested and working:
| Host | Result |
|---|---|
| Windows 11 (ASUS TUF F15, xHCI) | ✅ Full enumeration, keystrokes delivered |
| Android (Redmi Note 10 Pro, OTG) | ✅ Working |
| Android (Redmi 6, OTG) |
Built and tested on perfboard before PCB fabrication.
hardware/prototype/spider_config.jpeg shows the initial 6-wire ICSP
test harness soldered directly to the chip legs before any perfboard work.
final_cleanup_version_* shows the final prototype — PB4 (USB D+) pin
was damaged during rework and required a fine wire hot-fix, which held
reliably through all testing.
This project involved deep debugging across hardware, firmware, toolchain, and host-compatibility layers. Notable findings:
- OSCCAL drift caused intermittent enumeration — solved with runtime
calibration using
usbMeasureFrameLength() - Windows driver binding (libusb0 vs hidusb) was the root cause of keystrokes never arriving despite correct USB enumeration — identified by comparing working Digispark sketch logs against our own via UsbTreeView
- xHCI interrupt-IN scheduling behaves differently from legacy UHCI/OHCI for low-speed software-USB devices — characterized and worked around via double enumeration cycle and correct PID selection
Full debugging notes in docs/design-notes.md.
- Example firmware payloads (temp cleaner, credential injector, macro runner)
- PCB fabrication and SMD assembly at JLCPCB
- ICSP test pad verification on real PCB
- v2.0 PCB with ICSP pads on back copper for in-circuit reprogramming
Narendra Sagolsem — Electronics Engineer Portfolio: s69series-3-0.vercel.app GitHub: 69series
Firmware: MIT
V-USB library: GPL v2 — see firmware/usbdrv/License.txt
Hardware: CERN-OHL-S v2