AxxPD

AxxPD turns any USB-C PD charger into a programmable bench power supply. It negotiates the best voltage the charger offers — anywhere from 3.3 V to 48 V and up to 240 W — and delivers it through XT30 and 4 mm shrouded banana outputs, with 20-bit measurement, multi-layer hardware protection, a 1.47" color display, and SCPI automation over USB.

Built for embedded developers, ham radio operators, RC/drone hobbyists and electronics enthusiasts. The firmware runs on an STM32G491CCU6 and is open source under the GPL-3.0 license.

An assembled unit is available through Crowd Supply (campaign coming soon).

• USB-C PD 3.1 EPR sink — Fixed, PPS and AVS modes. Available voltages and power depend on the connected charger.
• 20-bit voltage/current/power monitoring (INA228, 6.8 mΩ shunt).
• Multi-layer hardware protection with sub-microsecond response — see Protection.
• 1.47" 320×172 IPS display (ST7789V) with 4-button navigation and six screens.
• USB-CDC command interface (SCPI + shortcuts) and 20 Hz CSV telemetry.
• Browser-based WebSerial dashboard — live readout, chart and CSV recording, zero install.
• Programmable presets (5 slots) and voltage sequencing, stored in flash.
• Energy tracking (INA228 Wh/Ah accumulators) and PPS-based constant-current mode.
• CNC aluminium enclosure with magnetic mount; dual XT30 + 4 mm banana outputs.

Plug a USB-C PD charger into AxxPD. After the splash screen, the boot PDO selector lists the voltages your charger supports — use UP/DOWN to pick one and SELECT to confirm (or wait 10 s for a safe auto-select of ≤ 20 V). The Dashboard then shows live V/I/W.

The four buttons are UP/DOWN (adjust / navigate, hold to repeat), SELECT (confirm / cycle screens / edit) and POWER (short: output on/off; long: off). The output starts OFF; the status bar turns green when it is live.

To control it from a PC, connect a USB-C data cable and open the dashboard in Chrome or Edge 89+ (WebSerial required), or use any serial terminal at 115200 baud and type help.

Safety: verify the voltage before connecting a load; output is OFF by default; the device warns at 60 °C and shuts down at 85 °C; a fault screen must be cleared with SELECT (POWER is blocked during a fault). Maximum output is limited by your charger.

Independent protection layers, fastest first. The hardware layers act autonomously, without firmware.

The LTC4368 hot-swap controller drives back-to-back MOSFETs; the COMP1/DAC backup OVP threshold tracks the negotiated voltage. Firmware adds OCP soft-start retry (default 3×) for hot-plug inrush, charger-disconnect shutoff, and a 1.5 s output-toggle cooldown.

Six screens, cycled with SELECT:

1. Dashboard — large V/I/W readout; set voltage/current with live edit and CC/CV indication.
2. PDOs — scrollable source capabilities incl. EPR AVS, plus cable e-marker info.
3. Graph — rolling V/I plot, one sample per pixel for smooth scrolling; selectable 5/10/30/60 s window.
4. Presets — five named slots, stored in flash.
5. Energy — session runtime, Wh/Ah, plus average and peak I/P (long-press SELECT resets).
6. Settings — grouped menus: Mode, Display, Sound, Protection, Tools, Calibration, System.

Defaults: output OFF at boot, OCP 5.5 A, OVP 55 V, 3-retry, 10 s graph window. Settings persist in the last 2 KB flash page (magic + CRC).

AxxPD exposes both interactive shortcuts and full SCPI over USB-CDC (115200 baud). See the complete command reference for every command, the SCPI subsystems, scripting examples and the telemetry format.

Two host-side tools in Tools/ (Python + pyserial) drive the device over USB-CDC and need no load connected:

python Tools/charger_test.py         # verify PD negotiation across all levels
python Tools/axxpd_selftest_full.py  # full standalone feature test (colour report, CI-friendly)

axxpd_selftest_full.py writes a timestamped pass/fail report and exits non-zero on failure.

A small custom bootloader lives at 0x08000000; the application runs from 0x08006000 behind it. Three ways to update, easiest first:

• Web dashboard (recommended) — open Tools/AxxPD_Dashboard.html in Chrome/Edge, connect, expand Firmware Update, and drop a released AxxPD.bin. The dashboard reboots the device into the bootloader over USB and flashes it — no button holding, no extra tools. It validates the image header/CRC before touching the device, and a failed transfer leaves the device waiting safely in update mode for a retry.
• USB DFU — for recovery, hold the boot button while applying power (or send dfu over serial) to enter the STM32 ROM bootloader, then flash with STM32CubeProgrammer or dfu-util. Write the bootloader at 0x08000000 and the app at 0x08006000.
• SWD — connect an ST-Link to the SWD pads and flash both images:

  STM32_Programmer_CLI -c port=SWD mode=UR reset=HWrst -w AxxPD_boot.bin 0x08000000 -w AxxPD.bin 0x08006000 -v -rst
  

Import Firmware/ into STM32CubeIDE, or build from the command line with arm-none-eabi-gcc on your PATH:

cd Firmware && make -j

Output is build/AxxPD.elf; -Os, ~77 % of the 256 KB flash.

Firmware/         STM32G491 firmware (C/C++17, CubeIDE project)
  axxpd_firmware/ Application code (GPL-3.0)
  Core/ Drivers/  HAL, CMSIS, LCD driver, uGUI
  pdsink/ etl/    USB-PD stack + Embedded Template Library (MIT)
Tools/            WebSerial dashboard, host-side test scripts
Documentation/    Schematic (SVG), command reference, photos

The schematic is published as SVG for reference.

The firmware is open source under the GPL-3.0, with the full toolchain (Makefile, linker script, CubeIDE project) included so you can build and flash it yourself. The PCB and enclosure design files are proprietary and not part of this repository; the schematic is published for reference only.

"AxxPD" is a trademark of Axel Johansson. Derivatives may state they are "based on AxxPD" but must not use "AxxPD" as a product name — see NOTICE.

AxxPD is provided as-is, with no warranty. It handles up to 48 V / 5 A — always verify your setup before enabling the output. The author accepts no liability for any harm or loss resulting from its use.