Selta's Keypad

Custom firmware for a CH552-based 3-button USB macropad with rotary encoder.

Features

• KVM switching via hotkey sequences
• Volume control (encoder twist)
• Mute toggle (encoder short-press)
• LED toggle (encoder long-press)
• RGB rainbow LED effect
• USB HID keyboard + consumer control

Current Configuration

Input	Function
BTN 1	KVM Switch → Input 3
BTN 2	KVM Switch → Input 2
BTN 3	KVM Switch → Input 1
Encoder Twist CW	Volume Up
Encoder Twist CCW	Volume Down
Encoder Short-Press	Mute Toggle
Encoder Long-Press (≥500ms)	Toggle LEDs On/Off

Note: Buttons are mapped "backwards" (1→3, 2→2, 3→1), and encoder rotation is "from the front" with vertical, USB port down orientation. Also noteworthy, my KVM (Level1Techs) uses Ctrl + Ctrl sequence for changing inputs.

LEDs

• Normal mode: Slow rainbow color cycle
• Bootloader mode: All red
• Long-press encoder to toggle LEDs on/off

LED settings can be adjusted in src/led.h:

• NEO_BRIGHT_KEYS - Brightness (0=dim, 1=medium, 2=bright)

Rainbow speed can be adjusted in src/led.cpp:

• cycle_counter_s >= 4 - Higher = slower cycle

Hardware

Based on a cheap CH552 macropad from Amazon:

• Amazon link

What's Inside

• WCH CH552 microcontroller (unsure which variant)
• 3 mechanical buttons
• Rotary encoder with push button
• 3 addressable RGB LEDs (WS2812/NeoPixel)

Pinout

Function	Pin
Button 1	P15
Button 2	P34
Button 3	P11
Encoder Button	P33
Encoder A	P32
Encoder B	P14
LEDs	P31

Note: Different PCB revisions may have different pinouts. Check your board if things don't work. The easiest way is to use a multi-meter, and check continuity between the key and the controller. You'll find continuity on the pin that the switch is connected to. Pin numbering usually is anti-clockwise, beginning with "1" where there is a notch or a small circle on the IC.

Building & Flashing

Requirements

• Arduino IDE
• CH55xduino board support

Setup

1. Install Arduino IDE
2. Add CH55xduino board support:
   • Go to Preferences → Additional Board Manager URLs
   • Add: https://raw.githubusercontent.com/DeqingSun/ch55xduino/ch55xduino/package_ch55xduino_mcs51_index.json
3. Install the CH55xduino board from Board Manager

Build Settings

In Tools menu:

• Board: CH552
• Bootloader: P3.6 (D+) Pull up
• Clock Source: 16MHz (internal) 3.5V or 5V
• Upload Method: USB
• USB Settings: USER CODE w/148B USB RAM

Entering Bootloader Mode

First time / without custom firmware:

• Short R12 or J4 (varies by PCB revision/brand) while connecting USB

WARNING: Shorting the wrong component can damage your board or computer. Double-check which pads to short using a multimeter or reference photos for your specific PCB revision. I am not responsible for any damage caused by incorrect modifications.

After flashing custom firmware:

• Hold the encoder button while plugging in USB
• LEDs will turn red to indicate bootloader mode

Flash

1. Get the macropad into bootloader mode
2. Click Upload (teal right arrow, or Sketch > Upload) in Arduino IDE

Customization

Button Mappings

Edit configuration.cpp to change button mappings.

Button Types

• BUTTON_SEQUENCE - Send keyboard key sequences
• BUTTON_MOUSE - Send mouse/media events
• BUTTON_FUNCTION - Call custom functions
• BUTTON_NULL - Disabled

Available Encoder Events

• SCROLL_UP, SCROLL_DOWN
• LEFT_CLICK, RIGHT_CLICK
• UP, DOWN, LEFT, RIGH (mouse movement)
• VOLUME_UP, VOLUME_DOWN, MUTE_TOGGLE

Example Configurations

Single Key

[BTN_1] = {
    .type = BUTTON_SEQUENCE,
    .function.sequence = {
        .sequence = {'a'},
        .length = 1,
        .delay = 0
    }
}

Two-Key Combo (Ctrl+C)

[BTN_1] = {
    .type = BUTTON_SEQUENCE,
    .function.sequence = {
        .sequence = {KEY_LEFT_CTRL, 'c'},
        .length = 2,
        .delay = 0  // 0 = hold all keys together
    }
}

Three-Key Combo (Ctrl+Shift+Esc)

[BTN_1] = {
    .type = BUTTON_SEQUENCE,
    .function.sequence = {
        .sequence = {KEY_LEFT_CTRL, KEY_LEFT_SHIFT, KEY_ESC},
        .length = 3,
        .delay = 0
    }
}

Sequential Keys (not held together)

[BTN_1] = {
    .type = BUTTON_SEQUENCE,
    .function.sequence = {
        .sequence = {KEY_LEFT_CTRL, KEY_LEFT_CTRL, '1'},
        .length = 3,
        .delay = 50  // delay > 0 = press/release each key separately, in milliseconds
    }
}

Volume/Media Control

[ENC_CW] = {
    .type = BUTTON_MOUSE,
    .function.mouse = {
        .mouse_event_sequence = {{.type = VOLUME_UP, .value = 1}},
        .length = 1,
        .delay = 0,
        .keypress = 0
    }
}

Mouse Scroll

[ENC_CW] = {
    .type = BUTTON_MOUSE,
    .function.mouse = {
        .mouse_event_sequence = {{.type = SCROLL_UP, .value = 2}},
        .length = 1,
        .delay = 0,
        .keypress = 0
    }
}

Disabled Button

[BTN_1] = {
    .type = BUTTON_NULL,
}

Re-enable Layer Switching

To re-enable the profile/layer menu on the encoder button:

[BTN_ENC] = {
    .type = BUTTON_FUNCTION,
    .function.functionPointer = keyboard_press_enc,
}

Hold encoder and twist to switch profiles. See configurations[] array for profile definitions.

Available Keys

Modifiers: KEY_LEFT_CTRL, KEY_RIGHT_CTRL, KEY_LEFT_SHIFT, KEY_RIGHT_SHIFT, KEY_LEFT_ALT, KEY_RIGHT_ALT, KEY_LEFT_GUI, KEY_RIGHT_GUI

Special: KEY_ESC, KEY_TAB, KEY_RETURN, KEY_BACKSPACE, KEY_DELETE, KEY_INSERT

Navigation: KEY_UP_ARROW, KEY_DOWN_ARROW, KEY_LEFT_ARROW, KEY_RIGHT_ARROW, KEY_HOME, KEY_END, KEY_PAGE_UP, KEY_PAGE_DOWN

Function: KEY_F1 through KEY_F24

Printable characters: Use single quotes like 'a', '1', ' '

Long-Press Threshold

Adjust in src/buttons.cpp:

• LONG_PRESS_MS - Default 500ms

Multiple Profiles

The firmware supports multiple configuration profiles (see configurations[] array). The layer-switching menu is currently disabled but the code remains if you want to re-enable it.

Credits

• Original CH55xduino by Deqing Sun
• KVM, volume control, and LED modifications by Selta

Resources

• CH55xduino on GitHub
• CH552 Datasheet

License

This work is licensed under Creative Commons Attribution-ShareAlike 3.0 Unported License.