medius

Custom firmware for MAKCU mouse-passthrough boxes, and the Rust library that drives it.

A MAKCU box sits between a mouse and a PC: the mouse passes through while a control program injects movement, buttons and scroll over USB-serial. medius replaces the stock firmware with a binary protocol; this crate binds its commands 1:1 (one call, one frame) and adds handshake, keepalive and reconnect.

Flash and test a box in the browser, with no drivers or install: medius.k4tech.net/dashboard. Docs: medius.k4tech.net.

medius vs stock firmware

Same box, different firmware. Both clone the mouse's USB descriptor byte for byte, since that's the hardware. The differences:

	medius	stock MAKCU
Your motion	Injection adds to physical motion; both arrive.	Injection overwrites it; at 1 kHz physical motion never arrives.
Detection	Timing, control values and USB conformance measured against the native mouse and matched.	Copies the descriptor; no published native-behaviour audit.
Reliability	Clears all injection after 1 s of host silence, so a crashed controller never leaves a button held.	No documented silence release; a forced button stays held until you clear it.
Link	Binary frames with CRC and request IDs at a fixed baud.	ASCII command prompt; replies matched by arrival order; a baud handshake lost on power cycle.

Quick start

[dependencies]
medius = "3.4"

use medius::{Button, Device, Result};

fn main() -> Result<()> {
    let device = Device::find()?;             // auto-detect by USB VID/PID

    println!("{}", device.query_version()?);  // firmware version
    device.move_rel(100, -50)?;               // relative move
    device.press(Button::Left)?;
    device.release(Button::Left)?;
    device.wheel(-3)?;
    device.reset()?;                          // back to passthrough
    Ok(())
}

Features

The base crate is the sync core. Optional features:

Feature	Description
async	AsyncDevice: async queries over the same core, runtime-agnostic
mock	MockBox: an in-process fake box for hardware-free tests
tracing	per-frame TX/RX tracing instrumentation

medius = { version = "3.3", features = ["async", "mock"] }

API

Connect

let device = Device::find()?;                 // first box by VID/PID (0x1A86:0x55D3)
let device = Device::open("/dev/ttyACM0")?;   // a specific port

open/find run a version handshake and reject a mismatched protocol.

Multiple boxes

for b in Device::list() {                     // every connected box
    match &b.device {
        Some(d) => println!("{} {:?} {} {}", b.id(), b.name(), d.kind, d),  // MAC, name, kind, vid:pid + product
        None => println!("{} is on protocol {}", b.id(), b.version.proto_ver),  // needs a firmware update
    }
}
let m = Device::find_mouse_box()?;            // the box cloning a mouse
let k = Device::find_keyboard_box()?;         // the box cloning a keyboard
let d = Device::open_by_id("5a4e00111e28")?;  // by device MAC (or CH343 serial)

A box's identity is its device-chip MAC, so a reopen reaches the same unit after ports renumber.

A box on a protocol other than PROTO_VER lists with device: None; open_by_id and find_* return Error::BadProtoVer carrying the protocol it reported. Update it from the dashboard.

Mouse control

device.move_rel(100, -50)?;          // relative move (+x right, +y down)
device.wheel(3)?;                    // scroll

device.press(Button::Left)?;         // force down
device.release(Button::Left)?;       // release our press (a physical hold stays)
device.force_release(Button::Left)?; // force up, masking a physical hold
device.inject(Button::Right, Action::Press)?; // the generic form

device.reset()?;                     // clear all injection → passthrough

Buttons are Left, Right, Middle, Side1, Side2. Move and wheel take a full i16; the firmware clamps to the descriptor with carry, so move_rel(2000, 0) lands exactly 2000.

Keyboard & media

use medius::{Action, Key, MediaKey};

device.press(Key::A)?;              // hold a key (a modifier like Key::LEFT_SHIFT folds in)
device.release(Key::A)?;            // release our press (a physical hold stays)
device.force_release(Key::A)?;      // force up, masking a physical hold
device.inject(Key::ENTER, Action::Press)?; // the generic form

device.press(MediaKey::VOLUME_UP)?; // a media key by 16-bit Consumer usage
device.release(MediaKey::VOLUME_UP)?;

Keys are HID keycodes (Key::A, Key::ENTER, the eight modifiers, F-keys, arrows…) or any usage via Key::new(0x04); media keys are Consumer usages (MediaKey::VOLUME_UP, PLAY_PAUSE, MUTE…). Like buttons, they take the tri-state Action (press / soft-release / force-release) and stay held across a reconnect. Both are present-gated: a key the board can't report is a no-op with no error; see caps().

Sustained motion

Send one fire-and-forget move_rel per tick. The firmware merges additively, never halves, and carries the remainder, so a 1 kHz loop lands the full distance; the box paces emitted reports to the native report rate:

for _ in 0..1000 {
    device.move_rel(1, 0)?;
    std::thread::sleep(Duration::from_millis(1));
}

Emit pacing

set_emit_pace sets two NVS-persisted settings in one OPTION(EMIT) frame: what paces injected motion, and the rate the clone advertises.

use medius::EmitPace;

device.set_emit_pace(EmitPace::Learned, None)?;
device.set_emit_pace(EmitPace::Fixed(500), Some(1000))?;
let s = device.query_emit_pace()?;  // mode, resolved_hz, force_hz, advertised_hz, force_active

EmitPace::Learned (default) paces to the learnt native report rate, EmitPace::Interval to the clone's bInterval poll rate, and EmitPace::Fixed(hz) to hz, snapped by the 1 ms frame clock to 1000/n Hz and capped at EMIT_MAX_HZ.

A non-zero force_hz re-clones the box with a bInterval the device did not advertise, snapped to 1000/n Hz; it needs allow_imperfect_clones. None keeps the native interval.

Rendering

set_render sets the texture, and whether native motion goes through it, in one NVS-persisted OPTION(RENDER) frame. The model is ABCurves (MIT).

use medius::RenderMode;

device.set_render(RenderMode::Despiked, false)?;
let s = device.query_render()?;  // mode, full, ready

RenderMode	What reaches the wire
Off	the paced fill, renderer off
Stock	rendered with the bit-exact triangular smoother
Despiked	rendered with the smoother's onset ramped, not stepped (factory default)
Unsmoothed	rendered with no smoother; the model receives raw injection

full extends the model to native motion, so one texture reaches the wire instead of an injected stream beside a relayed one. Rendering adds a little latency, which full puts on native motion too. full is off by default.

Nothing renders until the box learns a profile for the attached device (RenderStatus::ready); until then motion is relayed and injection takes the paced fill. The profile lives in RAM, so after a power cut every box is unready again until the mouse moves.

Input scale

lock blocks physical input on one field while injection still drives it. scale is the same command with the number exposed: the percent of the physical value kept, 0 (lock) to 100 (unlock) and every value between, up to 255 to amplify. Down to -255, a negative scale weighs the physical value and reverses it, so -100 on an axis inverts it. Only an axis takes a negative.

use medius::{Axis, Blanket, Direction};

device.lock(Axis::X, Direction::Both)?;          // block horizontal motion (scale 0)
device.scale(Axis::Y, Direction::Negative, 60)?; // keep 60% of upward motion
device.unlock(Axis::X, Direction::Both)?;        // pass untouched (scale 100)

Direction::With and Direction::Against are measured against the bearing (the direction the box is injecting), not a fixed sign, so motion along the injection and against it take separate scales. The box resolves them at the merge point, where the pending injection and the arriving report are both in hand. set_bearing sets how long a bearing is held; past that window the axis has no bearing, relative directions stop applying, and the physical delta reaches the PC unweighed, with no host command.

use medius::{Axis, BearingMode, Direction};
use std::time::Duration;

device.set_bearing(Some(Duration::from_millis(20)), BearingMode::PerAxis)?;
device.scale(Axis::X, Direction::Against, 40)?;  // motion against the bearing kept at 40%
device.scale(Axis::X, Direction::With, 130)?;    // motion along it amplified to 130%

A delta takes at most two scales, its fixed direction's and its relative direction's, and they multiply, so a block in either zeroes the product. Direction::Both writes the scale to the two fixed signs and a full pass to the relative pair, so a Both of 50 is 50% with or without a live bearing. BearingMode::Vector instead projects the physical delta onto the injected vector, leaving motion across it untouched; one relative scale, the lower of X's and Y's, then governs both axes, and query_locks reports it on both. Each axis's absolute scale applies to what the projection left, not the sign the report carried, so a block still covers motion the projection put on that axis.

Only an axis has a bearing, so With/Against on a button, key or media usage is Error::RelativeDirection instead of a frame the box would drop. A button, key or media usage carries one bit: a scale below 100 locks it, 100 or above unlocks it, and a negative is Error::LockScaleUsage. A media usage has no edges (it is suppressed whole), so an edge on one is sent as Both, which query_locks reports. lock_all(Blanket::Keys, ...) honours the edge: Positive blocks presses only, Negative releases only.

Clip playback

Preload per-frame input into a ring on the box, which drains one entry per native frame on its own clock, free of host scheduling jitter and the per-command send floor.

Motion is a per-frame delta, edges (buttons/keys/media) hold until changed, and a gap emits nothing for N frames. Pace top-ups by query_status().free.

use medius::{ClipBuilder, Button};

let mut b = ClipBuilder::new();
for _ in 0..1000 { b.move_by(1, 0); }  // 1000 frames of +1 dx, box-timed
b.press(Button::Left).gap(20).release(Button::Left);

let clip = device.clip();
clip.append(&b)?;
clip.start()?;                          // or a trigger: clip.bind(ClipTrigger::new(Button::Side1, Edge::Press, ClipAction::Start))?

Queries

let v = device.query_version()?;  // proto_ver + fw_major / fw_minor / fw_patch
let h = device.query_health()?;   // link_up, mouse_attached, clone_configured, injection_active, rate_confident, lock_on, catch_on, kbd_attached

let info = device.device_info()?;       // cloned device identity: vid:pid, bcd, flags, kind, product
let caps = device.caps()?;              // unified caps; caps.is_composite(), caps.mouse.n_buttons, caps.keyboard.nkro, caps.keyboard.has_consumer, caps.keyboard.n_keys
let rate = device.query_rate()?;        // live native report rate; rate.native_hz()
let stats = device.query_stats()?;      // delivery counters; stats.tx_drops / stats.tx_wedges / stats.link_rx_drops / stats.relay_drops
let locks = device.query_locks()?;      // active input scales; locks.scale_of(...) / locks.is_locked(...)
let catch = device.query_catch()?;      // catch table, drop counts, inter-chip clock

Catch

Subscribe to what the box carries: physical input, HID and vendor traffic in both directions, proxied control transfers, the bytes the clone emits, and bus lifecycle. Input is reported before lock suppression or injection, so one loop can lock an input and catch it to rebind it. Dropping the stream unsubscribes.

For input, input_events decodes the box's held-usage snapshots into edges:

use medius::{CatchFilter, Input, Key};

for ev in device.input_events([CatchFilter::watch(Key::ESCAPE)])? {
    match ev.input {
        Input::Press(u) => println!("down {u:?}"),
        Input::Release(u) => println!("up {u:?}"),
        Input::Motion { dx, dy, dz } => println!("moved {dx},{dy},{dz}"),
    }
}

CatchFilter::watch takes what lock takes. CatchFilter::all_input() covers every class; watch_class / watch_axis narrow it.

For traffic, catch_events yields raw frames. A Capture caps the bytes kept per packet; a vendor bulk pipe at whole packets saturates the 6 Mbaud control link by itself:

use medius::{Capture, CatchEvent, CatchFilter, TrafficClass};

let events = device.catch_events([
    CatchFilter::everything().with_capture(Capture::First(16)),
    CatchFilter::traffic(TrafficClass::VendorInterrupt, 3),   // this endpoint, whole packets
])?;
while let Ok(CatchEvent::Traffic(t)) = events.recv() {
    println!("{:?} ep {} {} bytes", t.class, t.id, t.true_len);
}

Both streams are bounded and drop under back-pressure (dropped()), stay open through the keepalive, and are re-asserted across a reconnect. Under async, recv_async().await. Timeline maps a box stamp onto this machine's clock, unwrapping the 32-bit rollover and both chips' domains.

Box management

device.reboot(RebootTarget::DeviceRun)?;  // restart a chip (run / ROM-download × device / host)
device.reconnect()?;                      // rescan VID/PID, reopen, re-assert held state
device.reapply()?;                        // re-send held overrides now

let fw = device.firmware_info()?;         // both chips' firmware versions and booted app slots
device.update_firmware(UpdateTarget::Device, &image, &mut |p| println!("{}%", p.percent()))?;

update_firmware writes the image to the chip's spare app slot over the control port and boots it; the box reverts an image that will not run. It needs a box this build can open, so update a box on another control protocol from the dashboard.

The reader reconnects by itself if the link drops. When the device chip restarts under a live link, or the box releases the program's state without a restart (a patch apply or clear, an opt-in toggle, a detached device, the inter-chip link dropping), the crate re-sends what it holds once the clone is up; ClipHandle::lost reports a dropped clip.

Observability

for line in device.logs() {       // device LOG stream
    println!("[{:?}] {}", line.level, line.text);
}

let c = device.counters();        // frames_tx / frames_rx / crc_drops / reconnects / restarts

Async (feature = async)

The same core; only queries await, and fire-and-forget commands are unchanged:

let device = Device::find()?.into_async();
device.move_rel(10, 0)?;                // instant, not async
let v = device.query_version().await?;  // awaits the correlated reply

It uses flume's async recv, so it has no runtime dependency and runs on any executor.

Mock (feature = mock)

use medius::{Button, Device, FrameType, Health, MockBox, PROTO_VER, Rate, Version};

let mock = MockBox::new()
    .with_version(Version { proto_ver: PROTO_VER, fw_major: 1, fw_minor: 2, fw_patch: 3, mac: [0; 6], name: "my-box".into() })
    .with_health(Health::from_flags(0x0F))
    .with_rate(Rate { native_period_us: 1000, poll_period_us: 1000, confident: true, change_driven: false });

let device = Device::with_mock(mock.clone());  // the real stack over a fake box

assert_eq!(device.query_version()?.fw_minor, 2);
assert_eq!(device.query_rate()?.native_hz(), Some(1000.0));
device.press(Button::Left)?;
assert!(mock.saw(FrameType::Inject));          // commands are recorded

Examples

cargo run --example basic                   # minimal usage (needs a connected box)
cargo run --example hw_full --all-features   # on-hardware validation suite (Linux)

Architecture

Four layers, protocol → transport → link → device, each using only the one below.

Layer	What
protocol	wire codec: framed binary (SOF, type, rolling SEQ, length, payload, CRC16), no I/O
transport	byte pipe (no unsafe) and VID/PID discovery over serialport; on Windows, serial2's overlapped COM handle keeps a read and a write in flight at once
link	live connection: reader thread, SEQ-correlated queries, keepalive, reconnect
device	typed API; each command is one link.send(...)

Device methods take &self; it is Send + Sync and clones cheaply. The link runs framed binary at a fixed 6 Mbaud, and queries correlate by SEQ. After ~1 s of host silence the firmware clears all injection, so a crash never leaves a button stuck; a keepalive thread keeps a deliberately held button held. Tested on Linux and Windows.

Other languages

The medius-capi crate exports the whole API as a C ABI. Its generated header compiles as C and C++, and a ctypes Python package wraps it. See bindings/.

License

MIT, see LICENSE.