diff --git a/fw/dtlogic/src/input.rs b/fw/dtlogic/src/input.rs
index 0ad0a63..be7dd97 100644
--- a/fw/dtlogic/src/input.rs
+++ b/fw/dtlogic/src/input.rs
@@ -1,4 +1,5 @@
 #[derive(Clone)]
+/// A stateful button debouncer.
 pub struct Buttons {
     state: [Option<u32>; 4],
 }
@@ -10,6 +11,8 @@ impl Buttons {
         }
     }
 
+    /// Update internal state from raw input state, with given millisecond timestamp. The input
+    /// state is proccessed/debounced into the internal state.
     pub fn update(&mut self, ts: u32, state: [bool; 4]) {
         for (&s, v) in state.iter().zip(self.state.iter_mut()) {
             if let Some(pressed) = v {
@@ -29,6 +32,8 @@ impl Buttons {
         }
     }
 
+    /// Return a watcher, which is used to detect edge-triggered button events. Multiple
+    /// independent watchers can exist, eg. for different downstream code.
     pub fn watch(&self) -> ButtonWatcher {
         let mut pending = [false; 4];
         for (i, v) in self.state.iter().enumerate() {
@@ -43,12 +48,17 @@ impl Buttons {
     }
 }
 
+/// A edge-triggered button event watcher. Updates events from a Buttons struct and yields button
+/// press events once per button press.
 pub struct ButtonWatcher {
     state: [Option<u32>; 4],
     pending: [bool; 4],
 }
 
 impl ButtonWatcher {
+    /// Update internal state from Buttons. If any events are missed, a best effort is made to
+    /// reconstruct missing presses - but for all presses to be available, they should be processed
+    /// as early as possible via the events function.
     pub fn update(&mut self, buttons: &Buttons) {
         for (i, (v, o)) in self.state.iter_mut().zip(buttons.state.iter()).enumerate() {
 
@@ -79,6 +89,8 @@ impl ButtonWatcher {
         }
     }
 
+    /// Returns an iterator over pending button presses. Button presses are registered as processed
+    /// one-by-one as the returned iterator is consumed.
     pub fn events<'a>(&'a mut self) -> ButtonWatcherPending<'a> {
         ButtonWatcherPending {
             w: self,
@@ -86,6 +98,7 @@ impl ButtonWatcher {
     }
 }
 
+/// Button press iterator, yielding a button number for each pending button press (edge-triggered).
 pub struct ButtonWatcherPending<'a> {
     w: &'a mut ButtonWatcher,
 }
diff --git a/fw/dtlogic/src/ui.rs b/fw/dtlogic/src/ui.rs
index 59b4399..cc2f0b5 100644
--- a/fw/dtlogic/src/ui.rs
+++ b/fw/dtlogic/src/ui.rs
@@ -17,31 +17,47 @@ use profont::{
 
 use crate::input;
 
+/// Color space that we expect to support: some form of RGB.
 pub trait ColorSpace = RgbColor + PixelColor;
 
+/// Context passed to state tick functions.
 pub struct Context {
     pub buttons: input::Buttons,
     pub ticks: u32,
 }
 
+/// Transitions (ie. FSM edges) returned by state tick functions.
 pub enum Transition {
+    /// Do nothing (keep current state).
     Keep,
+    /// Start the 'Running' state.
     Running {
+        /// Tickcount at which the timer started running.
         start: u32,
+        /// Number of milliseconds that the timer should run.
         msec: u32,
     },
+    /// Start the 'Time Select' state.
     TimeSelect,
 }
 
+/// State combining state logic, transition logic and drawing logic.
 pub trait State<D: DrawTarget<Color: ColorSpace>> {
+    /// Called when the state becomes active, before tick is called.
     fn enter(&mut self, tr: Transition);
+    /// Called on each tick of the application when the state is active.
     fn tick(&mut self, ctx: &Context) -> Transition;
+    /// Called whenever the application can/should draw. Called after tick.
     fn draw(&mut self, target: &mut D) -> Result<(), <D as DrawTarget>::Error>;
+
+    /// Return the size of the target display. Helper function for draw().
     fn size(&mut self, target: &mut D) -> (u32, u32)
     {
         let bb = target.bounding_box();
         (bb.size.width, bb.size.height)
     }
+
+    /// Fully clear the target display with a given color. Helper function for draw().
     fn clear(&mut self, target: &mut D, color: D::Color) -> Result<(), <D as DrawTarget>::Error>
     {
         let (width, height) = self.size(target);
@@ -49,6 +65,8 @@ pub trait State<D: DrawTarget<Color: ColorSpace>> {
             .into_styled(PrimitiveStyle::with_fill(color))
             .draw(target)
     }
+
+    /// Draw bottom soft buttons. Each button should be one-character string. Helper for draw().
     fn draw_softbuttons(&mut self, target: &mut D, labels: [&str; 4]) ->
         Result<(), <D as DrawTarget>::Error>
     {
@@ -80,6 +98,8 @@ pub trait State<D: DrawTarget<Color: ColorSpace>> {
     }
 }
 
+/// State used to select the time for the timer. Has +/- buttons and a start button (which
+/// transitions to Running).
 pub struct TimeSelect {
     cleared: bool,
     bw: Option<input::ButtonWatcher>,
@@ -195,6 +215,7 @@ impl<D: DrawTarget<Color: ColorSpace>> State<D> for TimeSelect {
     }
 }
 
+/// State when the timer is counting down.
 pub struct Running {
     cleared: bool,
     msec: u32,