Statecharts, one tick at a time

What you’ll learn

Twelve chapters, read in order. Each one introduces a construct with a chart you can run, states the rule exactly, shows the mistakes beginners make, and ends with exercises.

After that, Going further covers the rest of the language and Patterns works through complete charts.

1. 1Chapter 1: Thinking in ticksA chart runs in ticks: it reads its inputs, reacts in no time at all, and writes its outputs, the same way every time.
2. 2Chapter 2: Your first SCChartWrite a complete chart from an empty file, read its diagram, run it, and recognise the first compiler errors you will meet.
3. 3Chapter 3: Triggers, effects and priorityWhen a transition fires, what it does on the way, and which one wins when several could fire at once.
4. 4Chapter 4: Delayed and immediate transitionsWhen a transition is checked, how several transitions can fire in one tick, and how a chart finishes.
5. 5Chapter 5: Variables, types and expressionsBooleans, integers, floats and strings; inputs, outputs and locals; initial values, operators, assignments and arrays.
6. 6Chapter 6: SignalsPure and valued signals: present or absent in each tick, emitting and testing them, and combining values.
7. 7Chapter 7: SuperstatesPut a state machine inside a state: nesting, the scope of declarations, and finishing with a termination transition.
8. 8Chapter 8: Concurrent regionsSeveral state machines side by side in one chart: they react in the same tick, talk to each other within it, and can finish together.
9. 9Chapter 9: AbortsCutting a superstate short: weak and strong aborts, immediate aborts, priorities, and what the inside may still do in the tick it is left.
10. 10Chapter 10: Entry, during and exit actionsEffects that belong to a state rather than a transition: they run when the state is entered, while it stays active, and when it is left, however that happens.
11. 11Chapter 11: The previous tickWith pre(x), the value x had one tick ago, a chart can react to changes instead of levels: detect edges, fire one-tick pulses and count.
12. 12Chapter 12: Why the compiler says noWhat KIELER checks before it runs a chart with concurrent regions, why it refuses a causality cycle, what its error messages mean, and how to change the chart.

Ways in

• Start learningBegin with what SCCharts is, where it comes from and how this guide works.
• Short on time?A lab tomorrow? Set up, build one small chart and meet the surprises that catch everyone, on one page.
• PlaygroundWrite a chart, see its diagram and run it tick by tick, all in the browser.
• Cheat sheetAll of the syntax on one page, for when you know what you want to write.
• SCCharts Lab for VS CodeWrite charts in your editor with live diagrams, simulation and compiler diagnostics.
• Stuck?
  • Look up a compiler messageWhat each of KIELER’s errors and warnings means, a chart that triggers it, and the fix.
  • It compiles but misbehavesFrom the symptom to its cause: a transition a tick late, an output stuck at true, a press counted twice.

Credits

SCCharts was created by the Real-Time and Embedded Systems group at Kiel University as part of the KIELER project. It builds on Harel’s statecharts and on synchronous languages such as Esterel. The PLDI 2014 paper introduced it, and the KIELER wiki is its reference documentation.

This guide is an independent project, not part of KIELER or Kiel University. Its examples are checked against KIELER’s own compiler, tick for tick, so they behave here as they do in KIELER; where KIELER has a bug the guide does not copy, the example says so (how this guide is checked).