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This is your challenge : to implement in firmware the code to run the washing machine as described in this spec.
There's a high-level flow involved in operating the washing machine that's independent of the wash program that's selected. This is shown below as a state diagram (tip: treat this as a requirement rather than implementation design) (and another tip: if you open this picture in a new tab or window, you can zoom it for clarity).
Some additional narrative:
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The physical door switch has to be taken as an absolute. So, for example, if the switch is found to be open when the "Unloading" state is entered, the "open door" event is taken to happen immediately.
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Don't take "enable" and "disable" to imply that hardware is initialised or uninitialised. The spec is agnostic about implementation : this could mean that the event is simply ignored in this state.
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A lot of the complexity in requirements - and hardware interaction - is in the "running" state.
The door is simulated by a slide switch. In the upper position, the door is open, in the lower position it's closed.
When the program is being selected by the operator, the "Open" indicator has to be updated promptly to show the current state of the door switch.
A straightforward function - the operator can press the up and down buttons to cycle between selected programs. The current program is always shown on the left-hand two digits as "P". The "P" never changes.
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The available programs are clamped to the range 1-3. Pressing UP when the display shows P3 has no effect. Pressing the DOWN button when the program is on P1 has no effect.
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On startup, the currently selected program is P1.
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They're cheap momentary switches, so some degree of button bounce is inevitable. How will you debounce? And when will you do this?
The wash program is initiated by pressing "START". If the door switch is currrently open, this has no effect. Otherwise, if the door is closed, the wash program is commenced.
Check the high-level operation FSM again : there are many actions initiated when the program is started and ended.
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The program is locked in until it's ended.
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Pressing "START" has no effect when the wash program is already running.
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If the door slide switch is moved to open, it's disregarded and the "OPEN" indicator stays off. The door cannot be opened while the wash program is running.
The clock countdown does
In a wash phase, the powder valve is opened for the duration of the fill.
In a soften phase, the softener valve is opened for the duration of the fill.
There are three different programs the operator can select. There are variations in:
- what phases (wash, condition, rinse, spin) are in the program
- the timing of the phases
- the water fill level that's
- the target temperature of each phase (if heated)