Project: flag register

Bit week ends with a project you can use in every base: a flag register. Four buttons toggle four lights – one press on, the next one off. Instead of wasting four registers on four states, everything lives in one.

The setup#

A button reports 1 on Activate while it is pressed.

The idea#

Bit 0 belongs to button 1 and light 1, bit 1 to button 2 and light 2 and so on. Three things happen every tick:

1. Collect the buttons: all four Activate values go into new as bits with ins.
2. Find rising edges: pressed is only what is 1 now and was 0 before: new and (not old). With xor you toggle exactly those bits in the state.
3. Output: ext takes every bit out again and switches the matching light, the display gets the whole state as a number.

not tmp old
and edge new tmp
xor state state edge
move old new

Without edge detection a button held down would toggle the light every tick – it would flicker at 2 Hz.

Milestones#

1. Button 1 toggles light 1: press on, press again off.
2. The buttons are independent: buttons 2 and 4 only switch their own lights.
3. The display shows the state as a number: lights 2 and 4 on gives 2 + 8 = 10.
4. Holding a button toggles only once.