Project: warning blinker

Above the airlock door a light should blink as long as someone has flipped the “warning” lever. sleep could do that, but then the chip would only notice the lever every few seconds. So you build the blinker with a tick counter.

A small preview: define#

It should be easy to change the speed later without hunting through the code. That is what define is for:

define TICKS 4    # ticks per phase (4 ticks = 2 s)

Wherever TICKS appears afterwards, the chip uses 4. A define takes no register. define comes in detail in week 5 – today this one line at the very top is enough.

The sequence#

1. Read the lever. If it is not flipped: counter and light to 0, back to the start.
2. Otherwise increase the counter by one.
3. Once the counter reaches TICKS: counter to 0 and flip the light.
4. Write the light, back to the start.

Flipping means: 0 becomes 1, 1 becomes 0. You know that from the emergency light:

sub r2 1 r2       # flip the light state

Milestones#

1. Lever off: the light is off.
2. Lever on: the light is off for TICKS ticks first, then on for TICKS ticks, and so on.
3. Lever back: the light goes off right away.
4. Lever on again: blinking starts over with the off phase.

With TICKS = 4 the light is on for two seconds and off for two seconds.