The prompt-first pass
The original build below records how this idea first worked. The durable lesson now targets the M5Stack Stopwatch and lives in the prompt, behavior contract, generalized tests, and hand-review checklist. Generated firmware is evidence for those sources, not the source itself.
The 1996 Tamagotchi is one of the best-documented toys ever made — the P1’s timers, evolution rules, and care-mistake logic have all been reverse-engineered, and TamaLIB will run the actual ROM on a microcontroller.
That was the tempting road: real emulation, pixel-perfect nostalgia. It’s also Bandai’s ROM and Bandai’s sprites in a public repo, so the honest version of this lesson is a clean-room build: their rules, our code, our creature.
Offline aging is the whole toy
Strip away the art and the Tamagotchi is one mechanic: it needs you even when you’re not there.
Which means the simulation can’t live in loop() time — it has to live in calendar time.
The design answer is a single simulate(minutes) function.
The live loop feeds it one minute per real minute.
Boot feeds it the entire absence: read the RTC, subtract the saved last-seen epoch, replay.
There’s no separate “catch-up logic” to write or to get wrong — being powered off is just a large number of minutes. The function that ages the pet overnight is character-for-character the function that ages it while you watch.
Fractional progress persists too: decay accumulates in minute counters that save with everything else, so twenty short sessions can’t gift the pet immortality by resetting the clock on every boot.
Two buttons, four verbs
The original had three buttons: select, confirm, cancel. The StopWatch has two. The menu absorbs the difference — A cycles a wrapping selection, B confirms, and cancel stops existing because no action needs one. Feeding is idempotent, cleaning is idempotent, and play’s failure mode is the creature declining, which is the original’s charm anyway: the guessing game losing half the time is the personality.
Sixteen rows of hex
The art question answered itself at 16×16.
One uint16_t per row, five bitmaps, fat pixels with a one-pixel gut between them — the LCD-grid look the original had, achieved by drawing rectangles.
An egg is symmetry.
A baby is the egg with eyes.
Death is a flat line under a face — sixteen numbers that read instantly from across a room.
No converter, no assets, no pipeline. Sometimes sprite art is just typing.
What we learned
- Offline aging is one function with a big argument. Replay minutes; never write “catch-up” code.
- Persist the fractions. Decay accumulators must save too, or short sessions reset every clock.
- Menus absorb missing buttons. Cycle-and-confirm turns two pushers into any number of verbs.
- Rules aren’t copyrightable, expression is. The clean-room line: their timers, our everything else.