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Simulation

The TPU Button Video Is a Design Review, Not a Slow-Motion Test

A simulation is much more useful when it makes a design argument visible. In a static CAD view, a TPU key, a faceplate opening, and a switch location can all look compatible. The difficult part happens between those still images: a keycap drops, tilts toward a wall, transfers motion through the web, and either reaches the switch or spends its travel rubbing somewhere it should not.

That is why we made a cutaway video of the StackCalc TPU membrane. It is a short design-review tool built from the same membrane and faceplate STL geometry used for the prototype, not a claim that the unpopulated calculator has already survived a life test. The RP2350 board is in fabrication; the video lets us inspect the mechanical questions now and gives the first board session specific cases to repeat.

We Pressed One TPU Key 280 Ways Before the PCB Arrived

A calculator key rarely gets pressed straight down in the middle. A thumb catches an edge. A finger comes in from the side. A child presses a corner. The first question for our TPU membrane was not “does it move?” It was “which bad presses leave enough room for the key to reach the switch without scraping the faceplate?”

The StackCalc PCB is in fabrication, so the printed calculator is not yet a powered keypad. We used the interval to put numbers around the geometry that will meet the board: the printed faceplate aperture, the TPU web, the keycap stem, and the proposed tactile-switch interface. That produced a 280-condition press sweep and the plot below.

Fuzzing the Firmware Path in an RP2350 Emulator

The StackCalc calculator board is not assembled yet, but the firmware path is already rich enough to break in interesting ways. A random input sequence can cross number entry, ENTER, stack lift, shifts, menus, modes, clear, correction, and display updates in combinations that a hand-written demo never reaches.

We use the RP2350 emulator to run the firmware image and fuzz those combinations through its keypad contract. That is firmware-emulation evidence, not evidence of a physical switch matrix or display.