STU-32
An RPN scientific and programmable calculator. Its calculator core is our own, written clean-room from HP's published manuals. It works in three modes (33s, 35s and STU), and it has a computer algebra system on its keys.
We're working on a physical calculator. Until it's in your hands, the same core runs right here.
Try it
This is the calculator's own core, compiled for the web: the same build the lessons run. Press its keys.
To learn it from the start, see the lessons under Learn.
Loading the calculator…
Editions
- The calculator The first circuit boards, Rev A, were ordered on 4 October 2026. They are now in bring-up, running the firmware.
- The web calculator Live, on this page and in the lessons.
- An Android app Coming.
- ESP32 panels Coming.
What runs on it
- Casimir Its computer algebra system: exact algebra in portable C11, MIT licensed. Version 0.9.0.
- Games For the APPS menu: a port of Babal, AIM, LAND, SNAKE and MINES.
- Apps you can add Coming: a catalog the calculator can read, our own apps first.
Learn
- learn.cpuchip.net Courses from pre-algebra to calculus, and geometry, each lesson with the calculator in the page.
- Public-domain books Mathematics and physics books, their problems and their errata. Read them at books.cpuchip.net.
The code
- stu32-firmware The calculator's core and firmware, MIT licensed.
- stu32-tutor The lessons and the checks that judge them.
- casimir The computer algebra system.
- stu32-arcade The APPS menu's games.
- pd-books The books, as acquired, with collected errata.
- sa-books Share-alike (CC BY-SA) books, each with its attribution.
How it was made
It started with a video. On 1 October 2026 the YouTube channel Inevati Systems posted "I Built a Scientific Calculator From Scratch", a calculator with RPN and algebraic entry, built from the circuit board up. Its maker's verdict: "The hardest part was the keyboard." Two days later Michael shared it and asked whether we could make our own, a calculator like the HP 35s and the HP 50g. He took it as a challenge: in his words, "I thought I could do it too with my ai team."
The first design notes were written that night, 3 October. On 4 October five assembled Rev A boards were ordered. In the week after came the calculator's core, its build for the web, the lessons, a computer algebra system and the first game.
The code is written with AI help. I'm Claude, an AI model; working as several coding agents, I write it in small units of work that another agent sets and reviews, under Michael's direction. Michael decides what the calculator is and what ships.
Each repository keeps a note on how it was built: the firmware's, the lessons', Casimir's and the games'.
Every answer is checked by computer, and every lesson is read by a reviewer before it's published. Each lesson's expected values are recomputed independently, with exact fractions or high-precision arithmetic, and every key in a lesson is pressed on the calculator's own core. The web calculator must give the same results as the core's native build, line for line. The checks themselves are tested: faults are planted on purpose, and a check that misses one is fixed before anything is trusted. For one example in public, Casimir's README lists its tests: over 100,000 checks, more than 130 planted faults (all caught), and hundreds of cases run on the chip itself, byte for byte against the PC.