OpenRPNCalc
Open-source and open-hardware scientific RPN calculator
Introduction
OpenRPNCalc is a scientific calculator based on the STM32 microcontroller. Its source code, schematics, and PCB design files are released under an open license. The goal is to develop a hardware platform with the following features:
- Reproducible without special tools, using only boards produced by PCB prototyping services
- A customizable keyboard to accommodate specific needs (such as RPN or algebraic logic, custom functions)
- Low power consumption and long battery life
- A high-quality, compact and sturdy enclosure
The hardware is inspired by the SwissMicros calculators, but is designed from scratch. The calculator features:
- Reverse Polish notation with a large stack (up to 100 elements)
- Double-precision arithmetic
- "Standard" scientific calculator functions (trigonometric, logarithms, exponentiation, square root and power)
- Error function (erf) and its inverse (erfinv), Gamma and log(Gamma) functions, combinations and permutations, p-values for Poisson, Gaussian, and chi-squared distributions
- Fixed, scientific (SCI) and engineering (ENG) display modes (including SI prefixes in ENG mode), with adjustable precision from 3 to 10 digits
- Calculations with uncertainties using error propagation (UNCERT mode) - a feature rarely found in software calculators and almost unheard of in hardware calculators
- Formulas from relativistic kinematics (centre-of-mass two-body decay momentum, conversion between angle and pseudorapidity, beta and gamma factors)
- Statistical functions: mean (or weighted mean in UNCERT mode), RMS, chi-squared of a series of values (only in UNCERT mode).
- 100 memory registers
- Power consumption: 0.6mA running at 16MHz, 16 uA in standby mode with the LCD on, and 5 uA with the LCD off.
- Optional HP48 keyboard files, with the aim to run DB48x (this is still very much WIP)
Hardware overview
The calculator is built around a low-power 32-bit ARM microcontroller, the STM32U385. The display is a Sharp memory LCD module, the LS027B7DH01 (400x240 pixel monochrome). The keyboard uses light-touch tactile switches, specifically the Panasonic EVPBT1C4A000 (50g actuation force). All electronics are powered by a 3V lithium battery (CR2032), which should be sufficient for several years of operation.
The enclosure is designed as a stack of multiple PCBs. The front and back panels are aluminium PCBs with a white solder mask and black silkscreen. The keys are also made of PCB, with labels printed as silkscreen. All components are kept together with six M2 standoffs and twelve flat-head screws. The enclosure dimensions are 139 x 73 x 8 mm.
Repository contents
- Code: STM32 firmware developed using STM32Cube IDE
- Hardware: Schematic and PCB design files for KiCad, PCB production files
- Doc: Documentation and images
- Emulator: Online emulator files to be deployed to github-pages or other web hosting
Documentation
Emulator
Online emulator, based on the original calculator firmware extended with Emscripten and Raylib