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

• Schematics and parts of the PCBs and the list of electronic components and other parts with links to datasheets
• PCB design: images of the PCB layers and their functions
• Case and keyboard assembly: photos of the insides of the calculator, assembly instructions
• Firmware: code structure and flashing instructions

Emulator

Online emulator, based on the original calculator firmware extended with Emscripten and Raylib