The main features of the calculator are the following:
- Electronic schematics designs Open Source (GPLv3).
- 32-bit ARM Cortex-M33 MCU.
- Ultra low power MCU.
- Floating Point Unit (FPU). Floating-point arithmetic functionality.
- 274KB SRAM.
- 512KB FLASH MCU memory.
- 6MB FLASH storage Octo-SPI accessible via USB.
- 128-bit floating point precision implementation (IEEE 754-2008).
- USBC port, connects as USB mass storage device.
- Graphic LCD Display (128x64 dots).
- Possibility of creating/porting firmware for CAS or RPN calculator.
- Same keys as the legendary HP48.
Below is the list of electronic components used to manufacture the board.
| Component | Ref | Description |
|---|---|---|
| SoC Processor | STM32U535CB | ARM Cortex-M33 ultra low power with FPU |
| Display | EA_DOGL128L-6 | STN reflective display 128x64 |
| Flash 6MB | AT25SF641 | 6MB Flash |
| Voltage Regulator | ADP2108AUJ-3.3 | Vin 2.3 - 5.5, Vout 3.3 |
| Power by USB (5V) | LD3985 | Micropower Low-Dropout Voltage Regulator |
| Transistor | MMBT3904L | 200 mA, 40 V NPN Bipolar Junction Transistor |
This is the brain of our calculator. Ultra-low-power Arm Cortex-M33 MCU+FPU, 240 DMIPS, core clocked at 160 MHz and 274 KB of Static RAM.
STM32U535CBT6 datasheet Arm Cortex-M33 Developers Guide
This is the persistant memory of our calculator. This chip stores 64 Mbit of data, and serves it over a 104 MHz Quad-SPI bus.
The ADP2108 is a high efficiency, low quiescent current stepdown dc-to-dc converter. This part regulates the voltage from the USBC input.
The USB port can receive electrostatic discharges. Those very high voltage surge can permanently damage electronic components. This part evacuates the surge entering from the USB port and protects the whole circuit.
The EA DOGL128L-6 is a 128x64-pixel graphics display. The L model does not have LED backlighting
Supports the following batteries:
| Batteries | Capacity (mha) |
|---|---|
| AAA Alkaline | 900 - 1155 |
| AAA Ni-MH | 800 - 1000 |
The support for AAA batteries on the PCB is XXXXXXX
For the development of the hardware, the KiCad tool has been used.
The board can be ordered from PCBWay, JLCPCB, etc. In addition, 3D designs for the calculator case and keyboard are included.
PCB front:
PCB back:
General comparison, taking the maximum specifications between all the SOCs of each family.
| Serie | PW OFF | RAM | Flash |
|---|---|---|---|
| STM32L4 | 8 nA | 320KB | 64KB - 1MB |
| STM32L4+ | 8 nA | 320KB | 512KB - 2MB |
| STM32L5 | 17 nA | 256KB | 256KB - 512KB |
| STM32U5 | 90 nA | 768KB | 1MB - 2MB |
The CORDIC co-processor provides hardware acceleration of certain mathematical functions, notably trigonometric, commonly used in motor control, metering, signal processing and many other applications. It speeds up the calculation of these functions compared to a software implementation, allowing a lower operating frequency, or freeing up processor cycles in order to perform other tasks.
The CORDIC main features are:
- 24-bit CORDIC rotation engine
- Circular and hyperbolic modes
- Rotation and vectoring modes
- Functions: sine, cosine, sinh, cosh, atan, atan2, atanh, modulus, square root, natural logarithm
- Programmable precision
- Low-latency AHB slave interface
- Results can be read as soon as ready without polling or interrupt
- DMA read and write channels
- Multiple register read/write by DMA
Limit: maximum precision of the coprocessor is 20 bits in the best case.
Comparative performance CORDIC versus software (math.h functions), in CPU cycles:
| Method | sin,cos | atan | exp | ln | sqrt |
|---|---|---|---|---|---|
| CORDIC | 29 | 33 | 39 | 27 | 23 |
| Software | 416 | 332 | 319 | 260 | 58 |
DOWNLOAD STL FOR PRINT CASE DOWNLOAD STL FOR PRINT KEYPAD
The keyboard has been designed for FDM printing, with inserts in each key and post-processed with acetone.
Includes a separating piece.
Printing numbers on the keyboard is done using a sublimation transfer process, ordering the sheets from local companies and ironing them onto the keyboard.
The design has been designed for FDM printing, with inserts in each key and post-processed with acetone.
The front plate is printed on photographic paper and cut to size, with professional results. A photo-quality printer is recommended, such as the Epson Ecotank.
To create the faceplate, the Inkscape tool has been used to edit the SVG and the font used is Latin Modern Math, licensed by the GUST Font License, developed by the TeX user group in Poland, and can be used for academic use. , commercial or academic.
Comparison of the hardware used in the different reference calculators:
| Year | Model | Processor | RAM | FLASH | Display |
|---|---|---|---|---|---|
| 1988 | HP42S | Saturn | 8KB | 64KB | 131×16 |
| 1993 | HP48G | Saturn | 32KB | 512KB | 131×64 |
| 2006 | HP50G | ARM920T | 512KB | 2MB | 131×80 |
| 2013 | TI-Nspire CX II | ARM9-26EJ-S | 64MB | 128MB | 320x240 |
| 2016 | NumWorks | STM32F730V8T6 | 256KB | 6MB | 320x240 |
| 2017 | DM42 | STM32L476RG | 128KB | 6MB | 400×240 |
| 2019 | HP Prime G2 | i. MX 6ULL | 256MB | 512MB | 320×240 |
| 2021 | OpenRPNCalc | STM32L476 | 128KB | 1MB | 400x240 |
| 2024 | DM50 | STM32U535CB | 274KB | 6MB | 128×64 |
| Software | Detail |
|---|---|
| DB48X | DB48X is a modern implementation of RPL |
| C47 | C47 is a RPN Scientific Calculator (old WP43C project) |
| WP43 | Firmware for the WP43 pocket calculator (old WP43S project) |
| Free42 | Free42 is a software clone of the Hewlett-Packard 42S calculator |
| TCalc | Firmware for STM32 CAS calculator |
| TinyExpr | parser and evaluation for math expressions |