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Kitty Calculator
A small desk companion — a calculator built entirely from scratch in pure C99 for the ATmega328P.
What it does
Works like a normal calculator but with a dangerously-cute looking style
Demo
Photos
Enclosure in CAD
First Prototype
How it works
This project steps away from the standard Arduino (.ino) environment and relies on bare-metal C99 programming (AVR libc) to maximize performance and demonstrate what happens under the hood.
- Logic (FSM): The calculator logic isn't a messy
if/elseblock inmain.c. It's decoupled intocalculator.cand operates as a Finite State Machine (FSM) with an accumulator and a display register, mimicking real Casio chips. - Hardware Debouncing: Keypad inputs are heavily debounced in software (20ms delay logic on state change) directly in the scanning matrix, effectively ignoring both press-bounce and the notoriously tricky release-bounce.
- Direct Register Access: Uses direct port manipulation (
PORTC,PORTD, etc.) instead of slowdigitalWrite()wrappers for keypad scanning.
Wiring
The project uses an I2C module for the LCD to save pins, leaving enough GPIOs for the 4x4 matrix keypad.
| Component / Pin | ATmega328P (Arduino Uno) Pin |
|---|---|
| I2C LCD SDA | A4 (PC4) |
| I2C LCD SCL | A5 (PC5) |
| I2C LCD VCC | 5V |
| I2C LCD GND | GND |
| Keypad Row 1 | D2 (PD2) |
| Keypad Row 2 | D3 (PD3) |
| Keypad Row 3 | D4 (PD4) |
| Keypad Row 4 | D5 (PD5) |
| Keypad Col 1 | A0 (PC0) |
| Keypad Col 2 | A1 (PC1) |
| Keypad Col 3 | A2 (PC2) |
| Keypad Col 4 | A3 (PC3) |
[Add a wiring diagram image — make one in Fritzing or annotate a clear photo:]
Firmware
This project is written in pure C. The logic is divided into modular files:
main.c- The system router.calculator.c- FSM calculator logic.lcd_i2c.c- Custom bare-metal I2C driver for the display.keypad.c- Matrix scanning and debouncing.
To flash it, you can use avr-gcc with avrdude, or simply open the project in PlatformIO / Microchip Studio and hit upload.
Bill of Materials (BOM)
| # | Part | Qty | Link | Approx. price |
|---|---|---|---|---|
| 1 | Arduino Uno (used as dev board) | 1 | link | ~20zł |
| 2 | 4x4 matrix keypad | 1 | link | ~12zł |
| 3 | 16x2 character LCD (with I2C backpack) | 1 | link | ~25zł |
| 4 | Jumper wires (M-M / M-F) | ~15 | link | ~30zł |
| 5 | Random screws | 8 | [link] | FREE |
| 6 | 3D-printed enclosure | 1 | cad/ | filament |
Build notes
When I started building this project, I thought it was going to be easy—until I learned how hard it is to write raw asm bare-metal code for an LCD display originally designed for 16 parallel pins! That was the moment I switched my approach, utilized an I2C backpack, and structured the whole system in clean C99.
Another huge learning curve was physics: mechanical switch bouncing. Writing a custom matrix scanner that properly filters out both press and release noise without lagging the CPU was a major milestone for this build.
License
MIT License. Feel free to use the code and CAD files for your own desk companions!
Similar repos
Languages
C
72.3%
Assembly
22.3%
Makefile
5.3%
See what people are saying
README
Kitty Calculator
A small desk companion — a calculator built entirely from scratch in pure C99 for the ATmega328P.
What it does
Works like a normal calculator but with a dangerously-cute looking style
Demo
Photos
Enclosure in CAD
First Prototype
How it works
This project steps away from the standard Arduino (.ino) environment and relies on bare-metal C99 programming (AVR libc) to maximize performance and demonstrate what happens under the hood.
- Logic (FSM): The calculator logic isn't a messy
if/elseblock inmain.c. It's decoupled intocalculator.cand operates as a Finite State Machine (FSM) with an accumulator and a display register, mimicking real Casio chips. - Hardware Debouncing: Keypad inputs are heavily debounced in software (20ms delay logic on state change) directly in the scanning matrix, effectively ignoring both press-bounce and the notoriously tricky release-bounce.
- Direct Register Access: Uses direct port manipulation (
PORTC,PORTD, etc.) instead of slowdigitalWrite()wrappers for keypad scanning.
Wiring
The project uses an I2C module for the LCD to save pins, leaving enough GPIOs for the 4x4 matrix keypad.
| Component / Pin | ATmega328P (Arduino Uno) Pin |
|---|---|
| I2C LCD SDA | A4 (PC4) |
| I2C LCD SCL | A5 (PC5) |
| I2C LCD VCC | 5V |
| I2C LCD GND | GND |
| Keypad Row 1 | D2 (PD2) |
| Keypad Row 2 | D3 (PD3) |
| Keypad Row 3 | D4 (PD4) |
| Keypad Row 4 | D5 (PD5) |
| Keypad Col 1 | A0 (PC0) |
| Keypad Col 2 | A1 (PC1) |
| Keypad Col 3 | A2 (PC2) |
| Keypad Col 4 | A3 (PC3) |
[Add a wiring diagram image — make one in Fritzing or annotate a clear photo:]
Firmware
This project is written in pure C. The logic is divided into modular files:
main.c- The system router.calculator.c- FSM calculator logic.lcd_i2c.c- Custom bare-metal I2C driver for the display.keypad.c- Matrix scanning and debouncing.
To flash it, you can use avr-gcc with avrdude, or simply open the project in PlatformIO / Microchip Studio and hit upload.
Bill of Materials (BOM)
| # | Part | Qty | Link | Approx. price |
|---|---|---|---|---|
| 1 | Arduino Uno (used as dev board) | 1 | link | ~20zł |
| 2 | 4x4 matrix keypad | 1 | link | ~12zł |
| 3 | 16x2 character LCD (with I2C backpack) | 1 | link | ~25zł |
| 4 | Jumper wires (M-M / M-F) | ~15 | link | ~30zł |
| 5 | Random screws | 8 | [link] | FREE |
| 6 | 3D-printed enclosure | 1 | cad/ | filament |
Build notes
When I started building this project, I thought it was going to be easy—until I learned how hard it is to write raw asm bare-metal code for an LCD display originally designed for 16 parallel pins! That was the moment I switched my approach, utilized an I2C backpack, and structured the whole system in clean C99.
Another huge learning curve was physics: mechanical switch bouncing. Writing a custom matrix scanner that properly filters out both press and release noise without lagging the CPU was a major milestone for this build.
License
MIT License. Feel free to use the code and CAD files for your own desk companions!
Similar repos
Languages
C
72.3%
Assembly
22.3%
Makefile
5.3%