An ATmega2560 with 8 KB of RAM can support a multitasking desktop environment, complete with games, file management and video playback. That is what Vladimir demonstrated by building a DIY laptop around an Arduino-compatible board with an ATmega2560, the Mega 2560 Pro Mini. The device measures 12x11x3.4 cm and packs a QWERTY keyboard, a 128×64 monochrome display, audio, microSD, an RTC and an 18650 battery into a 3D-printed enclosure. The firmware runs on FreeRTOS and offers real multitasking, with task switching via Alt-Tab. So a video that has been started keeps playing its audio while you play Sokoban.

The architecture of independent tasks is the core of the project. Applications receive events and draw through the UI engine, while the shell handles registration, stacks, switching and events. As a result, adding a new application does not require touching the others. A new task is a class with setup and loop plus a couple of methods for its menu entry and icon.

The UI engine and the applications on FreeRTOS

The interface engine handles forms, buttons, checkboxes, lists, input fields and dialogs. It takes care of key presses, tabbing, redrawing and the events sent to the owning task. On top of it run the ready-made applications:

• file manager
• video player
• music player
• clock
• calculator
• Sokoban
• Snake
• Game of Life
• text editor

The ST7920 display is driven in parallel mode with a custom-written driver and a 1 KB video buffer in SRAM. The graphics functions range from a single pixel to text and XBM images. The AVR microcontroller runs at 16 MHz and everything runs on 8 KB of RAM, half of which is already taken up by global variables. That is why every byte of the buffer counts.

8×8 keyboard, debouncing and a 4-to-16 decoder

The keyboard has 64 keys in total, arranged in an 8×8 matrix that requires two 8-bit ports of the microcontroller. The keys were laid out in an 11×6 rectangle. Scanning happens column by column at hundreds of hertz, with software debouncing. In one revision a diode was added for each key and column selection moved to a 4-to-16 decoder. This way four pins are enough to select a column and the keyboard can expand to 16 columns.

The matrix rows are currently 6, but by reading a whole port of the microcontroller they can be brought to 8. The key labels were printed with a 0.25 mm nozzle. For the audio and memory side, the project relies on components that have a direct counterpart in the catalogue: an audio decoding shield for compressed formats, a microSD card reader with an SPI interface for the files, and a board that combines an ATmega2560 with Wi-Fi connectivity for anyone who would rather start from a different base.

24 fps video and XBM format plus MP3 in a single file

The video player uses a custom format with an XBM stream and MP3 in a single file. This way the player comfortably handles 24 fps video with audio, without keeping more in RAM than needed. The rest of the board is taken up by the RTC and the 18650 battery, which make the device self-contained. The enclosure was machined with a CNC3018 and 3D printed, while the schematics were drawn in EasyEDA.

Anyone who wants to rebuild the project will find the story of the work and the information to recreate it on the project page on Hackaday.io signed by Vladimir. Alternatively, an enclosure for an Arduino Mega can serve as a dimensional reference for the electronics, even though this laptop’s case is printed to measure.

Source: https://hackaday.io/project/206744-avr-laptop

Related products

• VS1053 MP3 Shield for Arduino
• MicroSD Card Reader module with SPI interface
• Board with ATMEGA2560 and ESP8266 Wi-Fi SoC Module

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