Amethyst: an 8-bit home computer based on ATmega1284
Amethyst is a great project by Matt Sarnoff.
It’s like a classic home computer: it has an integrated keyboard and can generate audio and video and has a built-in high-level programming language for users to write their own programs.
It uses just six chips: an ATMEGA1284P CPU, a USB interface, and four simple integrated circuits.
- ATmega1284 microcontroller (8-bit) running at 14.318 MHz
- 16KB internal memory (16352 bytes available for applications)
- 4KB nonvolatile EEPROM memory
- Mechanical keyboard with Cherry MX keyswitches
NTSC color and monochrome composite video output:
- 256-color, 16-color, and 4-color bitmap graphics at resolutions up to 160×200 pixels
- Monochrome bitmap graphics at resolutions up to 640×200 pixels
- 40×25 and 80×25 monochrome text modes
- 40×25 color text mode
- 40×25 color tiled graphics modes
- Single-channel audio output (pulse wave or PWM)
- USB serial communication at speeds up to 57600 baud
- Four SPI expansion ports for peripherals, controllers, and storage devices
- Only 6 chips (ATmega1284, FT320X, 2x 74HC157, 2x 74HC166)
The ATMEGA1284P has 128 kilobytes of flash memory for program storage and 16 kB of RAM. It can run at up to 20 megahertz, comes with built-in serial-interface controllers, and has 32 digital input/output pins. 
Complete schematics, PCB files, and system code are available on Sarnoff’s GitHub repository :