DRM65, a 6502 computer in a FPGA

SIMRETRO FPGA board (green) connected to a programming board (red), and to a VGA monitor

What is it? Details  here: drm65.pdf   Sources here: drm65.tgz

DRM65 is the design of a complete 8-bit computer in a FPGA board. It was more or less inspired by David Murray's "8-bit Dream Computer", but I'm not a big fan of Commodore, so this is a quite different (and a lot cheaper) design with these guidelines:

• Simple hardware without legacy, hard to get, parts. It is integrated inside the FPGA and all signals are 3.3V
• VGA monitor, PS2 keyboard, and PWM audio for human interface. SPI flash and/or SD card for storage.
• It should run BASIC.

The FPGA board was designed by Angel Díaz (tutored by Jesús Arias) to emulate these kind of computers and includes:

• An ICE40HX4K FPGA from Lattice with 7680 logic cells (not 3520, as stated in the family data sheet)
• An SPI flash memory for FPGA configuration and storage (N25Q64 from Micron mounted)
• A 64K x 16 bit SRAM
• VGA interface, 4-bit (16 levels) for each color component
• PS2 keyboard interface
• Joystick interface
• Single-bit audio, with power amplifier. Capable of PWM modulation.
• SD card connector (uses SPI mode)
• USB micro-b connector for power input (and, maybe, also for USB device/host interface someday)
• Serial port (through programming connector)

An additional board was designed in order to program the SPI flash contents, to upload bitstreams to the FPGA directly, and to provide a serial port to the main FPGA board. This board isn't needed after the SPI flash is programmed.

The DRM65 in turn includes:

• A 6502 core designed by Arlet Ottens running at 25MHz.
• A video controller with:
• a 512 x 400, 1 bit per pixel graphic mode
• a 256 x 200, 4 bit per pixel graphic mode
• a 16 x 12 palette table.
• An internal 8KB memory for boot code
• A 7-page MMU (page size: 8KB) for the translation of virtual 64KB spaces into a physical 128KB memory
• A PS2 keyboard interface
• A 115200 bps UART interface
• PWM audio (31250 Samples/s, 8-bit samples)
• Interrupts on UART RX, UART TX, keyboard scan code, video line, video frame, and single-step execution.

The contents of the internal memory are loaded along the FPGA configuration bitstream. This startup code has the following features:

• Interrupt-driven keyboard and serial port drivers.
• Full 6502 disassembler and debugger. Invoked by typing <crtl>-C in the serial port or <ctrl>-V in the keyboard.
• Two text modes for the 512 x 400 video mode: 64 and 85 columns
• 42 column for the 256 x 200 video mode
• Fixed address jump table for I/O routines and graphics.
• Can load programs via serial port or from the SPI flash.

Some applications were also developed for this computer, including:

• EH-Basic port from the Lee Davison's source. (No graphics, poke and peek them if needed ;) interrupt with <ctrl>-X )
• Picture display and sound playing form SPI flash
• A DOS-command-like file explorer for SD cards (limited to non SD-HC cards with FAT-16)
• A Tetris game compiled form C sources with cc65

And here are some screenshots of these programs running:

• Selecting files to load from SPI flash: