adumont / hb6502

My Homebrew 6502 breadboard computer with my own #FORTH implementation

Homebrew 6502 SBC

• Homebrew 6502 SBC
• Introduction
• Hardware
  • Schematics
  • Memory Map
• Software
  • Monitor
  • AlexFORTH for 6502
• Datasheets
• References

Introduction

This project is about building a single board 65c02 computer.

It's powered by a WDC 65C02s CPU driven by a 1.8432MHz oscillator clock, with 32KB of SRAM (only 16K used), 32K of ROM, a 65c22 VIA interface, a 6551 ACIA serial interface, and an 20x4 LCD screen.

The ROM is programmed using my self-made EEPROM programmer.

Hardware

Schematics

Schematics in PDF format.

Memory Map

The address decoding logic is following the one described by Garth Wilson in his 6502 Primer, Address decoding page. The corresponding schematics is:

(picture by Garth Wilson from his 6502 Primer)

Address	Component
$0000-$3FFF	RAM (16KB)
$4200-$4203	ACIA
$6000-$600F	VIA
$8000-$FFFF	ROM (32KB)

Software

Monitor

At the moment the monitor is very rudimentary, but it serves its purpose.

Monitor will show content at ADDR. You can type:

• a new ADDR (4 char, hex): this will set ADDR to the new value, and display the content. For ex. 04F0
• a value (2 char, hex): this will store the value at ADDR. For ex. FB
• ' followed by a char: store the char at ADDR. For ex. 'A
• j restore registers to saved values and jump to ADDR
• c continue (after a BRK). restore registers to saved values, set ADDR to saved PC (after BRK) and jump to ADDR

Registers manipulation (saved values):

• aXX : store XX (hex) in A
• xXX : store XX (hex) in X
• yXX : store XX (hex) in Y
• sXX : store XX (hex) in S
• pXX : store XX (hex) in P
• r : print registers value

Early demo of the Monitor in Kowalksi 6502 simulator:

AlexFORTH for 6502

I have implemented my own FORTH kernel for this Homebrew 6502.

Read more here hb6502/forth.

Datasheets

• W65C02S 8–bit Microprocessor
• W65C22S Versatile Interface Adapter (VIA)
• HD44780U LCD Controller/Driver
• UM61256FK-15 32K x 8 Bit SRAM
• AT28C256 32K x8 Paged Parallel EEPROM
• HD44780U LCD Controller/Driver
• SN74HC00 Quadruple 2-Input NAND Gates
• SN74HC138 3-Line To 8-Line Decoders/Demultiplexers
• SN74HC245 Octal Bus Transceivers With 3-State Outputs

References

• Build a 6502 computer | Ben Eater
• 6502 PRIMER: Building your own 6502 computer
  • Address Decoding
• ca65 Users Guide
• 6502 Instruction Set