SamCo's Single Board Computer (65C02)

This single board computer was constructed for fun. It has the following specifications:

• 65C02 processor
• 32K RAM (with 256 addresses reserved for I/O)
• 32K ROM
• 65C22 Versatile Interface Adapter (VIA)
• 65C51 Asynchronous Communications Interface Adapter (ACIA)
• 2x16 LCD
• Support for USB or external 5V power

After trying out writing programs in assembly and in BASIC, I found the Tali Forth 2 project, and that's what I've been using as an operating system/programming language ever since.

The memory map is currently as follows:

The EXSEL1 line is brought out to the expansion bus on the righ side of the PCB and is available for use. The EXSEL2 line is an active-high version of the /RESET signal. Both of these signals are not currently used by the SBC and can be repurposed by reprogramming the GAL.

The memory decoding is handled by a GAL22V10, programmed from the ADDECODE.PLD file. I used Wincupl (run under Wine on Linux) to generate the .jed (JEDEC) file, and then I used Galblast on an old Windows 98 machine with a parallel port adapter to program the GAL.

• Because I used a WDC65C22S (which has a totem-pole IRQ output rather than the WDC65C22N which has an open drain IRQ output,) I had to cut the IRQ trace and add a Schottky diode (cathode towards pin 21 of IC6) across the gap.

• I needed to add approximately 30pF of capacitance to the ACIA crystal to get it to start up and oscillate reliably. No provision was made on the board for this, so I added it on the back side of the board between pins 1 and 6. I also needed to add a 1Meg Ohm resistor across the crystal pins when using an RCA CDP65C51 (but did not need this when using a Rockwell part.)

The Eagle schematic board files are provided here under a Creative Commons BY-SA 2.0 (CC BY-SA 2.0) license.