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:
| Memory Range | Function |
|---|---|
| $0000-$7EFF | RAM |
| $7F00-$7F3F | VIA (65C22 - parallel I/O) |
| $7F40-$7F7F | EXSEL1 (External Select #1) (Active Low) |
| $7F80-$7FBF | ACIA (65C51 - Serial port) |
| $7FC0-$7FFF | LCD |
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.
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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.
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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.