[Coco] FPGA CoCos

[Dave Philipsen]

No, no. I'm just enhancing a project that Grant Searle initiated.  It's a 6809 synthesized computer on FPGA. His original design was on Cyclone II development board that can be purchased on eBay for around 16 bucks.  You add a power supply, DB15 connector, PS/2 keyboard DIN connector, SRAM, some resistors and you have a working computer with BASIC. Unfortunately, there is no practical way to save/load programs and the display is a character-based display with the IBM PC / CGA character set.

What I did was to port the project over to a DE1 board (actually I've got it running on three different foundations) and wrote some code to take advantage of the 512K SRAM. The MMU that I created is 100% equivalent to the MMU on the CoCo 3 but that's as far as it goes for compatibility.  I also interfaced a real-time-clock module I bought of eBay designed for the Arduino. In addition, I'm planning on a boot ROM in serial EEPROM (24FC512).

Currently still working on some timing tweaks for the onboard 512K SRAM of the DE1. The Alliance AS6C4008 SRAM on both of my wire-wrap proto board works perfectly, however. My plans are to get the SD card working so that I can load/save programs and data. One idea I have is to create hundreds of blank fixed-size files on the SD card which could be mounted or unmounted to the system appearing as a floppy. In theory it would be possible to use a stock Disk Basic ROM and just trap the accesses to the FD controller and relate them to the SD card.

Dave Philipsen

> On May 23, 2015, at 2:02 PM, Barry Nelson <barry.nelson at amobiledevice.com> wrote:
> 
> So, I am to understand that you have a fully working emulated CoCo 3 on a modern FPGA board? What is supported? Does sound work? What about graphics? What graphics modes are supported? Can I plug legacy joy sticks into it? Where can the files be obtained to program it? How dos disk access currently work?
> 
>> After about 3 hours of trial and error this amateur VHDL coder got it 
>> figured out! (I'm sure a more experienced person would have figured it 
>> out in no time)  A few very minor changes to Grant Searle's project  
>> (which was written for a pretty standard 128K x 8 SRAM) makes it work on 
>> the new DE1 board.  The 256K x 16 SRAM on the DE1 now looks like 512K x 
>> 8 to the Multicomp project and my MMU which works on another board with 
>> a 512K x 8 RAM now works on the DE1.
>> 
>> Next issue to tackle: make a file system to access the SD Card in FAT 16 
>> and eventually FAT32 formats.
> 
> 
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