A 6502 SoC Project using a Spartan 3 FPGA

ElEctric_EyE · Post by **ElEctric_EyE** » Tue Jan 04, 2011 1:15 pm

I agree, if I first read "6502 Multi-Core..", I would've thought that too.

Arlet wrote:

How are you producing the graphics ? I didn't see a VGA module on your list, and I didn't really understand the reference to PS2 with respect to the graphics software...

No VGA core, yet. I really wanted to keep everything moderately simple. Simple is fast. Besides connecting multiple I/O cores onto an FPGA, one of the main goals is to see top speed of your CPU design. Although, after refreshing my memory, and rereading BigEd's post after you added BCD mode here, I see the expected speed @~54MHz. That will go perfectly with the SSD1963, as that top speed is ~50MHz. Should make for some impressive 6502 controlled graphics methinks!

So I am relying on a hardware controlled TFT display for the video. It's a 640x480 pixel display with 18-bit color. I will have to explain abit more about the controller later, it's very nice. But for anyone interested it's an SSD1963.

I have 2 fonts, 3x5 & C-64. I use only 16 colors, same colors as the C-64, in a lookup table. Also, there are 7 sizes. These are the character attributes.

When you type on the keyboard, you see the characters. This much I've accomplished.

I'm working on the last piece of 6502 software for my PWA project in the hardware section. It is a hex dump from a banked 2Mx8 SRAM to the display. That PWA project is going to morph into a 65816 controlled system. I'll update that thread soon as well...

But as I said, I didn't want this effort into the software to be wasted, so what better way to use a known successful chunk of software to test the 6502SoC, especially since it is already using 1 of the I/O interfaces.

Earlier I said, this could take 6 months to a year. I don't anticipate getting the 6502 core and the PS2 core "up and running" to take very long, after the wiring.

Nothing is even on paper yet, but my little mind's eye is picturing 5 IC's:

The XC3S400 Spartan 3
The FPGA PROM
A 2Mx8 SRAM
A 512Kx8 EEPROM
An I2C programmable oscillator

Here's a block diag of what I'm thinking. As in the PWA, the "OS" will be copied from slow EEPROM @F1(2.5MHz), and run from SRAM @F2(20MHz+). The 2GB SD will hold "test data". I'm thinking bitmap pic info that can be readily copied from the PC to the SD card. Then plugged into 6502SoC and outputted to the display to test the SD core.

CPU core chosen. I'll list and credit the other cores next...

edit: Added more info after, "No VGA yet,..."

Arlet · Post by **Arlet** » Tue Jan 04, 2011 5:28 pm

Well, if you're interested, I don't think it would be too hard to add a VGA interface directly in the FPGA. I already have some verilog lying around from some earlier projects that I could clean up and donate. See below for a picture of my spartan-3 eval board, hooked up to a test board with a QVGA panel, showing some test font.

The frame buffer is stored in an SRAM chip (included on spartan-3 board). The SRAM interface ran at 100 MHz, but the pixel clock was only 25 MHz. I only had a simple state machine instead of a CPU to write the data. It would be an option to raise the pixel clock to 50 MHz, which would allow something like 800x600, combined with a 50 MHz 6502, alternating access cycles on the SRAM.

Lower speeds are possible too, of course.

ETA: advantage of a home-built VGA controller is that you can implement different color modes, or implement some 2D acceleration features like a bit blit.

6502.org wrote:

Image no longer available: http://ladybug.xs4all.nl/arlet/fpga/qvga.jpg

BigEd · Post by **BigEd** » Tue Jan 04, 2011 6:07 pm

(skipping over Arlet's interesting VGA story and kind offer...)

On the boot rom front, it would be tempting to ditch the conventional EEPROM and use an i2c EEPROM - 256kbit for $2 especially if you have an i2c port already, and an SD card interface nearby. The EEPROM only has to contain a bootstrap which should be small and stable, just enough to load application or OS code from SD. Actually if the bootstrap is small enough you could place it inside the FPGA, which is even simpler.

Arlet · Post by **Arlet** » Tue Jan 04, 2011 6:30 pm

BigEd wrote:

Actually if the bootstrap is small enough you could place it inside the FPGA, which is even simpler.

Yes, there's plenty of room inside the block RAMs, and you can preload them with code, use as a bootloader, and afterwards use them as regular RAM.

ElEctric_EyE · Post by **ElEctric_EyE** » Tue Jan 04, 2011 7:42 pm

All in good time my friends. I'm not as advanced as you guys might think! I have to start from a 'known' foundation. However, it's nice to throw ideas out there, so I will do the same.

As far as VGA, looking at some of the cheapest LCD monitor prices on newegg.com, it may be worth looking into as it would be more universal.

I've seen some HDL code for a VGA controller on FPGA4Fun website, and it looked pretty short. Not sure how to interface it to the 6502 though? What about your code Arlet? I could use the display I'm now using for development purposes, and have the VGA part as an add-on later...

I haven't tested I2C core yet, so I need to start small with that DS1085 programmable freq. generator. As far as RAM though, how much block ram would be available? Maybe I could get rid of the external 2Mx8 SRAM?

(Drank too much coffee this morning and my brain is fizzled out, so I can't even finish up my hex dump software)

Arlet · Post by **Arlet** » Tue Jan 04, 2011 8:30 pm

The XC3S400 you mention has 16 block RAMs of 2KB each. Each block RAM is a dual ported memory, and can be configured as 9, 18, or 36 bits wide. You can combine multiple blocks to make deeper or wider RAMs.

If you want on-board VGA controller, you'll need external SRAM for graphics frame buffer. If you just want text, you can run VGA from block RAM. Here's one of my very first projects with my Spartan-3 eval board, using 2 block RAMs. One for text buffer, the other for character ROM: http://ladybug.xs4all.nl/arlet/fpga/text2.jpg, because all block RAMs are dual ported, the other port is free for easy CPU access.

With my graphics version of the VGA controller, the data comes from external SRAM, and the 6502 can write to same SRAM. This is done with a dual port SRAM controller. From the 6502's perspective, part of the memory map would be mapped to the video memory. As you store data in that memory, it'll show up directly on the screen.

What kind of hardware are you planning to use ? Some standard FPGA board, or something home made ?

ElEctric_EyE · Post by **ElEctric_EyE** » Tue Jan 04, 2011 9:23 pm

Arlet wrote:

The XC3S400 you mention has 16 block RAMs of 2KB each...

I didn't think it would have been that limited. The SSD1963 LCD controller has 1.2MB of built in RAM. I'm sure it's some sort of FPGA. It's only 144-pin QFP... But anyway, that scraps the VGA idea. Only because I intend to eventually have 4-layer 3.5"x2.8" mini-pro boards made by ExpressPCB. So chip count needs to be as low as possible at this point.

Arlet wrote:

...What kind of hardware are you planning to use ? Some standard FPGA board, or something home made ?

Home made using wirewrap. I already experimented soldering the XC3S400 208-pin QFP using a PGA adapter and wire wrap headers. Been sitting on an anti-static pad gathering dust!

Edit: Corrected BGA to PGA!

kc5tja · Post by **kc5tja** » Tue Jan 04, 2011 9:45 pm

The LCD panel probably has a custom ASIC which is made from standard cores found on FPGAs. I know, when I worked at Hifn, our engineers used Verilog to design ASICs using a lot of the same cores as were found in FPGA libraries.

I don't know any of the details on how they did this though, as I worked in a different department (I was post-silicon, not pre-silicon).

Arlet · Post by **Arlet** » Wed Jan 05, 2011 9:40 am

ElEctric_EyE wrote:

Home made using wirewrap. I already experimented soldering the XC3S400 208-pin QFP using a PGA adapter and wire wrap headers. Been sitting on an anti-static pad gathering dust!

Ok, well, I may still work on this stuff when I have some free time. My goal would be something that runs on a simple FPGA eval board, and that has a 6502 core, video output, keyboard input, and whatever seems useful, so it can be used as basis for other projects.

ElEctric_EyE · Post by **ElEctric_EyE** » Wed Jan 05, 2011 9:24 pm

I have the same desire, but wish to make custom boards in the end, and save money by not purchasing an eval board.

Couple questions though...

Which version of Spartan 3 is on your eval board?
What kind of eval board do you have?

BigEd · Post by **BigEd** » Wed Jan 05, 2011 9:27 pm

Would it be good to have a thread with a first post which tabulates various dev boards, with links and price and maybe some indication of capability?

A bit like the 6502 FPGA core thread...

Of course it would need revisiting once or twice a year as things change, but if it carries a date that's no problem.

Cheers
Ed

Edit: oops, mistyped 6502 and caused a Terbium fest

ElEctric_EyE · Post by **ElEctric_EyE** » Wed Jan 05, 2011 9:54 pm

Interesting read, a few good links there about the 65832...

history:
http://homepage.mac.com/jorgechamorro/a ... /65xxx.txt
spec sheet:
http://www.obelisk.demon.co.uk/files/W65C832.pdf

But Biged, aren't most of the Xilinx dev boards made by digilent?

BigEd · Post by **BigEd** » Wed Jan 05, 2011 10:05 pm

ElEctric_EyE wrote:

But Biged, aren't most of the Xilinx dev boards made by digilent?

well, there are Xilinx' own offerings, and then there are Xess' products.

$249 http://www.xilinx.com/products/devkits/ ... P601-G.htm

http://www.xess.com/prods/prod035.php

And then, there will be various Altera-based products - I've settled on xilinx but of course others will have the opposite preference.

And then, there are modules like OHO's, which aren't dev boards but allow experimentation with minimal circuit and soldering expertise.

And, just checking at fpga4fun.com, I see there's an outfit called KNJN
http://www.knjn.com/FPGA-FX2.html

And, sparkfun have something: http://www.sparkfun.com/products/8458

So, amongst other things, getting an idea of which boards people have used and would recommend might be good!

Cheers
Ed

Edit: added quote to make this self-contained

Edit: see this new specific thread about dev boards

ElEctric_EyE · Post by **ElEctric_EyE** » Wed Jan 05, 2011 10:12 pm

I, for one, think that would definately be useful. I forgot about Xess...

I would be posting the other cores I plan on implementing, but opencores.org is down... So...

I'm gonna ramble on here, because I've been reading that previous thread about the 65032/832. Probably the reason it never got off the ground was anticipated lack of software support.

The 6502 has loads of useful software, be it assemblers, etc. When I made the jump to the 65816 recently, I become a little more restricted in taking advantage of the chip. Then the 65032/832? So what if you have a piece of silicon. Unfortunately, you can't develop with it unless there is software to support it. I think there is a shortage of M. Kowalski types around...

I guess what I'm saying is: It is nice to have backward hardware compatibility, but I shouldn't have to pay for a $500 software dev kit. I don't want a $500 IC either... Why not make the IC cost more and make the dev software free? I would pay >$50 <$100 per '816, if I had a good software product to develop it with...

Mass production doesn't start instantly... It all starts with the individual tinkerer. I think someone has lost sight of this in their accounting dep't formulas.

Look at the success of Xilinx FPGA's/CPLD's. Their software is readily available, and easy to use to develop their IC's, even without the need for evaluation boards...

Arlet · Post by **Arlet** » Thu Jan 06, 2011 7:41 am

ElEctric_EyE wrote:

I have the same desire, but wish to make custom boards in the end, and save money by not purchasing an eval board.

Which version of Spartan 3 is on your eval board?
What kind of eval board do you have?

I have this board, which I bought several years ago from Xilinx, and isn't in production anymore: http://www.xilinx.com/products/devkits/ ... -UNI-G.htm

It's got a X3S200 FPGA, 8-color VGA interface, PS2, RS232, 7-seg displays, 8 leds, switches, and 1MB worth of SRAM. It came with a parallel-JTAG cable, software, and documentation. I think it was $99 for the kit, which is a pretty good deal. It probably would have cost me more to buy all the components separately. The size of the FPGA is plenty for hobby projects, especially if you write most of the code yourself it will be hard to fill up even the small device. I don't think I ever got it more than 1/3rd full for a project that included a dual USB interface, 16 bit CPU and SDRAM controller.

It looks like digilent still sells this board, or at least something very similar for $109: http://www.digilentinc.com/Products/Det ... od=S3BOARD but they've dropped the free programming cable.