6502 + 6845 + SRAM

[jgharston]

I've been teaching myself how to use KiCAD over the last couple of weeks. After fighting to stop component pads overlapping tracks and trying to turn snap-to-grid off, I realised I'd set my board to be 35mm instead of 35cm.... All those decimal places bypassed my brain - who uses 0.01mm PCB measurements???

Anyway, I feel confident enough to tackle my personal project, YA602SBC. Inspired by things like Greant Serles minimal chip count systems, the core is 65xx02 + low RAM + high ROM&IO. However, I want to build a system that uses a 6845 to generate video timing, along with SRAM for memory rather than the complications of DRAM.

My initial sketches suggest going along these lines. Is that going along the right lines?

[Martin_H]

I'm confused by the chips in the middle of the diagram. What are they and what is their function?

[floobydust]

I believe the boxes represent some buffers/latches, like the 74LS244 and the upper being bidirectional like the 74LS245.

As for the basic circuit block level, it looks fairly high level. You'll also need a dot clock to drive the shift register (74LS166 ?) and the rest of the circuitry to provide horizontal and vertical sync to whatever display you're planning to drive.

Not sure if you might have it, but Gerry Kane's CRT Controller Handbook is an excellent resource and includes details on the 6845 and the 6545 CRTCs. I have both a hard-copy I bought long ago and also found a PDF of it many years ago as well.

I had done a design with the 6545 decades ago, which was hand-drawn on graph paper... no idea if I can find this, but it was planned to be an add-on to the Vic-20 and C64 machines. It was also pretty complete. but I was using a 2KB EPROM to hold a character set and another 2KB SRAM to hold the text screen data.

As your drawing shows that you'll be using SRAM as a graphic display, perhaps using a single 128KB SRAM might be useful, as you could have multiple "screen pages" by driving the upper address lines separately, just a thought.

[jgharston]

floobydust wrote:

I believe the boxes represent some buffers/latches, like the 74LS244 and the upper being bidirectional like the 74LS245.

Yes, I thought the triangles made it obvious.

floobydust wrote:

As your drawing shows that you'll be using SRAM as a graphic display, perhaps using a single 128KB SRAM might be useful, as you could have multiple "screen pages" by driving the upper address lines separately, just a thought.

Yes, I just shows 32K there for the display memory addressed by the 6845 and 6502, omitting the other address lines.

The main thrust is not wanting to the hassle addressing and selecting DRAM, the object is to have a 6502/6845 system to play with, not to be messing about designing and constructing messy memory systems.

It's like back in the '80s it was so frustrating that so much of my electronics construction projects seemed to end up 90% building a power supply. I had less than zero enthusiasm for building power supplies, I wanted to build the thing I wanted to build, not the damn power supply. Decades later I realised how much easier things would have been if wallwarts had existed.