ABNielsen.com 6502 SBC r0

Thank you so much to everyone who followed and liked this project. 

Even though this project is finished, the journey is far from over. 

The project page for Revision 1 has been updated with lots of logs and I hope you will all check it out here: https://hackaday.io/project/184725-abn6502-sbc-r1

-Anders

I can proudly announce the project made it the hackaday.com blog right before it was officially completed and superseded by Revision 1. 

https://hackaday.com/2022/04/05/wireless-bootloader-saves-you-from-swapping-rom-chips/

All my efforts will now be focused on the new project page - you're welcome to follow that too :)

https://hackaday.io/project/184725-abn6502-sbc-r1 

Github has been updated with R1 of my SBC.

https://github.com/AndersBNielsen/abn6502

Since this actually turned out to be a complete rerouting of the whole board, I finally got rid of the - now unobtainium - PMS171b, and the firmware also got quite an overhaul I'm finishing up this project and moving on... To R1! 

R1 will get it's own project on hackaday.io because they are simply too different - naming-wise I'll just call it R1 :) 

Check out one of the upgrades here - as you see the board is quite different:

I'll post a final log here when R1 has it's own project page. 

As some people have noticed I managed to rainbow some WS2812B LED's with my SBC.

Since I'm writing a wireless bootloader(surprise!) I think I will take the rainbow code and move it to "userland" instead of keeping it as a permanent part of the ROM..  Or maybe I will keep the basic "send pixel" subroutines so the firmware will still reset LED's on boot and you can call subroutine addresses from userland by simply setting R G and B zero page locations.

Either way I don't feel code organization is my strong side and the current zero page layout can really use a manual defragmentation. 

I always meant for my SBC to have a simple "monitor" firmware that is barely able to let the user type in assembly programs. 

I have a feeling the upper limit of the firmware will be a(very) simple resident assembler + disassembler - typing mnemonics straight to RAM might be "fun". 

As for hardware changes I still haven't had a chance to test the "VGA" output reset modification  and I really want to switch out the 4 bit counters with tristate 8 bit counters ( 74HC590's should work.. Right?) so I only need '245's on 6502 side... And for now sadly the PMS171B since I can't tristate that either.  That thing really has to go :) 

Lot's of things to do and not much time to do them...

I always tend try to get things into production fast. This has a tendency to involve limited redundant information - said another way: I don't add silkscreen information already present on the schematic.

 
I believe I've reached a point where I know another revision of this board is coming eventually , so I decided to add some silkscreen text - if nothing else, so I don't forget that the broken out data and address busses have LSB on the left when the VGA port is pointing away from you :) 

Note: The Gerbers.zip hasn't been updated this time - if you are about to send them off to manufacture, maybe replot them from Kicad :) 

Yay! I finally pulled myself together and made a short video about my 6502 SBC :)

The new revision on Github has a few differences to the last tested version. 

On r0 pin 15 of U20 (Chip enable of the VGA signal) is connected to ground. On r1 it is connected to the inverted reset signal(through a spare inverter on U14) through a pullup resistor. 

Technically this should make it possible to RC-delay the VGA output signal until the VRAM is cleared by code - instead of having random data in the blanking periods that could damage a real CRT screen. 

If this was really ~1979 I would definitely implement the VGA blanking periods in hardware.

The project files are now on Github!

This included Kicad board files

https://github.com/AndersBNielsen/abn6502

Photos of my SBC have now been uploaded - on r0 traces have been cut and A14 bodged to the RAM OE.

Not that I'm particularly proud of my 6502 assembly, but I just managed to remove enough stub code that it only does the bare minimum - and doesn't have half-written subroutines for things coming in the future. 

I guess it's ready for github :)