warning: LOTS OF TEXT AND IMAGES AHEAD!

This is my prototype 6502 8-Bit Computer. The first of many to come! My project is called the 8x2 Captain, a 8-Bit Computer with 2, 8-bit CPUs for more efficient and stable performance. However, I have a long way to go before I can even create a concept schematic for such a device! The Prototype I've designed is called the 8x1 Trooper because it only has 1 CPU.

Before I go on and describe this Prototype Motherboard and it's features, I want to talk about my inspirations for starting this project. After watching YouTubers like 8-Bit Guy, Adrian's Digital Basement and Ben Eater. I had this feeling that I was missing out on amazing Retro Technology! like I have nostalgia for something I've never experienced before. And it's that feeling that drove me to do some amounts of research on the 6502 CPU and some of the hardware on 8-Bit computers and general!

The schematic I've created is based on Several sources, all of which I'll link below:

Dan Grice YT video and Schematics on GitHub
Wilson Mines Co. 6502 Primer
Texas Instruments Video Display Processor Data Manual
Both the MAX3100 and MAX3223 Datasheets
And some information from replies from my previous posts!

The specs and hardware features of the 8x1 Trooper goes as follows:

SPECS:
CPU: W65c02S
VIA: W65C22S
EPROM: Windbond W27c512 64K x 8
SRAM (x2): 62256LP-70 32K x 8

VDP: TMS9918A
VRAM (DRAM) (x8): Mostek MK4116N-2 16,384 x 1-Bit (16K x 1)

Serial UART: MAX3100
Serial Driver and Receiver: MAX3223

HARDWARE FEATURES:
Address Decoder that allows the use for 64K of memory
Components: 74HC688 + 74HC00

VDP Select Logic
Components: 74HC238 + 74HC00

Video read/ Write Timing and Logic
Components: 74LS74 + 74HC165 + 74HC00

Overall System Specs:
CPU: W65c02S
ROM: 64K
RAM: 64K
VDP: TMS9918A
VRAM: 16K

Power Supply
ABLEGRID 12 Volt Power Supply - 2.5 Amp Standard (12V 2.5A DC)

That's the list with all of the ICs on the board, I'll leave an image of the schematics below!

I made the schematics and PCB designs in EasyEDA. Here are the PCB trace widths below:

+12v: 1.060 mm (Current: 2.5A)
+5v: 0.781 mm (Current: 2A)
-5v: 0.781mm (Current: 2A)
everything else: 0.300 mm (Current: 1A)

Now, here are some questions I have regarding my schematics and PCB designs!

1. On several places on the PCB, CPU to Logic IC connections are borrowed by the pins on the 3 CPU memory chips. For an example, Pin A8 is connected to U7 (74HC688) and the Memory Chips. U7's trace connection to A8 is borrowed from the A8 Pin on the ROM chip. Is that a good way to connect some of the logic chips to the CPU? or is the current going to be higher for the Memory Chips and lower for the Logic ICs like U7?
   
2. Do the traces connecting to and from the Crystal Oscillators need to be thicker?
   
3. U13 (MAX3223) uses 5 Capacitors to convert the 3.3v serial voltage to 5v for TTL and CMOS circuits. Do the traces to these Caps need to be thicker as well?
   
4. According to a comment on my previous Post, I can use a 6522 VIA to interface with a MAX3100 (U11) UART. While I was reading and looking through the Datasheet for the MAX3100, I saw a Typical Operating Circuit (Page 23) and a Quick Brown Fox Generator (page 17) that shows that the MAX3100 needs a 4-Bit bus to work. However, Pin 6 SHDN, the Hardware-Shutdown Input is in none of the schematics or similar diagrams in the datasheet, leaving me confused whether or not to tie it to Port A on the VIA just like the other pins (1 through 4). According to Page 5, when Pin 6 is pulled low, It turns off the Crystal to lower the voltage current. Is it alright to tie Pin 6 (SHDN) to PA4 from the VIA?
   
5. The voltage Regulator I'm using to convert 12v to 5v is the classic 7805! However, It's voltage current output is 1.5A... I feel like that's barely enough for a 6502 Computer like this. Is that enough current for my system, if not is there another voltage regulator IC i should use that use that has higher current output than 1.5A?
   
6. Since I can't find any 64k SRAM chips, I decided to use 2 HM62256 32K SRAM chips and just connect them together. However, I'm doubting if that'll add the memory size of both chips tp 32K for some reason. I've seen other 8-Bit computers from the 1970's and 1980's multiple SRAM chips before, but they where probably not connected to the 6502 CPU like this. Will connecting 2 SRAM chips like this increase the 32K of one SRAM chip 64K by adding another SRAM chip? Both SRAM chips have the same exact connections.
   
7. The 6522 VIA on my System Board has a 20 Pin Female Header, So I can connect jumper wires to a breadboard to interact and experiment with Hardware I'd like to add to the next Prototype System Board. But PA0, 1, 2, 3 and 4 are connected to the UART that's used for the Serial Port. Is it okay to use those pins on Port A for something else as long as they aren't being programmed for the serial UART, or should I just use the pins for Port B for experimenting?
   
8. There are these small TFT LCD screens that are used for Microcontroller projects, Cameras and other electronic products. Can the TMS9918A to output it's video Output to a TFT LCD module? Can I made an external device that has a TFT LCD screen and some sort of composite video decoder to display graphics from the VDP? The reason why ask this is because testing the graphics will be easier if I use those TFT screens.
   
9. near the top left of the board, Between VR1, VR2 and VDC1 there is a Via (An element of a PCB that allows a Trace to go from the top layer, to the bottom layer of a PCB) that allows the top 3 VRAM chips to have connectivity to GND. Is this Via bad for a PCB like this? On a similar note, There is a Via on a +5V line right under the CPU that connects to the RDY pin, Is that bad for the +5V supply traces?

My PCB designs and schematics are attached to this post blow!

That's it for my Post here! I'm sorry if there is a Lot to read, I just want to make sure I didn't many any mistakes and I needed some questions I have answered. Any help, Information and Advice will be thanked and appreciated in Advanced!

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

A few points:

First, as you are asking about wiring a pair of 32KB static RAM chips in parallel and not understanding what the result is (32KB vs 64KB) tends to highlight that you don't quite understand how this works.

Second, while I've not looked at the various links you reference for design influence, combining parts of other disparate designs without a detailed knowledge of how everything should work, generally results in nothing working as expected.

Third, as this is an obvious first build, my advice would be to scale a first project to a much smaller and less complicated circuit and less complicated parts to use.

I think you need to take a more detailed look at your memory map. You should also list the memory map, showing where you address the static RAM, the ROM and the various I/O devices, as an example:

RAM could start from $0000 and go up to $7FFF, which is 32KB.
ROM could start from $8000 and go up to $FFFF, which is also 32KB.

Now you need to pick an address for your I/O window, which will take addresses away from either RAM or ROM. Also note you can only access 64KB of total addressing... which includes all RAM, ROM and I/O devices.

An I/O window could be from $FE00 - $FEFF, which is 256 bytes. Then you need to further decode the 256 bytes I/O window to provide chip selects for the 6522 and the video controller. Also, is the goal to have direct access of the video memory from the 6502?

Also realize that the 6502 requires a 16-bit address vector at $FFFC/$FFFD after reset which tells the CPU where to start executing code.... which in your system (and most others) will be in ROM.

_________________
Regards, KM
https://github.com/floobydust

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I'm going to join in on the chorus of advising to slow down and simplify.

Your first build, of a trusted design, may well not work. You'll be learning how to debug, on something which is simple and is trusted.

Your first build, of your own design, probably will not work. You'll be learning how to debug, on a design which might not even be right in the first place.

Adapting a comment I made elsewhere:

I want to echo other's sentiments, that this seems like a complex design for your first build, particularly the video circuitry. Far be it from me to try to dissuade your enthusiasm, but you'd probably be better off getting the "computer" part working and talking over serial before worrying about video outputs.

I haven't had a chance to do a thorough inspection of your schematic, but a couple of things that immediately jumped out on a glance:

1. You've got the MAX3100 (UART)'s s IRQ output and the W65C22S (VIA)'s IRQ output tied together. Normally you can tie IRQ pins together on devices where the IRQ pin is "open-drain", as it is on the MAX3100. However the W65C22S's IRQ output is totem-pole and therefore you cannot do that in this case. As it is, what will happen is when the MAX3100 wants to indicate an interrupt whilst the VIA is not, you'll create a short between power and ground through the VIA to the MAX3100. This is not good. Best case the interrupt won't work, worst case you'll cook some chips! Additionally, you need a pull-up on the IRQ line for open-drain pins, because open-drain outputs only pull low: When they're inactive you need a pull-up otherwise it'll just float.

The classic solution here would be to put a pull-up (3.3K ohm would be a reasonable value) on the UART's IRQ output, and then run the VIA and the UART's IRQ signals separately into an AND gate, the output of which is now your IRQ input to your CPU.

Another solution that doesn't involving adding another gate, would be to cascade the UART's IRQ into the CB2 or CA2 inputs on the VIA, as these can be configured as interrupt inputs. Then when the UART triggers its IRQ, that cascades into the VIA triggering the VIA to interrupt too. Obviously you'll have to check the VIA's IFR register to see which device triggered the interrupt, but you'd have to do that anyway with the other configuration.

2. 7805 is absolutely fine for your power regulator, but as others have mentioned it is extremely inefficient and, at higher currents, will dissipate a lot of heat. You probably won't get to the point where it matters, but if you did you'd need a big heatsink and, again, its very inefficient so if you're on e.g. battery power, you're wasting a lot of your charge. Switching regulators are much better, but harder to implement, however RECOM make drop-in 7805-replacement switching regulator modules, which I've used before and highly recommend for hobbyist use.

_________________
Want to design a PCB for your project? I strongly recommend KiCad. Its free, its multiplatform, and its easy to learn!
Also, I maintain KiCad libraries of Retro Computing and Arduino components you might find useful.

NOPE! The CPU needs to access VRAM through the VDP. It says so in the Data Manual for the VDP, though the CPU needs to send just 1 byte to the VDP, and tell the VDP when it is writing to VRAM via the MODE, SCW and CSR pins on the VDP itself.


I'll make a detailed memory map when I'm 100% done designing the schematic and PCB


Knew something felt off while making the schematic ... thx for the heads up!

Thx for the info and lovley advice! I'll follow through the best I can!

How do I just use half of a 128KB SRAM chip?


The TMS9918A requires DRAM. It can automatically allocate it's own VRAM and Refresh the DRAM chips on it's own!


It outputs Composite Video, Is it still possible for the VDP to desplay video on an LCD? even If I build another device that decodes or converts the Composite Video signal into a signal that can be used for LCD screens?

Thank you very much for the info and advice! I'll try to implement as much as I can from what I learned from this!

Alright. The only part that I don't 100% understand on this schematic is the Video RW circuitry. Do you know a simplier way to create the Video WR circuitry?

Just tie off one of the SRAM's address pins to either high or low. It doesn't actually matter which voltage level, or even which address pin, but "typically" most people would tie the highest order address bit - A16 in this case - to ground.

_________________
Want to design a PCB for your project? I strongly recommend KiCad. Its free, its multiplatform, and its easy to learn!
Also, I maintain KiCad libraries of Retro Computing and Arduino components you might find useful.

Hmm. In five minutes - let's say twenty - you could write ten lines of text - let's say twenty - which explain your memory map. And that would help you - and also help us - in understanding what you intend to build and how you intend to build it. No schematic or PCB is needed. A tried and tested approach is that the design comes first, the implementation second.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

THank u so much!

Thx for the advice