The next step for my 6502 project is to convert it from bread boards to a PCB (printed
circuit board). There are many benefits to moving the project from bread boards to a PCB. For example it looks more professional, it is more stable as wires wont fall out, and it makes it easier to transport. The most important reason for me was it will make it easier to add additional chips/boards such as a sound board or vga (video) card.
(picture of my 6502 bread board computer that I am trying to convert to a PCB)
The first step was just to learn how to make a PCB. I decided to use a free program called EasyEDA. EasyEDA is free and you can use it in your web browser. There also is a downloadable version which I used. I decided for my first test board I was going to make an sr latch.
The first step to designing a PCB is to make a schematic. For this circuit I had 1 chip, the 7400 (this would cause problems), 2 buttons for input, 2 LEDs for output, and pins for 5v power and ground.
After making the schematic I know have to lay out the components on a board. My board was very simple so it was easy to place the components where I wanted. Then I used the auto router to route the traces (red and blue lines).
Once I was satisfied with he design I used JLC PCB to purchase the boards. It was very cheap, $2 for all 5 boards. After waiting a week I received my boards in the mail. I was so excited to test the boards but I didn't have a soldering iron, so I but in each component and bent the wires. Once I built the board this way I tested it. It didn't work which made me sad. I started thinking of all of the possible problems. The first that came to mind was because I didn't solder on the components there wasn't a stable connection and this was leading to issues. I then also thought about how the buttons weren't grounded so when the button wasn't pressed it was outputting no signal. This could lead to some problems. Both of these assumptions were correct. When I soldered everything together there was no flicker and I also added on some resistors to ground the buttons. Even though this was improvement, it didn't fully fix the problem. The board still wasn't acting like an sr latch. I then did some testing on a breadboard and figured out I couldn't used the 7400 chip. Instead I needed to use the 7402. When I was testing on the breadboard I was able to switch the two chips without changing the wire. Unfortunately it didn't work out that easily on the chip. My wiring on my breadboard was ever so slightly different from the wiring on my board leading to issues. I decided I was going to have to redesign the chip and fix all of the issues.
Here was the new schematic. It had a different chip (7402 instead of 7400), it grounded all of the buttons, and it had more inputs and outputs. It also had some luxury features such as a power switch and a power indicator LED.
Here was the layout of the board. It was slightly bigger than the first version. It also has holes for standoffs (to elevate the board over a table)
I ordered the boards and they arrived early! I was so excited and quickly soldered one together. Surprisingly it worked the first time! I think it looks very professional and I love the matte black. One problem is that the power LED is a bit bright.
Now that I was confident in my PCB design skills I started to work my a PCB for my 6502 computer.
As it can be seen above, the schematic is way more complicated than the earlier boards. It took a lot longer and it is slightly different than my actual 6502 bread board computer. It has some additional features such as the ability to disable ram using an input pin. This may seem random but it will allow me to do memory mapped IO. This will make it very easy to add extra components such as my sound chip.
Because this PCB had so many more components I went through many different designs for the board. As seen below I have shown 3 different versions. Each version is pretty similar but they have slight changes. The changes could be aesthetic or fixing silly mistakes.
Once I found a design I was happy with I sent it off to JLC PCB. Below you can see the board I ordered! It is a 3d model of my board so the real thing will be slightly different. The real one will be matte black because I think that looks the coolest.
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