8-bit CPU: design the PCBs

The idea is clear, now the hard part: design the boards.

I've been long time CatSoft Eagle user, but since they've been acquired by Autodesk, things have gone downhill. It did not even wanted to start on my machine (I use Linux as my primary OS). Well, so long and thanks for all the fish! KiCAD seems to be popular in the community, let's learn something new.

As planned before, I started with ALU module. The schematics part was quite easy, there are no space constraints when drawing that. PCBs on the other hand...

After board outline was set and connectors placed, it started to feel quite terrifying. Can I really fit ALL THAT on this small board? Eventually I did...

Next I have to route the tracks, a lot of them. Felt a little lazy to do it all by hand and wanted to see what autorouter produces. Turns out, there is no such feature in KiCAD. Everybody appears to to hate autorouters!

There was an option to export the board, autoroute it in FreeRouting and then import it back. The board was complicated enough and autorouter struggled to find a solution. Eventually it did, but I had to move chips around, swap gates and input pins. Not sure if it saved me any time.

ALU done, next Register and Flags. Boards are less complex, but it still took some effort to do them. I still had some hopes for autorouter's help, but it was not easy anyway. Not sure if I'll bother with it next time.

Here's how the register will look like when it's done (front and back).

 
 
 

It actually takes quite an effort to design a board like this, especially when space is tight. Well, I chose those constraints for myself.

I decided also to design a PCB version of the EEPROM burner - as a shield for Arduino Uno. That should be more stable and convenient to use, and I'll be able to reclaim the breadboard. Added additional headers for external connections - it should double as a control logic for module testing. One more extra - a PLCC socket for smaller footprint EEPROMs.

Ordered the boards at JLCPCB, should get them quite soon. Hopefully, the parts from Farnell will arrive roughly at same time.

Comments

Post a Comment

Popular posts from this blog

8-bit CPU: ALU and Flags

Image
Now that I have a pair of devices that can store a byte, I can proceed to do something more interesting. Build a device that add them together, for example. In other words - let's build an ALU. The design follows same basic idea as Ben's - two 4-bit adders, XOR gates and an output buffer. I'm using HC logic here, so I used 74HC283, 74HC86 and 74HC245 chips. Since I've built only one Register and did not want to build another, I connected my Program Counter board as register B. If I keep the counting function disabled, it serves the purpose just fine. It took a lot of wires, but I'm just getting started, let's build Flags register as well.  There were no surprises with Zero flag - 4 NOR and 3 AND gates does the job - just following the Ben's design. Another latch (74HC173) chip to store it, done. The fun starts with the Carry flag: I wanted to support a carry-in signal for the ALU. It is very useful if one wants to perform the addition or subtraction for wid...
Read more

8-bit CPU: Clock module

Image
The parts are finally here an I can have some fun! Since nothing runs without clock, I decided to start with that module. It's probably not the most fun one, but I definitely need it, before doing something else. I was planning to follow Ben's design to some point. I had some TS555CN timers at home, so I decided to try those first, before opening the package with the new ones I ordered. Wired up the 555 in basic astable configuration with capacitors, potentiometer and decided to test if it works. So I connected a LED to the output and powered it up. It worked. Well, sort of. LED blinked and I  could adjust the frequency, but then suddenly it started to blink very fast, then again back to normal. What's even stranger I could trigger or suppress the effect by touching either chip or capacitor (just top of the chip, not the leads). I did accidentally connect my power backwards on first try 😓, so I figured it caused some damage or something. Replaced the chip with one of my n...
Read more

8-bit CPU: Memory

Image
Time for another important part of the computer - memory. Here I think I have to come up with my own design. Ben's version has only 16 bytes, uses nowadays-exotic 74189 memory chips and there are DIP switches for programming. For the memory I plan to use 62256 SRAM chip. It can store 32 KiB of data, I initially plan to use just 256 bytes, so I'll connect remaining address pins to GND. I'm also not in a mood for any DIP switch based programming, so I'll add an EEPROM instead. There are several ways how computer can utilize such setup: split the address space into program and data memory. This is how Ben's 6502 breadboard computer works. Some additional logic gates are used for chip selection, the main issue however is that it is not very flexible. use Harvard architecture , where program and data memories are completely separate. Reading and fetching instruction are two different operations: former accesses RAM, latter - ROM. This is how AVR microcontrollers (heart ...
Read more