Monday, May 28, 2018

Building the Nestronic Prototype (Part 3)

(For background, please read the [Introduction] and [Architecture] blog posts.


I eventually got to the point where the schematic was mostly finalized and the whole thing was working on a breadboard. While a big achievement in and of itself, it was far from a finished product.

First, breadboards usually look kinda messy. After all, they tend to evolve as you tinker with the circuit design. Also, breadboard wires are never the exact right length for any given connection. While you can make a clean-looking breadboard, it often requires far more time and effort than one is willing to put in for a simple prototype.

Second, breadboard connections are a little flaky. Breathing on your circuit the wrong way can cause this wire or that resistor to have just enough of a contact issue that something starts or stops working reliably.

Third, breadboards are noisy and full of stray capacitance. I could detect clock signals from one end to the other. Digital connections sometimes had just enough noise to make the difference between a high and a low. And to quote Dave Jones, you sometimes have to "hold your tongue at the right angle" to get things to even start up correctly.

Breadboard Nestronic
At this point I exported the netlist for my schematic, pulled it into KiCad's PCB tool, and began the layout and routing process. After a lot of work, I eventually ended up with a complete board design. The core of this layout was the NES CPU section, with all its parallel bus traces. The rest was kinda squeezed in around it. Perhaps not the most optimal of layouts, but good for the general size and shape of the enclosure I wanted to put it in.

PCB Layout