qwertykeyboard wrote:
Now, I'm reading over the 6502 primer and having trouble understand how clock generation works. What are the different phase clocks for and how come there isn't just a single clock pin like on the Z80? Is it possible to hook up a 555 timer at a few hertz to the 6502? because the primer has no mention of this.
My apologies if this isn't the clearest it could be. I have a lot of work to do today and I'm in a hurry.
For simple systems, phase 2 is all you need. The primer's
clock-generation page gives a few clock options. I'm not sure the 555 will have adequately fast edges (ie, how quickly it gets from high to low, or vice-versa, regardless of frequency) for the 65c02's being produced today. A couple of members here ran into trouble with inadequate slew rate when using WDC processors. Also, heed what it says there about non-WDC 65c02's needing to stay below a certain maximum time on the phase-2-low time, meaning you would not be able to use a 1Hz square wave for example. (The WDC ones can be stopped indefinitely with phase 2 either high or low; but other brands can only be stopped with phase 2 high.) There's a circuit there for single-clocking which lets you stop indefinitely in any cycle, but only with phase 2 high, and the phase-2-low time stays below the maximum allowable time for Rockwell, CMD, Synertek, and other manufacturers. For an oscillator, a crystal oscillator can is usually the best way to go. The
circuit at the top of the circuit potpourri page shows different clock options too. The notes below it tell what you need to know (particularly Note 1 in this case). Someone suggested separating out the different options of that part of the diagram, with individual very small diagrams of just the clock circuit, and offered to do it for me so I could edit the page and insert this part with less work; so that may be coming.
Quote:
Honestly, the goal of this is just to build a computer completely from scratch to understand the architecture. I was planning to use a breadboard and slowly add stuff like RAM and I/O as time goes on to see where that takes me.
I do not encourage using solderless breadboards for this kind of work. Many people do it, and more than half of these creations work, but that still leaves an awful lot that don't work, and an awful lot of frustrated builders. One of the chapters in the primer is about wire-wrap. Unfortunately WW supplies have gotten more expensive in the last ten years; but you can certain get a circuit that's better behaved with WW. A custom PCB is best (assuming it's laid out well), but not everyone can go that route. None of the chapters in the primer are frivolous. I put a lot of time into making it as concise as I could, answering questions and problems that kept coming up on the forum years ago, and I frequently make little edits to improve it. The sections are also organized more or less in a logical order; so I recommend going through it that way, including paying attention to the section on avoiding AC performance problems. That way if you do feel a solderless breadboard is your only option at this point, hopefully you can still do it in a way that will get working results.
Best of luck, and again, my apologies for having to punch this reply out without taking the time to make it more clear.