Subject: Re: [stella] Re: 2600's TIA|
From: Kevin Horton <khorton@xxxxxxxxxxx>
Date: Tue, 28 Aug 2001 00:28:01 -0500
On Mon, 27 Aug 2001, Kevin Horton wrote:
> The CLK input line would be A0, and the divide by 6 would generate A1,2,
> and 3. the ROM would be 12 bits wide, and 12 words deep. Each bit line
> could be used to generate the required phases by "storing" a square wave (6
> 0's then 6 1's in a row); each off by 30 degrees (or exactly 1 ROM location).
Yeah, ROM lookup table are Good Stuff(tm). The need for 12 words is obvious. But why 12 bits? Do you need to generate 12 signals? I don't know anything at all about NES architecture. Oh...duh. Nevermind I got it. When I do this type of thing to create a phased waveform, I usually use a loadable barrel shifter driving a regular shift register. Was in the wrong mindset for ROM lookups. :)
> This same idea could of course be condensed down into logic, too. This is > more likely than a lookup table which is "expensive" in regards to chip > real estate.
Yep. ROMs are good for experimenting with until you know exactly what you want.
> If you're seeing a large AC signal, that tells me you didn't ground your > scope, so you're seeing the AC line on it due to the open ground loop. Fix > that and check again.
No, it really was just a ripple. Like the chroma in a composite NTSC signal. A couple hundred mV at most. It had a crawling phase was crawling, but it was very frequency stable. And it wasn't at 60Hz or 120Hz...it was somewhere around 7.5MHz. So it wasn't line noise from poor grounding. I don't remember
- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/