Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!cbmvax!daveh From: daveh@cbmvax.commodore.com (Dave Haynie) Newsgroups: comp.sys.amiga.hardware Subject: Re: A3000/16 vs. A3000/25 non-CPU speed Message-ID: <19639@cbmvax.commodore.com> Date: 8 Mar 91 17:44:23 GMT References: <4101.27d2e586@miavx1.acs.muohio.edu> Reply-To: daveh@cbmvax.commodore.com (Dave Haynie) Organization: Commodore, West Chester, PA Lines: 34 In article holgerl@amiux.UUCP (Holger Lubitz) writes: >In article <4101.27d2e586@miavx1.acs.muohio.edu> jwwalden@miavx1.acs.muohio.edu (Darc Tangent) writes: >>I know that the new >>custom chips in the A3000 are clocked at the same rate as the main processor >This is the first time I read about custom chips running at a higher rate >than 14.32 MHz (and even slightly slower in PAL Amigas) OK. The original "Amiga" chips in the A3000, Agnus, Denise, and Paula, work from a two phase clock, nominally 7MHz -- 7.16MHz for NTSC, 7.09MHz for PAL. Since the two clocks, called C7M and CDAC, are 90 degrees apart, the chips are effectively clock at 14MHz, and the basic chip memory cycle is equivalent to a 14MHz 68000 cycle. The new A3000 system chips, Buster, DMAC, Gary, and Ramsey, are clocked by the CPU clock (16MHz or 25MHz). At least Buster and Ramsey are actually clock by this clock, called CPUCLK, and another 25MHz clock delayed by 90 degrees, called CLK90. Buster actually uses both edges of both clocks at various points in the logic. There are a few reasons for using a two phase clock rather than a single clock running at double speed. First of all, the 68030 runs from the 25MHz clock, so it's much easier to feed that same clock to the custom chips that need it than it is to build a 50MHz clock that's exactly synchronized to that 25MHz clock. Secondly, real fast clocks are harder to keep clean on the PCB, and noisier when it comes time for FCC testing. Finally, the gate array technology used for these parts is much happier with a couple 25MHz clocks on input than a 50MHz clock. From the point of view of the actual circuit design, you really don't care. -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy "What works for me might work for you" -Jimmy Buffett