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From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Fix that baby! (RAM Board clarifications)
Message-ID: <7988@cbmvax.UUCP>
Date: 25 Sep 89 16:28:38 GMT
References: <840@unsvax.NEVADA.EDU>
Distribution: na
Organization: Commodore Technology, West Chester, PA
Lines: 53

in article <840@unsvax.NEVADA.EDU>, maniac@arrakis.nevada.edu (ERIC SCHWERTFEGER) says:
> Keywords: memory bus

> 	While it is true that the memory cycle on the amiga (for the cpu)
> is 560 ns, the CPU doesn't give the memory that much time to read.  

True -- it looks like this:

        +------------Cycle Time = 560ns---------+
        |                                       |

          s0   s1   s2   s3   s4   s5   s6   s7
        +----+    +----+    +----+    +----+    +---
        |    |    |    |    |    |    |    |    |
7M     -+    +----+    +----+    +----+    +----+

                       |                   |
                       +-------280ns-------+
                           Access Time


> Now, the data is expected in by the start of the fourth clock cycle.  This
> gives the memory 2 clock cycles, or 280 ns to respond.  

But, we are talking only _access_ time for memory, too, not just the 68000.
Both have cycle times, both have access times.

> Now, you can figure on another 100 ns of overhead for memory decoding, 
> buffering, etc.  

If you have 4 levels of buffering between the 68000 and the board (as with
autoconfigured boards), you might get at worst 50ns of delay.  It's not
even as bad as all that.  If you're building a DRAM board, you _could_
assert RAS* as soon as AS* falls, even before you're certain of SLAVE* 
being valid.  Give the DRAMs 180ns from RAS* (room for buffer delays and 
slop), and you're left with over 70ns from the time AS* is valid on your
board before RAS* need be asserted.  That, my friend, is FOREVER.

> In the Ram board I have (EXP -1000), I have a mix of 150 ns and 120 ns 
> memory, and unless I pull all my 150 ns chips, I need to let my Amiga 
> warm up for a few minutes in order to boot.

Like I've said before, it's up to the designer to insure his/her design
will work with any specific speed part.  You can certainly simplify your
design in many cases by requiring a faster part.  I'm sure you don't have
to pull the 150ns DRAM from your Amiga's motherboard to power up :-).

>  
> Eric Schwertfeger, UNLV, maniac@arakis.nevada.edu
-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
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                    Too much of everything is just enough