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RE: [N8VEM-S100:991] 512MB DRAM / S-100 Card

One thing that struck me Mike, perhaps we don’t have to run DDRAM chips at anything close to their "normal specs".  Do they work for example on only one clock transition. 
As to word size, we may just want to consider wasting half the capacity. We would be still ahead in terms of board capacity.

Thanks so much for looking into all this.  I will be delighted to take on the project if you could get it to a workable schematic. 

Also we may want to branch off the group and do e-mails directly back and forth for now and come back with a final outcome. 
Anybody else "listening" or would like to be involved please send me your e-mail address.
Terry, Rob, Paul could you send me your e-mail addresses. Any others?

John



John Monahan Ph.D
e-mail: mon...@vitasoft.org
Text:    mon...@txt.att.net


-----Original Message-----
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of mike
Sent: Tuesday, July 17, 2012 12:09 PM
To: n8vem...@googlegroups.com
Subject: Re: [N8VEM-S100:991] 512MB DRAM / S-100 Card


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Yes, I agree, let's chew on that for a little bit and see if we can't come up with something.

I was actually thinking along those same lines myself as well the last couple of days, but wanted to finish fleshing out the other option first so we have at least one realizable plan under our belt.

I don't know with any degree of certainty what it would take to interface with modern DDR memory. I'd have to catch up on the intimate details of modern DDR memory controllers.

What I do know is that timing relationships are more complex for sure because you have the data coming in (or out) on both the rising and falling clock edges...etc...

It seems that most of the DDR and DDR2 controllers that you can purchase are implemented in a FPGA and there are a number of them available apparently, I know little about them (i.e. to they do voltage level translation, and so on?)..

If I'm not mistaken DDR I/O operates in the ~2.5V range and DDR2 in the ~1.5V range (I don't think we should even look at DDR3). So there's that issue to contend with as well.

Also data is transferred 64 bits at a time. So how to contend with mapping a 64 bit bus to a 32 bit bus.

I'm sure I'm just scratching the surface of the issues related to interfacing with these bad boys.

In any case, to answer your previous question, yes, I am pretty much up to my neck, but I would be willing to commit to doing some further research and a first pass of the ram board schematic, beyond that, I'll do what I can.

- --Mike

On 07/17/2012 02:03 PM, John Monahan wrote:
> Mike thinking some more, before we go for this route are you sure 
> there is not a way to refresh one modern day PC style Gigabyte DDRAM 
> type SIMM with a crude binary counter slamming wait states back to the 
> CPU in the process.  The clock speeds we would be using are way lower 
> that what these DRAMS are expecting.  It seem such a shame to have to 
> be using 4X132 pin chips and 8 DRAM SIMMS with what is available 
> today.
> 
> John


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