[Coco] [Color Computer] Re: POKE 65495,0

[jdaggett at gate.net]

On 4 Feb 2007 at 4:21, James Diffendaffer wrote:

> Well, I just ran a test on my E (I think) board Coco 1 and it ran over
> 8 seconds in high speed mode but it was a small BASIC program so I'm
> not sure if all RAM is ok or if it would consistently work with that
> long of a period with refresh off.
> 
> As for me having "good dram"... we have no record of what good or bad
> DRAM is capable of so there's no basis for that statement. We do know
> some Coco's can't use this mode but we don't know if it's the DRAM of
> the board design.
> 
> 
***********

James

DRAM chips are specified for a period of time in which  the memory cell needs to 
be refreshed. The basic construction of a dynamic ram cell bit is a well something 
like a capacitor that holds charge and a NMOS transistor that acts as a switch. 
The well that stores charge has a leakage path through the p type material of the 
IC substrate to gorund. That leaskage path in combinatin with the low value of 
capacitance that storage well presents has a defined period in which charge is 
bled off to a level that is in the undefined region of standard TTL range for logic 
levels. Depnding on IC construction this period is between 4 milliseconds and 
about 100 milliiseconds. 

By "good" I mean that  you have a set of ram chips that hold the well charge 
longer than the specified time period. Still I have reservations that any dram can 
retain a logic "one" charge for upwards to 8 seconds without refresh. Also I have 
to point out that there is a remote possiblity that there is an error in the MC6883 
data sheet. That is to say that switching to high speed may "not" disable the 
refresh grants. That is the first t hing that comes to mind when  you report such 
long periods of dram retention. 

james

 
> ---------------------------------------------------------
> On 3 Feb 2007 at 22:37, James Diffendaffer wrote:
> 
> 
> > I was able to run my Coco 1 in FAST mode for several seconds before
> 
> > DRAM started to lose data... not a few milliseconds. I think it took
> 
> > well over 4.5 seconds for RAM to start to corrupt but I haven't run
> 
> > the tests since I was 16.
> 
> >
> 
> ***********
> 
> You had good dram then. Because your dram didn't loose data does not
> preclude that all dram will work after 4.5 seconds of no refresh .
> 
> james
> 
> 
> 
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