[Coco] the amazing memory persistance

[Luis Antoniosi (CoCoDemus)]

I tried to put a resistor on VCC and GND with no avail.

Then I replaced the chips with the ones that came with the coco3 and
now it retains data for only 5 seconds. So it's the memory chip which
is retaining data.

I've heard this is common with NMOS drams and it becomes worse with the time.

I noticed also that this DRAM has a different "default" pattern when
completely drained. It will create several horizontal lines (00 00
00..00 FF FF FF .. FF) (as you can see in the demo) opposed to
vertical strips 00 FF 00 FF as I get with the coco3 drams. It might be
related to RAS/CAS adressing ?

But since the coco don't clear the memory and relies on position 113
to determine if its a cold or warm boot, if the memory if filled with
FF what I get ? warm boot ? Didn't test that.


On Mon, Dec 9, 2013 at 11:24 AM, John Kowalski <sockmaster at gmail.com> wrote:
> Luis Antoniosi (CoCoDemus) wrote:
>>in case you don't believe me:
>>http://www.youtube.com/watch?v=vno_T2-FP2E
>>what can i do to solve this ? Put a resistors some place?
>
> That's pretty insane.  I would suspect that it's the capacitors
> internal to the DRAM itself that are holding their charges so long
> that it's acting like battery backed-up memory.
>
> I've had a CoCo that retained it's memory for a couple of seconds, but
> this is just astounding.
>
> The chips are MB81464?? You could try maybe placing a 1 KOhm resistor
> across pins 9 and 18 of (just) one of the memory chips and see if that
> changes anything, but that might not help if the DRAM's internal
> capacitors are isolated from the power input.   If it doesn't, maybe
> try the resistor across pins 2 and 18 to see the charge could be
> drained from one of the data lines when it's powered off.  If this
> works but only slightly, you may try adding a resistor to each chip -
> and then if still only slightly, one for each data line (between pins
> 2&18, 3&18, 15&18, 17&18).
>
> --
> John Kowalski / Sock Master
> http://users.aei.ca/twilight/sock
>
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