[coreboot] Disabling Auto-Refresh of DRAM Using Coreboot

[Zoran Stojsavljevic]

> Quantify data retention of unpowered memory.

Let us asses/investigate what is the technology of C (capacitor) in
the presented
diagram
<https://upload.wikimedia.org/wikipedia/commons/b/bd/DRAM_Cell_Structure_%28Model_of_Single_Circuit_Cell%29.PNG>
in
my previous email!

The capacitor in the stacked capacitor scheme is constructed above the
surface of the substrate. The capacitor is constructed from an
*oxide-nitride-oxide
(ONO) dielectric sandwiched in between two layers of polysilicon plates*
(the top plate is shared by all DRAM cells in an IC), and its shape can be
a rectangle, a cylinder, or some other more complex shape. There are two
basic variations of the stacked capacitor, based on its location relative
to the bitline—capacitor-over-bitline (COB) and capacitor-under-bitline
(CUB). In a former variation, the capacitor is underneath the bitline,
which is usually made of metal, and the bitline has a polysilicon contact
that extends downwards to connect it to the access transistor's source
terminal.

Well, let us search more (on the ONO - Oxide-Nitride-Oxide dielectric
material):

 *The commonly used technology for non-volatile Flash memory application
consists of a stacked-gate transistor with dual gates. The
Oxide-Nitride-Oxide (ONO) stacks constitute the inter-poly dielectric layer
between those gates.* These top and bottom polycrystalline silicon plates
are also known as the control gate (CG) and the floating gate (FG)
respectively. During read and write operation of a flash memory device, a
high electrical bias needs to be applied through the control gate in order
for electrons to be tunneled through the thin tunnel oxide towards the
floating gate which is surrounded by dielectric material.

Although the same material is used in DRAMs and FLASHes (as in one case for
dielectric in Cs, in other channel material for the FETs with dual gates),
the design of DRAMs and FLASHes are essentially very different, as I see.
It seems to me that in case of C, ONO is dielectric which holds the
capacitor charge, and leaks it through dielectric, in the case of dual gate
FETs we have here The Tunnel Effect, which captures some number of free
electrons inside the ONO.

Well... I am also curious (as you, Peter), what will be the retention time,
but, as a difference to you, I think that after maximum of 7.8us x 2 some
bits (maybe 5% of them, even less, but certainly more than 0.1%) will be
corrupted.

Now, after the quick analysis/assessment I made, now I understand why all
the DRAM companies are trying to pack future DDRs as FLASHes. Never came to
me before to investigate this... But there is always the first time
(courtesy of Mr, Chilingirian, Massachusetts Institute of Technology). ;-)

Zoran
_______

On Fri, Mar 24, 2017 at 12:19 PM, Peter Stuge <peter at stuge.se> wrote:

> Zoran Stojsavljevic wrote:
> > I am not sure what are you really trying to do,
>
> Quantify data retention of unpowered memory.
>
>
> > and, mostly WHY you are trying to do what you are trying to do?!
>
> To challenge the assumption that data is lost without power.
>
> It is an interesting area of research because that assumption - or
> simplification - is so widespread, although not at all correct.
>
>
> //Peter
>
> --
> coreboot mailing list: coreboot at coreboot.org
> https://www.coreboot.org/mailman/listinfo/coreboot
>
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