Hi
In NOR-based flash memory, once a bit has
been set to 0, only by erasing the entire block
it can be changed back to 1. I am eager
to know the reason for that behaviour/design ?

Further, i wonder why can't NOR-based flash
memory offer arbitrary random-access rewrite
or erase operations ?

Any ideas ?

Thx in advans,
Karthik Balaguru

On Jan 2, 11:30=A0am, karthikbalaguru
<karthikbalagur...@gmail.com>
wrote:
> Hi
> In NOR-based flash memory, once a bit has
> been set to 0, only by erasing the entire block
> it can be changed back to 1. I am eager
> to know the reason for that behaviour/design ?
>
> Further, i wonder why can't NOR-based flash
> memory offer arbitrary random-access rewrite
> or erase operations ?
>
> Any ideas ?
>
> Thx in advans,
> Karthik Balaguru

The FPGA subgroup isn't the best place to ask about Flash
fundamentals.  Have you tried researching the issue?  It comes down to
how the floating gate transistors are arranged.  See for instance
http://www.eeherald.com/section/design-guide/esmod16.html for "Flash
memory basics and its interface to a processor" where you'll see how
the source lines (where word lines and bit lines also exist) are used
in the erase.

On Jan 2, 10:03=A0pm, John_H
<newsgr...@johnhandwork.com> wrote:
> On Jan 2, 11:30=A0am, karthikbalaguru <karthikbalagur...@gmail.com>
> wrote:
>
> > Hi
> > In NOR-based flash memory, once a bit has
> > been set to 0, only by erasing the entire block
> > it can be changed back to 1. I am eager
> > to know the reason for that behaviour/design ?
>
> > Further, i wonder why can't NOR-based flash
> > memory offer arbitrary random-access rewrite
> > or erase operations ?
>
> > Any ideas ?
>
> > Thx in advans,
> > Karthik Balaguru
>
> The FPGA subgroup isn't the best place to ask about Flash
> fundamentals. =A0Have you tried researching the issue? =A0

Okay, i have looped in comp.arch.embedded.
Yeah, I have been searching regarding this & got other
links but did not get a link that clearly conveys this info
and hence this query.

After lot of searches, got the below links -
http://electronics.howstuffworks.com/flash-memory.htm
http://www.embedded.com/columns/technicalinsights/165701775?_requestid=3D63=
5173

It seems that Flash memory uses in-circuit wiring
to apply the electric field either to the entire chip or to
predetermined sections known as blocks.
This erases the targeted area of the chip, which can
then be rewritten. Flash memory works much faster than
traditional EEPROMs because instead of erasing one
byte at a time, it erases a block or the entire chip, and
then rewrites it.

> It comes down to
> how the floating gate transistors are arranged. =A0See for instance
> http://www.eeherald.com/section/design-guide/esmod16.htmlfor "Flash
> memory basics and its interface to a processor" where you'll see how
> the source lines (where word lines and bit lines also exist) are used
> in the erase.

This link is good !
I understand that erasure operation is done through
Quantum tunneling.

Interesting to know that in NAND-flash, cells are
connected in series resembling a NAND gate, and
so the name. The series connection prevents the
cells from being programmed individually. These cells
must be read in series.

So, it finally boils down to the design of flash
memory that has dependency on predetermined
sections known as BLOCKS or entire chip for
its in-circuit wiring that applies electric field
either to the entire chip or to predetermined
sections known as BLOCKS !!

Thx,
Karthik Balaguru

karthikbalaguru
<k...@gmail.com> wrote:

> In NOR-based flash memory, once a bit has
> been set to 0, only by erasing the entire block
> it can be changed back to 1. I am eager
> to know the reason for that behaviour/design ?

First there was EPROM, which erased the whole chip with
an external UV light source.  Using avalanche breakdown
to allow writing (storing charge on the floating gate),
but no electronics to remove that charge.

For some time there were EEPROMs that were electrically
erasable, but the design was complicated and density not
so high.

The easy answer is that flash allows one to share 
the complication of the erase circuitry over a block. 
That allows for a significant increase in density.

Some years ago there were ideas of using magnetic bubbles
for non-volatile storage, and CCDs for volatile high density
serial access storage, but neither caught on.  

Flash allows for very high bit density at reasonable access rates.
Since many times block erase is needed, it fits in well with 
current needs.

-- glen