[sdiy] SRAM question

[David Brown]

The S in SRAM stands for static.  It does not need to be 
refreshed.  A battery will keep the ram powered on.  Some type of 
circuit is typically needed to power the ram from the logic supply 
when on.  On older 5 volt systems, often times the battery would be 
at a lower voltage, say 3.75 volts.  A couple of diodes would be used 
to form an "or" between the battery and logic voltage.  This makes 
sure that none of the input signals are ever at a higher voltage than 
the ram.  If the battery was rechargable, then the charging circuit 
could provide this function.

Since the ram is kept alive, special attention is needed on the 
inputs so a random write does not occur on power down or power 
up.  Sometimes pullup resistors are added to the inputs to make sure 
they are pulled "down" on power down to reduce power 
consumption.  Very special attention is given to the write signal to 
make sure it doesn't glitch.  Sometimes this can be a specific driver 
that behaves well, or in some cases, a signal is provided from the 
power supply that indicates mains voltage has been lost.  This signal 
can be used to qualify the write signal.

Dave

At 09:36 AM 2/19/2010, cheater cheater wrote:
>I think the SRAM doesn't need to be refreshed? So basically you just
>keep voltage to the chip and that's it.
>
>But, bear in mind I know nothing of electronics. =)
>
>D.
>
>On Fri, Feb 19, 2010 at 18:25, Ian Smith <taciturn_unquiet at hotmail.com> wrote:
> >
> > Hey all,
> >
> > For those of you that are still here and haven't unsubed and gone 
> over to the yahoo list, I have a question about SRAM chips and 
> batteries for memory.
> >
> > How exactly does this work? using a battery to retain the data 
> saved in the chip?
> >
> > Thanks,
> >
> > Ian (still here) Smith