🚀 go-pugleaf

    It is *incredibly* easy to build a small backup circuit for static RAM.
    You simply use a 0.27F (that's FARAD) capacitor for the 'backup'
    supply.  The RAM power bus is connected to the main board +5V (BAR) via
    two transistors and a couple of resistors (voltage drop is only about
    0.01V between main BAR and RAM BAR).  Or, to skimp on parts you can
    drop incomming power through a diode to the main bus and incomming
    power through a diode to the static RAM (this prevents the backup power
    from trying to drive the rest of the board).  Using diodes uses more
    power though since you loose 0.7V.

    A single 0.27F capacitor (footprint is a little larger than a dime)
    will backup 8 32Kx8 static RAMs for a heck of a long time -- several
    days easy.	A static RAM can generally go down to 1.5V when inactive
    before it starts to loose its mind.

    You must do a couple of things (even with FRAMs, most of these apply):

    (1) you probably want to ensure that all tri-stated pins are pulled up
    or down (usually w/ 100K resistors) to reduce wear on the drivers.

    (2) Any inputs must brought low or high (usually low) via a 100K
    resistor to prevent oscillations (CMOS/HCMOS parts).

    (3) the RAMs must not be chip-selected during a power down.  If they
    are, you will generally wipe the entire RAM to garbage instead of just
    a single location.

    (4) Any ICs that run off the backup supply my never output a '1' state
    to ICs that do NOT run off the backup supply when main power is off.
    This is because static protection diodes will turn that '1' into a
    short since they are connected to BAR and GND.

    And, as has been mentioned, there must always be some kind of ROM or
    EPROM, even if it is only an extremely small amount, to handle a cold
    boot situation.

    A CMOS or HCMOS part will generally pull picoamps when in standby (no
    inputs changing), certainly not more than a microamp.  Most static RAMs
    are CMOS.



In article <1156@teslab.lab.OZ> andrew@teslab.lab.OZ (Andrew Phillips) writes:
>In article <183539cc.ARN0ccd@Chucla.CAM.ORG> stephane@Chucla.CAM.ORG writes:
>>   Not so long ago I remember reading about a new kind of RAM, called
>>FRAM.  It looked like a new technology but then again, I don't know
>>much about memories...	The authors of the paper (Sorry, I can't remember
>>who they were) basically stated that their FRAMs were similar to a EEPROM,
>>but with reads and writes at DRAM speed.
>
>Yes, FRAM is non-volatile RAM.  I believe the technology is not very
>mature with only very small capacity chips currently shipping
>(16Kb?).  Also FRAM is much more radiation hardened which has caused
>a lot of military interest.
>
>>   This memory, if available, would make a great replacement for ROM/EPROM
>>chips or true recoverable ram disks.
>
>There will always be a (small?) need for non-writeable memory and ROM
>will probably always be cheaper to produce.  But FRAM will eventually
>do away with the need for hard disks.
>--
>Andrew Phillips (andrew@teslab.lab.oz.au) Phone +61 (Aust) 2 (Sydney) 289 8712

					    -Matt
--


    Matthew Dillon	    dillon@Overload.Berkeley.CA.US
    891 Regal Rd.	    uunet.uu.net!overload!dillon
    Berkeley, Ca. 94708
    USA