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PICList Thread
'Serial RAM Connected to a PIC'
2000\02\15@150750 by Martin McCormick

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       How many PIC pins does it normally take to support a serial
RAM chip?  What are the easiest SRAMS to use with a PIC?

       I can imagine that one could get away with two wires (clock
and data) if the SRAM requires a word of X bits with the first bits
being an address and the last 8 or so bits being data, but I have
never used one of these yet so I don't know what is available.

       In the present project, I need a SRAM of at least 8192 bytes
or 65536 bits.

Martin McCormick

2000\02\15@152613 by Maverick

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<x-flowed>At 02:06 PM 2/15/2000 -0600, you wrote:
>         How many PIC pins does it normally take to support a serial
>RAM chip?  What are the easiest SRAMS to use with a PIC?

For that matter, does anyone actually know of a serial SRAM part that you
can actually find for sale?  I was looking for such a critter about a year
ago, but had little to no luck finding any such part that I could actually
buy anywhere.

ND Holmes
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2000\02\15@154708 by Sean Breheny

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One good source for just a few is old (386 or 486) motherboards with cache.
That's how I got the one I used in my PIC-O'Scope prototype. They are fast
and often (in earlier MBs) are DIP.

Sean


At 02:23 PM 2/15/00 -0600, you wrote:
{Quote hidden}

| Sean Breheny
| Amateur Radio Callsign: KA3YXM
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2000\02\15@161031 by Martin McCormick

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       That's great.  I have a couple of dead mother boards.  What am
I looking for?

       It seems kind of strange that they would use serial RAM for a
cache because the tradeoff between lots of wires is lots of clock
cycles.

Martin McCormick

Sean Breheny writes:
>One good source for just a few is old (386 or 486) motherboards with cache.
>That's how I got the one I used in my PIC-O'Scope prototype. They are fast
>and often (in earlier MBs) are DIP.

2000\02\15@161444 by jamesnewton

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Huh? x86 cache ram is not serial AFAIK. What mfgr and make are you looking
at and what do you mean by serial?

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{Original Message removed}

2000\02\15@164609 by Martin McCormick

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       It just occurred to me that we may have a nomenclature
problem.  I used the expression SRAM meaning Serial RAM.  Old X86
mother boards do have SRAM in their cache all right, but the S means
Static.  That is fine to know if we need to build something that has
some RAM in it and we don't want to have to deal with the refresh
problem that one has with DRAM or Dynamic RAM, so
what Sean said is correct but what I meant was the little 8-pin or so
serial RAM chips that do not use more than one pin for data and
possibly address information.  AS I previously said, you pay for that
in access time.  It will take at least 8 clock cycles to get or write
a byte and X clock cycles to address it depending on the width of the
address bus.

James Newton writes:
>Huh? x86 cache ram is not serial AFAIK. What mfgr and make are you looking
>at and what do you mean by serial?

2000\02\15@165430 by Sean Breheny

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Sorry, you are correct,I read the message too quickly and all I saw was
SRAM, I didn't see "Serial".

MB's are great for parallel SRAM, but not for serial ram. I was not aware
that serial RAM was commonly available. I have seen threads about this here
before and the general conclusion was that with a few rare exceptions, the
only serial memory which exists is either EEPROM, FLASH, or FRAM (the
ferromagnetic memory). None of these can withstand an extremely high number
of writes (and I think FRAM has a read limit, too).

It seems to me that some chip maker should start making serial static RAM.
It should be easy enough.

Sean

At 03:44 PM 2/15/00 -0600, you wrote:
{Quote hidden}

| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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2000\02\15@172719 by jamesnewton

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FRAM is supposedly not write limited and are available in serial (8Kx8) see:
http://www.ramtron.com
easy to interface to a PIC. Appnote at
http://www.ramtron.com/products/appnotes/EDTecAppNote.zip

Also of interest:

OKI "Serial voice register" series if 1 bit data registers: MSM6685 (8Mx1),
MSM6389C (1Mx1). Internal address counter provides continuous data stream
presetable in 1k blocks.
http://www.okisemi.com/communicator/public/fm/docs/SpeechTables-2.html

IDT 5V Sequential Access - Random Access Memory (SARAM) (4Kx16)
http://www.idt.com/products/pages/Multi-Port-PL98_Sub235_Dev204.html

More (including all I know) at:
http://techref.massmind.org/mem/srams.htm

Are there not Video Frame buffer chips or LIFO cache chips out there?

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{Original Message removed}

2000\02\15@194128 by John Orhan

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Hi all,
Can someone explain FRAM to me as I'm a newbie to the concept of
ferromagnetic RAM and how it works. Many thanks

John

{Original Message removed}

2000\02\15@195832 by Robert A. LaBudde

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<x-flowed>At 02:23 PM 2/15/00 -0600, you wrote:
>At 02:06 PM 2/15/2000 -0600, Nathan wrote:
>>         How many PIC pins does it normally take to support a serial
>>RAM chip?  What are the easiest SRAMS to use with a PIC?
>
>For that matter, does anyone actually know of a serial SRAM part that you
>can actually find for sale?  I was looking for such a critter about a year
>ago, but had little to no luck finding any such part that I could actually
>buy anywhere.

You can get them from Jameco Electronics (http://www.jameco.com) or DigiKey
(http://www.digikey.com).


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2000\02\15@204207 by Somasundaram Meiyappan

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Dallas Semiconductors have DS1200 - It is a Serial RAM wit 1KB memory. I am
not sure if there are serial RAM with 8KB. May be he can use 8 of them with
a chip select pins.

Regards,
Somasundaram Meiyappan

At 02:23 PM 2/15/00 -0600, you wrote:
{Quote hidden}

2000\02\15@205456 by Matt Burch

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At 07:02 AM 02/16/2000 +0500, Somasundaram Meiyappan wrote:
>
>Dallas Semiconductors have DS1200 - It is a Serial RAM wit 1KB memory.

A quick search seems to indicate that this is an obsoleted part - I
couldn't immediately find it on Dallas's website, and a search of
findchips.com reveals no distributors who have it in stock. Hmmm.

I have been wrestling with this problem ever since I found it's tough to
get ahold of small quantities of the Philips PCF8570 (I2C SRAM).  Maybe I
should take the Parallax approach and put an SMD Pic and a parallel SRAM on
a DIP-size package and call it an ASIC.  ;)

mcb

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2000\02\15@211406 by Sean Breheny

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Hi James,

At 02:26 PM 2/15/00 -0800, James Newton wrote:
>FRAM is supposedly not write limited and are available in serial (8Kx8) see:
>http://www.ramtron.com
>easy to interface to a PIC. Appnote at
>http://www.ramtron.com/products/appnotes/EDTecAppNote.zip

If you take a look at the datasheet at ramtron, it says that it DOES have a
write AND READ limitation, although it is high (10 billion cycles), it
isn't as high as SRAM. I think the reason why I sorta had it in the back of
my mind that there was a big limitation to FRAM was because it has a read
limitation. However, considering that writes only take a few 100ns instead
of several milliseconds with EEPROM, there are some advantages I guess.

Sean


|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
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