(Woops, Jan, your reply field is set incorrectly so that replies go only to
you - I sent this yesterday but it went only to Jan)

Thanks everyone for the advice.  Does anyone no why there are no SPI RAM
chips?  Is it lack of demand or is there a technical reason than makes RAM
unfit for a serial chip (I can't imagine that, but then again IDKE:)

Ramtron:
The ramtron is interesting but expensive (2.5 times the cost of the same
amount of eeprom).  @100 pieces, 16kx8 is about $4 and 32k is about $6.
Ramtron has an I2C 8 pin part that's 32kx8.  FM24C256

Microchip:
I just checked the data sheet for the Microchip 25LC640 (SPI, 8Kx8).  It
states 100k writes cycles - that's the same as the Amtel parts.  The I2C
series are 1,000K cycle.  Larger densities too.  One order of magnitude
gives me a little more time before customers start sending them back to me,
but still not a great solution.

Parallel RAM:
If I use a parallel RAM I wonder if it would be worth it to include an
address latch chip, so that I could stick with the 28 pin 18F252 or if I
should switch to a 40 pin 18F452 and use all the additional I/O as address
lines?

--BobG



-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On
Behalf Of Jan-Erik Soderholm
Sent: Monday, January 13, 2003 3:49 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: Looking for SPI RAM to replace SPI EEProm

Igor wrote (to  Bob) :
>Microchip guarantees one million write cycles to their EEPROM. Isn't enough
>for you ?
>Igor

Ramtron says 10**12 (1 Trillion)  read/write cycles for the FM25C160
device (2K byte). At 5000 operations/second, thats 15 years...

Another device (FM25640, 8 kb) says 10**10 (10 Billion) cycles.
Note that plain reads counts also !
A read from FRAM is internaly a read-rewrite operation, that's why
read counts also.

And, with FRAM, writing is a no-wait-state operation. You can write
as fast as the SPI interface can transfer data (up to 5Mhz in the case
of these two devices).

I'v got four FM25640-P I got from the Ramtron sample service, but havn't
had time to run any test with them yet...

Jan-Erik Soderholm.
S:t Anna Data

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