semiconductor eeprom serial/parallel info

[sdeng]

I wanted to know why parallel eeproms only come 64Kbits or higher? Why can't I get a parallel 2K eeprom? Is it because the chip size is too small for the number of pins? Also, how do you figure out timing for serial eeproms? They have a certain bus speed, lets say 100 MHZ, meaning there is a 10 ns delay in between pulses. That means it will take 8(10 ns) to send input and 8(10) ns to receive output? Just seems like an excessive amount of time to receive an output. Thanks

 

[cbarn24050]

Hi,you will be hard pressed to find any kind of 8bit wide memory smaller than 64kbit. Serial eeproms usually run at less than 1 mhz, i dont know of 1 running at 100mhz. Bus speed is not an issue normaly as most devices have a greater than 1 mS write time.

 

[richs]

Hi-

You might find an old 1702 EPROM, but although it's a
2K device, I don't think that you will be happy with it.

Quoting from my old "intel memory design handbook" circa 1977, the 1702 used both a +5V and -9V in their read mode, which was irritating to say the least to provide another voltage level to the board. The programming voltages
mentioned something about a -40V supply.

Errr, that was a 2K bit device.

You can also look at older EPROMS like the 2708 8K eproms
or the 2716 16K eproms.

BUT, you will pay a premimum price for them, even if you
can find them. The low cost of EPROMS these days don't
really warrant the use of the small eproms.

However, I found in the "electronics express" catalog

http://www.elexp.com

Still have the 2708s and 2716s listed in their catalog.
From my catalog:

2708 $3.95
2716 $3.25

while
27C64 $2.95

Oh, and it also says to check for current prices.
The 2708s and 2716s are rated as 450ns parts while
the 27C64 is a 250ns part.

Hope that this helps. You can always ground an address
pin of one of the larger ones and just not use that portion
of the memory.

Hope this helps.

Cheers,

Rich S.

 

[IRstuff]

Don't forget that almost ALL of the older EPROM's required a separate programming voltage supply. Ditto some of the older EEPROM's.

Other than board space and grounding unused inputs, there should be no reason not to use reasonably current technology. You'll get better pricing and some hedging against obsolescence.

TTFN

 

[Heydave]

The basic answer is supply and demand. The demand is for bigger memories, not small ones. The remaining applications for small NV memory are mostly in association with microcontrollers. In this market, the microcontrollers integrate the NV memory on board so no external memory is needed. In more raw logic, FPGAs and CPLDs often can store enough bits to eliminate external data memory. An example is digital filter constants. What application are you looking at that needs small parallel access NV memory?