Re: Flash Memory Speed

> This is a hardware question. We are using 80c400.  Does anyone know if it
> would be ok to use 120 speed flashes instead of 70?  We are having
> difficulty finding 70ns speed flash.    I looked in the data sheet of the
> 80c400 but I must have just missed it.  Any idea?
> 
> Thanks
> Ray B.

No, you cannot "just use 120ns" memories. At least not if you want to
run at 28 MHz. Here are a few calculations previously posted to the
list:

On page 27 of the AM29LV081B data sheet, look at the "output enable to
output delay" time, t_ELQV. It is 30ns for the 70ns part. This is the
timing that corresponds to "PSEN low to valid instruction in" on the
DS80C400. 

Another important DS80C400 characteristic is the address to valid data
in (t_AVIV1) which is 3*clcl - 19. In the case of 29.5MHz that computes
to 82.5 ns.

In other words, the 70ns flash allows you to run at 33MHz. 90ns
flash at 27.5MHz. 120ns flash at 21 MHz.

Keep in mind that you need a good baud rate generator. Even the common
11.0592 MHz crystal (multiplied by 2) is too fast for 120 ns Flash
memories. Also, you cannot operate on a 100 MBit network if your clock
speed is below about 27 MHz.

The DS80C400 data sheet can be found on the Maxim web site at
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3609



-- 
  Robert Muchsel - Dallas Semiconductor - http://www.maxim-ic.com/TINI/

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