SPI Tolerance

Bruce Adler [Contractor] Bruce.Adler at west.sun.com
Fri May 27 05:31:41 PDT 1994

>From asami at dt.wdc.com Fri May 27 18:51 PDT 1994
>You are right, that you can buy 10.000MHz +/-250ppm crystal, but not 0ppm!!!
>You can buy a 9.75MHz +/-250ppm crystal so you can be 100nsec worst case,
>but that probably is a special custom frequency and will cost you $$$.

Did you mean 9.75MHz +/- 250KHz? I think 9.75MHz +/- 250ppm is the
same as 9.75 MHz +/- 2500 KHz.  So if your crystals are actually +/- 250
ppm and you want to handle the worst case without exceeding the 100 nsec.
minimum then you need to find a crystal at 9.9975 Mhz +/- 250 ppm.
Or if your crystals really are +/- 250KHz then you need 9.75KHz +/- 25641ppm.

I'm not certain what 100 nsec +/- 250 ppm means. How long is a
part in this case? Also if you specify it as ppm then your margin
changes as you vary the timing.  Wouldn't you be better off with a
fixed tolerance such as +/- 1 nsec.

adler at west.sun.com

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