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Re: Frequency and ns???
- Subject: Re: Frequency and ns???
- From: mjmahon@aol.com (Michael J. Mahon)
- Date: 14 Jun 2002 20:15:41 GMT
- Newsgroups: comp.sys.apple2
- Organization: AOL http://www.aol.com
- References: <%VnO8.238738$Gs.20838317@bin5.nnrp.aus1.giganews.com>
- Xref: archiver1.google.com comp.sys.apple2:20421
Bryan Parkoff asked:
>Frequency and ns???
>
> How can I calculate CPU's 2.8MHz into nanoseconds. For example: 65816
>has 28MHz Crystal that shows fixed number -- 28636360 that do every 65th
>1MHz cycle that can be translated into 140ns. (I wonder where the fixed
>number come from because of 28MHz crystal)
Period in nanoseconds = 1/frequency in gigahertz.
Since the crystal is 28.636360MHz, or 0.028636360GHz, the period is
34.9206393ns, or 1/4th of the nominal 140ns period you mention.
The particular frequency is chosen because it is eight times the standard
color subcarrier frequency for an NTSC color signal, 3.579545MHz.
Timing in an Apple II is somewhat complicated by the "stretching" of the
clock at the end of each horizontal line of the video refresh (every 65
cycles).
The clock is extended by 1/2 period of 3.579545MHz, or 139.68255ns,
to obey the phase rules for NTSC color. (The subcarrier frequency was
chosen to be an odd multiple of half the horizontal scan frequency so
that the frequency components of the sidebands of the chroma subcarrier
would fall between the frequency components of the luminance baseband
signal--but that's another story. ;-)
-michael
Email: mjmahon@aol.com
Home page: http://members.aol.com/MJMahon/