Mail Archives: djgpp-workers/2003/05/30/14:04:30
Ben Peddell wrote:
> I'll have to try to investigate that. I'll see just what frequency the
> RTC 1024Hz interrupt actually is.
>
> I know that the RTC is supposed to use a common 32768Hz crystal, and
> the PIT is supposed to use 14318180Hz / 12 (=1193181.66Hz).
If you are concerned about small frequency errors, you may be interested
in the origin of the 1.19-MHz rate that is used in the PC timer chip.
In the early days of personal computing, it was considered important to
maintain compatibility with NTSC, since people used NTSC monitors on
their computers. According to the EIA RS 170 A spec, the chroma
subcarrier is at 3.579545 MHz +/- 10 Hz, and the PC clock was chosen to
be 1/3 of that, or 1.19 MHz approximately. The specified 3.579545 MHz
rate itself is an approximation; it is exactly 455/2 times the
horizontal sweep rate, which was originally 15750 Hz in the early
black-and-white days, but was changed to 15750/1.001 when NTSC was
devised. Thus, the chroma subcarrier works out to 39.375/11 MHz, and
the PC timer chip runs at 13.125/11 MHz. As I recall, the system clock
on the original IBM PC was 4 times that, or 4.77 MHz.
-Eric Rudd
rudd AT cyberoptics DOT com
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