[ntp:questions] how do I lock in average frequency correction

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Wed Feb 15 08:00:52 UTC 2012


Dave Hart wrote:
>> Historically, the PC used frequencies which were convenient multiples
>> of the NTSC colorburst frequency, because NTSC crystals were really
>> cheap.
>
> I believe it had more to do with the Color Graphics Adapter circuitry
> needing to operate at NTSC-compatible frequency.l

Both, sort of:

The 8088 was spec'ed to run at 5 MHz, by taking the ~14.3 MHz needed by 
NTSC and divide it by three, they ended up with 4.77 Mhz as the original 
IBM PC cpu frequency. Two needs, one crystal!

(I believe the 8088 needed a 67% duty cycle, in which case they had to 
start with something close to 15 MHz anyway?)

Terje
-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"



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