Re: Todd's ELF-ish gets gets a Video Interface System (VIS) #ELF #Homebrew

Lee Hart

taf123 wrote:
Hi Lee -

Thanks for the right up about crystal oscillators. I don't know very
much (anything?) about this topic, so I looked around for application
notes, etc, to help me out here...

Intel's AP-155, "Oscillators for Microcontrollers" has a big discussion
about the built-in oscillators in their MCS-48 and MCS-51 MCU families.
"Logic gates tend to have a fairly low output resistance,
which destabilizes the oscillator. For that reason a resistor
Rx is often added to the feedback network... At higher frequencies a
20 or 30 pF capacitor is sometimes used in the Rx position, to compensate
for some of the internal propagation delay...
I think that's good advice; but old. The internal inverters weren't all that fast back then. I suspect they needed the capacitor in the Rx position because the inverter was barely fast enough, and needed help.

But today's 74HC is an order of magnitude faster. A 74HC04 has a propagation delay of only 7-8 nsec, or about 75 MHz. 11 MHz is "slow" in comparison. :-)

I know this is a "just try it and see approach" which would make my old
engineering professors turn to drink, but seems to be okay.
All you need is for *one* to work. And, you don't care if the oscillator is pulled somewhat off frequency by a less-than-ideal design. So go for it! :-)

Fools ignore complexity. Pragmatists suffer it. The wise avoid it.
Geniuses remove it. -- Alan Perlis, "Epigrams on Programming"
Lee Hart, 814 8th Ave N, Sartell MN 56377,

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