Electronics > Beginners
replace 3.58MHz with 3.579545MHz
schmitt trigger:
--- Quote from: edavid on December 30, 2019, 05:53:37 pm ---
Here is a datasheet for a 3.580MHz crystal sold by Mouser:
I have no idea why it exists.
--- End quote ---
My favorite theory? Because of non-technical purchasing people and lazy engineers who did not accurately specified it.
That is why our engineering director required that we should always call it as a NTSC Chroma frequency crystal.
mcovington:
I've never seen it referred to as "3.580" either, but that's simply 3.579545 rounded to 3 decimal places.
amyk:
A lot of early microcomputers in the late 70s/80s used this as a timing reference, allowing direct connection to a colour TV.
Zero999:
--- Quote from: edavid on December 30, 2019, 05:51:07 pm ---(About the MM5369)
--- Quote from: schmitt trigger on December 30, 2019, 04:40:35 pm ---To obtain 60 Hz from an oddball frequency such as 3.579545 MHz, it alternated the frequency’s division ratio.
--- End quote ---
No, it did not, it just divided by 59659. You had to tweak the crystal load to get it closer to 60Hz.
--- Quote ---As such, the output wasn’t a 50% duty.
--- End quote ---
Yes, that is correct.
--- Quote ---Sadly it is now obsolete, but can be emulated with any modern microcontroller.
--- End quote ---
It was an old PMOS part that wouldn't run at 5V, let alone 3.3V, so it wouldn't be very useful today.
--- End quote ---
You're right about the duty cycle, but according to the data sheet, it was CMOS, not PMOS. It will work at 5V, but not at 3.579545MHz, which requires a 10V supply.
http://pdf.datasheetcatalog.com/datasheet/nationalsemiconductor/DS010820.PDF
SeanB:
Only reason those crystals are around is because they had 2 major uses, TV IF oscillators and filters since 1954, and then, because they were a cheap and available item, they were then used in DTMF tone generators, developed by 1960, as a low cost precision reference oscillator for tone generation. Thus they were both a very cheap, and very precise, quartz crystal oscillator, and thus were used in the first microprocessors as a clock generator, because they were cheap, worked well with a single transistor oscillator, could be divided down easily with a 7474 to give you a roughly 1MHz clock with 50% duty cycle, and ran the processor at a decent speed.
That the clock was so close to TV line rates also helped with video generation, but the leading contributor to use was that you could get them very cheaply, and they were widely available, unlike any other crystal, and were a no set up device unlike a LC oscillator or any other form of oscillator. you have the design, apply power and, within the limits of your test equipment, it would work, and you could test it just using a regular TV set.
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