EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: havard on February 09, 2014, 02:35:50 am
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So I decided to build Grant Searle's simple 6502 machine, and started with the oscillator. Rather than getting a clean 1.8MHz clock, I had a not-quite-square wave that was somewhere between 40 - 50MHz, depending on tongue angle, regardless of load (into a 6502's clock input) or termination (tried 100 ohm and 10k pull up and pull down). I figured my crystal was a dud, so I dropped in a 32kHz watch crystal soldered onto some header pins and still had a 40 - 50MHz not-quite-square wave with a 32kHz modulation. Assuming I seriously screwed up, I began pulling out components until I got to the point where there was a resistor across the input and output of two gates. I no longer had a 40-50MHz not-quite-square wave. I had a 75 - 100MHz sine wave. I chalked it up to gremlins in the cheap breadboards I had at my disposal.
A few weeks go by. After watching eevblog #568, the thought hit me... what would happen if I soldered resistors directly across the leads and held the thing in the air? A few more weeks go by. I finally gave it a shot tonight. Grabbed a few 100 ohm 1206s, soldered them across the appropriate leads and sure enough, I have a 75 - 100MHz sine wave.
What is going on here? Is this expected? A bad 74hct04? Or most likely, did I miss some basic fundamentals?
edit: the oscillator originally started out as the one from http://searle.hostei.com/grant/6502/Simple6502.html (http://searle.hostei.com/grant/6502/Simple6502.html)
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Try building this: http://www.electro-tech-online.com/attachments/cmos_oscillator_761-png.7103/ (http://www.electro-tech-online.com/attachments/cmos_oscillator_761-png.7103/) instead... The topology you have used relies on the propagation delay of gates only, which vary significantly with temperature... So it's not a stable circuit... The one in this schematic relies on an external capacitors and 2 resistors... And the propagation delay doesn't count much at all...
If you want an even better one, with very sharp edges, I recommend you build one with a Schmitt trigger: http://i.stack.imgur.com/INtb9.png (http://i.stack.imgur.com/INtb9.png)
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Needs HCU04.
Unless otherwise noted, all HC series chips are input and output buffered. An HC04 is not a pair of MOSFETs, but a chain of three inverters. This gives so much gain and phase shift in the threshold region that it cannot be stabilized with simple resistor bias. Read up on 'ring oscillators'. ;)
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Needs HCU04.
Unless otherwise noted, all HC series chips are input and output buffered. An HC04 is not a pair of MOSFETs, but a chain of three inverters. This gives so much gain and phase shift in the threshold region that it cannot be stabilized with simple resistor bias. Read up on 'ring oscillators'. ;)
Thank you! It all makes sense now.
I actually read up on ring oscillation when I first stripped down the crystal circuit into what I have now, so I figured it would be something like this. It's working as it should, just not as I would expect! That will teach me to read and reread the datasheets, even for something as "obvious" as a hex inverter.
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There is also an episode of The Signal Path Blog that explains ring oscillators and shows some measurements.
Frequency Generation: Crystals and Ring Oscillators (Part 1/4) (https://www.youtube.com/watch?v=yYGwfVnGAdg#ws)