Has anybody implemented a divide by 2 using just Basic SMD IC ?
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MikeDB 5 days ago
Getting fed up of the $3 Extended components charge as the HC74 isn't in basic components so looked at doing a divide by 2 using just the basic logic components.  Obviously can be done with a HC04 and a HC573 but can anybody see a way to do it with just a single IC ?
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andyfierman 5 days ago
I have checked through all the 74 and 4K series parts that contain or can be used as +2 counters, including single gate packaged versions but none of them are listed as Basic parts in JLCPCB. About the only thing I can think of is to use a 555 timer monostable as a divider but that's of limited use as it's for a fixed frequency. I did once use a 555 and a diode or transistor pump circuit as a divider which will at least track the input frequency for a synthesiser many years ago. That might get you out of a hole if you have 2 spare transistors or diodes and a couple of caps available. By varying the voltage on pin 5 you can have some control over the divider ratio: [https://oshwlab.com/andyfierman/555-timer-frequency-divider](https://oshwlab.com/andyfierman/555-timer-frequency-divider)
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MikeDB 5 days ago
@andyfierman Yeah that was where I ended up as well, but the signal is at 24.576 MHz (MCLK = 96kHz audio * 256).   But audio things also need half that frequency as well so every board ends up with a HC74 next to the crystal oscillator, and hence the $3 charge, as well as the $3 for the world's most expensive electrolytic capacitor next to it.  Even for production runs of 50, the extended costs are still significant, making the blank PCB cost itself almost irrelevant, though they have promised to handle V-cut and larger runs sometime in the future. Anyway thanks for checking it I'd missed anything.  I thought I'd managed it with a HC165 shift register using the negative output fed back and the load inputs set to 10101010, but found an initialisation timing hole in that idea. They seem to have lots of weird parts as basic, yet staples like a HC00 and HC74 are extended.
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MikeDB 5 days ago
@andyfierman Also you do wonder about what sort of things they load.  I can understand having 138000 HC595s as it's a great MCU extender, but they also have 26000 HC573s in stock, which I've never even considered using in a design, but only 2500 HC00s.
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andyfierman 5 days ago
@MikeDB, I should have thought of this as soon as I thought of using a monostable... As it's a fixed frequency can you make a non-retriggerable monostable using the gates in a 74HC00 gates to retrigger every other cycle: [https://oshwlab.com/andyfierman/monostable-as-frequency-divider](https://oshwlab.com/andyfierman/monostable-as-frequency-divider)
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andyfierman 5 days ago
Might be safest to add a series resistor between the RC net and the gate input (pull the unused input to VCC) to stop any overshoot on the RC voltage turning the gate into a thyristor. NXP parts already have a series current  limiting resistor on their inputs but other manufacturer's parts may not. I may add it to the sim later...
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MikeDB 5 days ago
@andyfierman  Unfortunately the HC00 (surely one of the most used logic devices on the planet) isn't in basic parts :-)    Might be possible to rejig it with HC08 though
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andyfierman 5 days ago
@MikeDB, Sorry, thought I'd seen one listed under Basic parts last night. I don't think you could make that work with a 74HC08 because you need an inversion in the loop. What about a building it as a 2 transistor monostable? Maybe using jfets to make it faster? Ah! The 74HC14 is available as a basic part: you can used that and again put a series resistor between the feedback from the Q output and the Qn gate input to create the trigger input forming an RC differentiator. May need another diode to clamp the swing to ground or rail depending on trigger polarity.
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andyfierman 4 days ago
Close... ...but no cigar. Sheet 2 in: [https://oshwlab.com/andyfierman/monostable-as-frequency-divider](https://oshwlab.com/andyfierman/monostable-as-frequency-divider)<br> <br> is a way to make a non-retriggerable monostable out of 74HC14 gates. Trouble is it looks like it won't work much above 1MHz, maybe 10MHz at most (I built the 74HC models to as closely as possible, model the propagation delays and rise and fall times of the real parts vs. supply voltage and load...).
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MikeDB 4 days ago
@andyfierman  Yes unfortunately I think it's a bit the bullet and stick with the HC74 charge.  Also forgot to mention the mark-space ratio has to be >45% < 55% so a monostable would need pretty accurate timing components. Thanks for trying anyway.  If I can get them to put an electrolytic in basic parts then that will knock $3 off every board anyway.  Does anyone NOT put an electrolytic on the power lines ? Mike
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andyfierman 4 days ago
Me in my day job. They are crap at 400bar. :) One other idea I had later on was to make the non-retriggerable monostable using a fast comparator, which of course has to be in the Basic list...
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MikeDB 4 days ago
@andyfierman  Ah ok.  Yes we were banned from using them at HP apart from in the PSU which had a "replace after ten years" statement on it.   But I've fallen into bad habits since then :-)
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andyfierman 4 days ago
Done it. Needs a comparator with a roughly 5ns tpd. And is messy, eats current and would probably fail the 45:55 duty cycle unlike a divide by 2. Have a look at Sheet 3 in: [https://oshwlab.com/andyfierman/monostable-as-frequency-divider](https://oshwlab.com/andyfierman/monostable-as-frequency-divider)<br> <br> Even so at 12.5MHz, might a 74HC be a bit close to the duty cycle limits?
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