74HC74 pulse shaping

Lol, tell that to my digital logic professor! I took a course in multi-valued logic once which turned out to be a thinly disguised course in abstract algebra! FFs were *very* much mathematical objects!!! I think I skipped that question on the comprehensive exam.

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Rick

An SRFF _has_ positive feedback. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Essentially the problem is: It would be nice for the "gate" synchronization circuit in this piece of equipment I'm modifying to always be fed a neat square wave, but audio guys like to abuse equipment for fun.

In use the "gating" circuit may be fed with any old garbage signal in the audio range, and I'm working on a circuit that will make a best effort to clean up whatever it is fed and at least attempt to output some kind of sync pulse based on whatever the input is.

So "best effort" means using a left over '74 in a moderately pointless circuit? I guess I'm not much used to even having a '74 on the board anymore, much less considering various weird ways to use it. I literally can't remember the last time I designed in an MSI logic device other than a buffer/level shifter. If you are talking audio rates, there is a *lot* you can do that would not take up much space or $$$.

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Rick

Do you understand how this is being used? With one input always asserted it isn't an SRFF anymore.

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Rick

Oh, also, the feedback is not analog, an SRFF is purely digital. A Schmitt trigger requires analog feedback.

--

Rick

Well the total cost of all the ICs used so far in this mod in quantities of 100 is about 27 cents so...

No real circuit is digital... all circuits are analog >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

So pennies are the overriding concern? How do you define "best effort"?

BTW, if you are talking about quantities of 100, 27 cents per part is only a few dollars less than using a 50 cent MCU. Why are you even worrying about parts cost? A handful of '74 type parts will use more $$$ in board space than the cost of an MCU.

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Rick

But you still can't make a silk purse out of a sow's ear and a '74 is not a Schmitt trigger.

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Rick

What he said. The idea that a SR flop is purely digital is a comforting illusion, not reality. Purity is rarely real.

Consider the humble '555 timer: it implements a very accurate Schmitt trigger function, using a SR flip flop. Because, that's sometimes the best way.

I don't get how the SRFF provides hysteresis. I thought that came from the *two* separate thresholds and comparators.

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Rick

I don't understand the question. Hysteresis requires history-dependence, and an SR latch holds one bit of historic information.

So does a comparator with positive feedback. Without the positive feedback (of the analog variety) to adjust the input threshold a FF is just a digital device. Please show me how to use a '74 FF without any additional active components as a SRFF to implement a Schimitt trigger which seems to be what the OP was asking for.

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Rick

Okay, so use the 'HC74 inverter that Rick and I suggested, follow it with a 2N7002 inverting stage, and connect it as a Schmitt trigger. That's one $0.008 transistor and three $0.002 resistors, total.

Cheers

Phil Hobbs

Hysteresis does require history dependence, but not everything with history-dependence is hysteresis. The on-off switch will eventually wear out, which is history dependence, but not hysteresis.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

I had considered a microcontroller. The issue is that it's a modification to a battery powered circuit - using a CMOS logic part and a couple low-power op amps I can probably keep quiescent power draw well under 100 uA. A uC running at any reasonable clock rate is going to be drawing hundreds of uA to a mA or more.

Sure, I guess there are ways that I could set it up so that the uC drops into low power sleep mode after not getting pulses for a while, and then wakes up on an external interrupt, does its thing for a while, and then goes back to sleep and so on. It's likely still going to be using more dynamic power when active than the circuit using dumb parts, and then you have to do a lot of testing like, does it always come out of sleep mode cleanly regardless of the input signal and so on.

I've attempted to do some experimenting with low power modes on the AVR at least before and it's one of the most pain-in-the-ass features to write code for - you have to power sequence everything just right to make sure the device really is in the deepest sleep level.

Microprocessors are great for a lot of things but for this particular application, even though it might simplify the circuit a lot, setting it up correctly all just starts to feel like a lot of work

If he is worried about microamps, then I would just bite the bullet and add a Schmitt trigger buffer. They come in 1 gate packages (more likely

2 gate for a buffer) which are about the same size as a SOT-23 transistor. The CMOS buffer will draw virtually no power current.

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Rick

At audio rates, you can use megohm resistors.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net