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4hv.org :: Forums :: General Science and Electronics
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Frequency counting without a microcontroller

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ragnar
Sat Apr 21 2007, 06:42AM Print View
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Hi all,
here's an unconventional frequency counter circuit I've designed. I'll try and explain how it works:
(This circuit does not include a prescaler)

A schmidt-triggered inverter gate is used as a relaxation oscillator. You adjust R and C to get 512Hz. I call this signal "REFERENCE" because the displayed frequency will be proportional to this.

The 512Hz REFERENCE signal is divided by 128 using a CD4024 7-stage divider chip to give a 4Hz signal. I call this "REFRESH", because this inherently defines the refresh/readout rate of the LED displays.

I take REFRESH and squish it down from a 4Hz squarewave into a 4Hz pulse with a pulsewidth of more than 200nS. This is done using my pulse-squishing circuit; a diode, resistor, capacitor, and another schmidt-triggered inverter gate. The resulting pulse is called "RESET" because for those 200nS, the display will go blank and the counter will reset to zero.

I take the positive edge of RESET, and allow it to positively trigger another schmidt-triggered inverter (with the input floated at V/2 so it maintains its state). I take the negative edge of REFERENCE, and allow it to negatively trigger the same gate. The output of this gate is now the "INHBIIT" signal.

To count frequency:

Say we are measuring a 1MHz signal with no prescaler:

1) The "reset" pulse hits the counter, blanking the display and resetting it to zero. I will consider the 200nS infinitesimally small for purposes of clarity.
2) The positive edge of the same "reset" pulse "uninhibits" the CD4026s and they start counting.
3) The negative edge of the next "reference" pulse "inhibits" the CD4026 and they stop counting.
The CD4026s will have counted (1/512) * 1,000,000 pulses, and the clock will end up displaying "1953"
4) The clock will hold and display that value for 1/4 of a second, until the process repeats.

Now, to calibrate the frequency counter, you tweak the frequency of the reference oscillator (which affects the display refresh rate too, but who cares) until the display reads the appropriate value.

You can add a prescaler for higher frequencies. I recommend a CD4020 or a 74LS93.


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Sulaiman
Sat Apr 21 2007, 06:58AM
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Very nice,

except for the reference frequency generation,
a quartz crystal oscillator would be more accurate/stable.
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Hazmatt_(The Underdog)
Sat Apr 21 2007, 08:41AM
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Are there any contrast issues with the 0000 readout then the 1953 readout? What I mean by that is do you see the segments dimly lit?
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Dr. Who
Sat Apr 21 2007, 10:34AM
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Cool design. You could divide down a 32.768 KHz crystal to get a nice stable 4 Hz reference signal.
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ragnar
Sat Apr 21 2007, 10:39AM
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Sulaiman, I cannot use a crystal with my design unless its frequency lines up perfectly with any prescaler and the width of the pulse-counting part of the 'read' cycle. In due course, I may make a version with the pulse-squisher and dividers to bastardize it so it works, but for now, it has to be manually calibrated. I guess that's OK for me, because I don't mind a +/-1% reading.

Hazmatt, I haven't made the circuit yet -- I don't even know if it will work. I'm just doing the PCB right this minute. :)
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Ken M.
Sat Apr 21 2007, 03:04PM
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It sounds like it will work and be moderaterly stable, minus maybe having to calibrate it every time you turn it on, but since that won't bother you its ok. As for the PCB desing looks nice except I don't see how your gonna be able to do the white jumpers that in between some pins, also these 3 red traces (I'm presuming) unless they were meant to make contact with those other or you still have some trace editing to do, all in all it looks great and sounds like it'll work.
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Hazmatt_(The Underdog)
Sun Apr 22 2007, 12:59AM
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Okay, I'll just sit-tight until you can test it. I'd like to know because every time I did a 4026 design I had that issue.
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Ken M.
Sun Apr 22 2007, 01:10AM
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I had a similar problem with my capstone, or final project for college ( It was a clock that displayed miliseconds as well as the usual time frames), even though it was based off a ucontroler, I solved it by doing a "Chained display" (Can't remember the acyual term for it). i.e since the clock displayed 12:00:00:00, lets say the clock showed 12:34:56:78, the actual display would light up 8 then 7 then 6...etc, i believe every display was shown for ~2-5ms, any faster and the display wouldn't light any slower and it would hang on a digit.
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ragnar
Sun Apr 22 2007, 01:14AM
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WhiteArc, it only has to be calibrated once, ever. Then I'll record the values for Rfreq, Cfreq, Rsquish and Csquish, and I will be able to crank them out repeatably. Crystals are also expensive. The CD4026s should be good to at least 8MHz counting frequency, but with a prescaler, they won't need to count that fast.

Dangermouse, a 32,768Hz crystal will cost me a few dollars here, and it is imperative that I significantly change the reference frequency in order to calibrate the meter. (The initial readout value will have a completely arbitrary proportion to the input frequency -- you have to unbastardize it (and the prescaler) by adjusting the timing period)

Those white jumpers I will carefully stretch between pins. The 'red traces' will in fact be actual wires on the bottom of the PCB. At some point in time, when I can make these up commercially and get a double-sided board done, I won't need any links.

Matt
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Ken M.
Sun Apr 22 2007, 03:12AM
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Oh ok, I was just concerned about the jumpers because In a tech prep course I took in High school, we had to make PCB's, design, layout, print, etch, and test them, and putting solder points that close to each other just makes me nervous, so I was concerned, but I guess if you know how good you are at doing that kind of stuff then by all means go for it.
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Bored Chemist
Sun Apr 22 2007, 08:40AM
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"Dangermouse, a 32,768Hz crystal will cost me a few dollars here,"
How much would a broken digital watch cost? Also, if, rather than 512 Hz you used exactly 1000Hz derived from a crystal clock, would you need to calibrate the thing?
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ragnar
Sun Apr 22 2007, 09:40AM
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BoredChemist, I understand you are practically- and economically- minded. I am too, but for me that includes repeatability. I cannot use a crystal yet for the simple reason that the counting portion of the timing period is of an arbitrary length. That's how I can get away with doing this with only six chips.

OK, perhaps later it may land within 1% of a useful dividable value, then of course I'd spend a couple of dollars on crystals to pretend it has some semblance of accuracy. Until I can tweak the circuit and determine those possible values, I cannot use a crystal. The reference frequency has to be determined after I build it, and it depends on how squished the "ALLOW" pulse is. =)
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Steve Conner
Sun Apr 22 2007, 10:03AM
Joined: Fri Feb 03 2006, 10:52AM
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The counting period is not an arbitrary length, it's 1/512 of a second, if you ignore the reset pulse from your squisher.

If you replace your 512Hz oscillator with a quartz crystal and divider that outputs 1000Hz, you will get a counting period of 1/1000 of a second, so the display reads directly in kHz and doesn't need adjustment at all. If the pulse from the squisher is shorter than 1 microsecond, the display won't notice it.

If you can find the quartz crystal cheaper than you could find the resistor, capacitor and trimpot, you may still win. And if you want repeatability, this should either work accurately out of the box or not at all... The Schmitt gate oscillator will probably be unstable with changes in supply voltage and temperature. If you can't use a crystal, even a 555 with a good quality plastic film capacitor would be an improvement.

BTW, that trick of using a Schmitt gate as a flip-flop is astoundingly cheeky. I don't think you would get away with it in a commercial product though, because the threshold voltage and hysteresis varies between devices, between batches, and with temperature.

PS: If the reference waveform has a duty cycle of 50%, does the counter only count during the high period? In that case you need 500Hz, not 1000.
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ragnar
Sun Apr 22 2007, 11:34PM
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Hi Steve,
the "RESET" pulse has to be of significant length that it is recognised by the CD4026s... as a result, it will eat into that 512Hz period, so the allow/inhibit period is going to be affected by that.

Yep, that 74C14 is extremely cheeky. ^_^

REFERENCE is a 512Hz squarewave,
REFRESH is a 4Hz squarewave,
RESET is a 4Hz pulse,
ALLOW is a 4Hz pulse longer than RESET and shorter than REFERENCE,
INHIBIT is the inverse of ALLOW.

I have the board with me in Sydney today, so I'll let you guys know what happens as I build it. I have no idea whether it will work, it just looked like a good idea on paper. =)





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Steve Ward
Mon Apr 23 2007, 04:24AM
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Just out of curiousity, why did you specifically *not* want to use a microcontroller? Id been considering a similar project, and id definately be using a microcontroller unless there is some reason not to (nothing that i can see, provided i use a crystal oscillator for higher accuracy).
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ShawnLG
Mon Apr 23 2007, 04:49AM
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"Just out of curiousity, why did you specifically *not* want to use a microcontroller?"
Are microcontrollers expensive? Discrete components are much more cheaper I believe.
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Wolfram
Mon Apr 23 2007, 05:04AM
Joined: Sat Feb 04 2006, 01:31PM
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Microcontrollers are actually not expensive at all. The PIC16F716 would propably manage the task, and it is under two dollars in smaller quantities from DigiKey. If you buy over 25, it's under the dollar. I'd be surprised if you could make it cheaper with discrete logic, especially considering all the board real-estate you'll save when using one microcontroller instead of 6 other chips. Then you could also make it autoranging, and add other practical solutions, and it would be very accurate. From looking at other PIC-based frequency counters, it could propably do over 50MHz without a prescaler.
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ragnar
Mon Apr 23 2007, 08:41AM
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My two excuses(?) are:
A) I'm trying not to have to get into microcontrollers just yet,
B) All these microcontroller frequency counters seem to use LCDs, which are pricey, here, too.

I'll gladly admit that I've learned a whole lot more by thinking through the circuit and designing it all with logic chips I've never touched before, than I would have by copying somebody elses assembler code (which I wouldn't understand) onto a microcontroller (with a programmer I don't have, nor do I have the inclination to build).
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Wolfram
Mon Apr 23 2007, 11:25AM
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Excuses, excuses.

Here's a simple frequency counter using LED displays. Here's an other one.

LCDs can be very cheap at ebay.

A programmer can be as simple as two resistors.

The argument about learning still holds.
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Steve Conner
Mon Apr 23 2007, 11:57AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6749
Meh, you have to learn about discrete logic AND microcontrollers some time, does it really matter which you start with?

I started with discrete logic the same as BP, because I was studying a joint degree in electrical and mechanical, and one of the things they left out of the electrical curriculum to make room for the mechanical stuff was microprocessors.

The most complex thing I built was a coin recognition mechanism for a vending machine, as part of a uni team design project. There were three of us, so we made one sensor each, and I did the control board and display too.

It used all 74 series logic, apart from a large EEPROM that stored a lookup table telling what ranges of sensor outputs corresponded to which coins. We calibrated it by flinging about 100 different coins through, writing down all the sensor outputs, and then typing the ranges we got into a simple C program that spewed out a binary file. We then put this on a floppy disk and took it to the electronics tech who burnt it into the chip for us. Nowadays it would all be done with a 5 dollar PIC programmed from a laptop, and each coin you tossed in would post a LiveJournal entry

The second most complex thing I made from discrete logic was a temperature controlled soldering station: Again, a 5 dollar PIC would have replaced 90% of the guts of this unit, however it has worked fine since I made it in 1996 and I can't be bothered rebuilding it.

Again, microcontrollers are incredibly powerful and useful, but if you don't understand discrete logic, you may find it harder to understand how the guts of a uC work, which may make it harder for you to program in assembler. (It's just a big heap of flip-flops, gates, and memory.) A knowledge of logic circuits is also very important for working with PLDs and FPGAs.

So I think BP is doing the right thing by starting simple. The only thing I would say is to try not to rely too heavily on one-shots! (that's essentially what your pulse squisher is) Synchronous logic is much preferred over stuff that relies on little RC delays here and there. (Try getting a one-shot on board a FPGA, and you'll see why!)

Anyway, BP, your board pr0n looks great as always, now build it and let's see it working! :P
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Ken M.
Mon Apr 23 2007, 08:12PM
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As a matter of fact you can even get samples of Micro controls namely PIC's I have a few, my personal choice for a high res digital display would be the mamoth PIC16f777-17p Its a 40 pin but it can handle driving 8 digits at a very high accuracy, and thats with 7segment displays. As for programming software, you can demo Pic basic and pro at compilerspot.com i think...its been a while and besides I have Picbasic here at home and its great being able to type in basic phrases to run a system. As well as Steve and the others...your board looks awsome , you make it, or did you get a PCB manufacture to make it?
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Steve Ward
Mon Apr 23 2007, 08:50PM
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Well, my school is very heavy on digital design (even though its not my area of focus). Now that i know how to program uCs and FPGAs (field programmable gate arrays), i dont think i could ever do electronics without them. Up until about 4 months ago, i think i was right along with you BP... thought i could just manage all the stuff i wanted to do with gates.

Anyway, you are certainly capable of learning these devices, so maybe in a few months you can start the "Frequency counting WITH a microcontroller" thread, where you can discuss the advantages and disadvantages you find with each design .

Funny Conner mentions synchronous design... In my digital electronics lab i once got "made an example of" by my TA for BAD design practice for gating a clock signal to one of my flip flops (despite this, the circuit actually did work exactly as required, as i expected). If he saw this frequency counter, i think he'd blow a gasket :P. Personally, while i think true synchronous design (where you dont mess with the clock!) is always better, i dont frown upon little RC tricks and stuff as long as its for your own uses.
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ragnar
Wed Sep 19 2007, 07:47AM
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Well, you might think of this as letdown of the year. Steve reckoned in April that in a couple of months I could have got into microcontrollers. Shamefully, I didn't. I'm still doing it with 40-series logic.

I made this frequency counter once before, and then after assembling it, discovered that the segments on the LED displays I used didn't line up with the datasheet. Wrong datasheet. My fault. I got thoroughly frustrated and tossed the board into my junk projects tray for a rainy day.

Now, in September I needed a frequency counter. Sure, I've got one in my oscilloscope, and I've got one in my multimeter, but for some reason, I wanted to build a signal generator with one in there too. But what actually reminded me was when I saw the newly available BLUE seven-segment LED displays at my store. They're so pretty.

So, I redesigned the circuit, and, without the help of an autorouter, was able to get the four CD4026s connected to the displays, their clocks/carries together, their disp.enbl.in/disp.enbl.outs together, their resets together and their inhibits together, on one layer without a single jumper.

Oh, and because I only have a hand-drill here, everything is surface-mounted. I surface-mounted the chips directly, because if I used sockets I wouldn't be able to get the solering iron tip down between them to solder their legs to the board.

Now I use a CD4060 (with oscillator) to get my "reference" clock. I divide it down, Q10 & Q9 & Q8 & Q7 & !Q6 to give me a the "inhibit" period. I use "inhibit" & !Q5 & !Q4 to give me my "reset" pulse. Now the relationship between reset and inhibit is non-arbitrary, and everything is a lot more predictable and a lot more stable.

The one-off calibration involves tuning the CD4060 oscillator until the numbers displayed match the frequency of your test signal. The display refresh rate will change, but who cares?

The two improvements I made whilst calibrating it were realising that a ~15Hz refresh rate made the "counting pulses" stage almost unnoticable. It might only be 1/100th of a second, but the '0000' segments were juuust momentarily visible. By hooking the "display enable" pin to "inhibit", the display is only turned on when the numbers aren't changing -- with a suitable refresh rate, my eyes' POV makes the display look like it's on all the time. Beautiful.

Now I prescale it as much as I want, and it doesn't matter if it's I use a binary or decimal chip because I can just factor it out by recalibrating. Squee!

Who wants one?




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Wed Sep 19 2007, 02:25PM
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sweet!

It always amazes me that you can cram circuits so closely on a 1layer board (although it looks like you needed a little rework over on the clock generator side...).

As soon as I get a signal generator I will start looking into a decent frequency counter
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Marko
Wed Sep 19 2007, 06:43PM
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Matt those blue displays are beautiful.
Apart from maybe being a bit inaccurate the circuit is cool.
Have you considered to use une of those 32khz watch crystals for reference?
It would add a lot of IC's, though.



My fault. I got thoroughly frustrated and tossed the board into my junk projects tray for a rainy day.


That's why I use donutboards.. and still I find more than enough things to get frustrated with
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Hazmatt_(The Underdog)
Wed Sep 19 2007, 07:19PM
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I never could get the right frequency divided down because my local store never had the right crystal for a 100Hz reference, so I got some from Mouser. The other frequency counter thread has it, which is like 3.27MHz or something... that's the one you would want to use for your standard.

Glad to see it working! I was wondering how you would solve the missing latch problem.
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ragnar
Thu Sep 20 2007, 02:01AM
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No offense intended, but I did want to compete a bit with your "World's simplest frequency counter" thread.

I mumbled under my breath that it couldn't possibly be simple -- it has TWENTY chips!

I just wanted to brag that I could do it cheaper and dodgier with six chips.
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Hazmatt_(The Underdog)
Thu Sep 20 2007, 03:26AM
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But Matt, I can do it in 4 and it won't be dodgy, it all depends on what chips you select and how you implement it.

The reason I did the 'sledge hammer to squish a fly' version is for everyone to see how the technology works, and for a few other reasons too:
1. sequential logic
2. latching and how memory works
3. no timing capacitors, its all gated so the only deviation is from the crystal, which by the way could be scaled to 100MHz and in an oven for perhaps 1ppb tolerances.

As far as rock solid design goes, I think that's pretty much there.

I liked the 4026's because conceptually they would do the same job in fewer IC's, so yea I liked them too. But when I found the combination of the 4011 and 4553, I could do a 3 1/2 digit frequency counter that would be rock solid with those from the 4000 family, then use the 4060 with the right crystal, fed to the 4017 as my timing engine, and bam... rock solid in 4 chips. So if you want to beat me, you can try but I think that pretty much sums it up.

Conversely I could use the 4060 and 4017 to generate my timing and send that to the 74C925 and do it in 3 chips if you want to be beat out by 2:1, so its your choice. heh.

I was really interested in the strobe situation though.
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...
Thu Sep 20 2007, 03:54AM
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Using a multiplexed display driver is no fair

I guess we need to change the competition to 'chips not including the display drivers'

Also hazmat, I doubt you would be able to get 9 digits out of your design (enough to be able to see the 1ppb stability you quote), due to pratical limitations in the max frequinecy you can use If you run the clock at 1hz (much slower than that and counter wouldn't be very usefull...), and your input frequinecy tops out at 40MHz (although the chips are only reated for 15MHz even at 15v), you stiil only get 7.5 usable digits... Unless you found a way to get subHz resolution with resonable refresh rates that I overlooked.
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Hazmatt_(The Underdog)
Thu Sep 20 2007, 08:49PM
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1ppb clock stability is what I was talking about.

Multiplexed driver chips are fair because he's using a divider decoder. If divider decoders aren't fair then what I did is the only 'fair' way to build the counter, 1 chip per function.

Anyway, that doesn't matter, I'm glad to see interest in frequency counting because it requires some thought and I havne't seen much discrete stuff out there that actually works.
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