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Forum Index : Microcontroller and PC projects : 74HC14 question for the experts
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| PhenixRising Guru  Joined: 07/11/2023 Location: United KingdomPosts: 857 |
I want to square-up my encoder signals with this Schmitt trigger which is 3.3V compatible. This device features clamping diodes and can accept my 5v encoder signals via current-limiting resistors. Max input clamping current is 20mA per the datasheet. Therefore, I'm wondering; 1K or 10K resistors on my 5v signals? 74HC14.pdf |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4223 |
Phenix, Voltage dividers. Don't rely on the clamping diodes. Remember that the current pushed into the clamping diodes ends up in the power rails. And these do not have protection, only the current drawn by the chips can absorb this energy. Typically this is enough, but that depends on the amount of current, and the number of inputs, and if there is a sleep mode for chips (no current consumption) Simply use a 1k/2.2k divider to get from 5V to 3.3V levels. Volhout PicomiteVGA PETSCII ROBOTS |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 857 |
Oh sure but I use TH resistors and want to keep this board to the bare minimum. I figured that because this is a stated feature, I could use it:  |
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CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2075 |
note from the datasheet:"Input Clamp Current ... ±20 mA" so 1K would be fine, but to save a bit of juice I'd go higher. it's a CMOS input so not terribly low impedance beyond the clamp anyway, we are only sparing the diodes. Whereas Harm's reply above is perfectly valid, in the interests of reducing the component count I would use a 4K7 in series with the inputs. Excess voltage into each input (assuming 5V with Vcc=3.3) would be 1.7v, under 4K7 is around 360uA so even if all six inverters are used, it will dump just 2mA into Vcc across the whole chip. I wouldn't lose sleep over that. I would use whatever resistor 2K2 - 10K I had most of  Edited 2024-05-16 22:09 by CaptainBoing |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 857 |
Great to see you posting again, Cap'n  Just so happens that 2K2, 4K7, 10K are the ones that I have ridiculous quantities of  |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2075 |
cheers Phenix.  |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
G'Day you lot. High value R's will give slow rise & fall times which may or may not matter. Other than that, the higher the value the better until input impedance matters. A slow rise / fall time can be corrected with speed up c's but that way lies madness. I should stay out of these things but Capt. inspires me 😊 Petee |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 857 |
Yeah, signal is more important than board real-estate...Divider it is |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2075 |
oh dear.There's probably some voluntary organisation that can help you rehabilitate |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
Capt. Probably not. Thirty years ago I decided to give up smoking, that was easy compared to staying out of the shed. The problems I have now are housework, lack of energy, brain decay I forget what else. And when I look at the shed I have no idea what people are talking about but once in a while an interesting topic like this one comes up and I remember the fun we had in the past. I wonder if PaulL is still hanging around out there with his particular brand of humor. Stick with it. Peter😘 |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2075 |
yes it keeps me sane. Prob is, I lose stuff... I have a bee in my bonnet about my "sensible cartesian co-ordinates" graphic pack, and I want to finally get round to adding the ORIGIN sub... can I find my F4s? I have two of these beasts but they aren't where I thought... so this weekend - the hunt is on.  |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
So, did you find it / them? Just to indicate where I am at, I've got no idea what you are talking about, but then I probably never did but you are at least still trying, Keep it up for as long as you can. I keep finding things and wonder what it is. perhaps an F4 who knows? Peter Edited 2024-05-31 18:50 by PeterB |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2075 |
Hey Peter sadly no. They have formally been assigned upgraded status from "mislaid" to "lost". it really irks me... I was convinced they would be with the others... went straight to my MMXs and H7 and plethora of smaller MMs. F4 & H7 are ST based "megamites". H7 is almost half a gig clock  - monster. same cpu as in the CMM2. F4 is the same architecture, different peripherals and 300Mhz clock - not to be sneered at. Both have a nice LCD backpack and are excellent dev platforms for screen based stuff.I had a bit of a mare of Saturday, everything I touched turned to mud Subsequently I have a big repair to do which resulted in some re-design - so overall it was a "fun" experience... bit of thought, new PCB... personally, I am still in the "ignore the b'stard" stage... good days & bad,but meh. I seem to have re-found my mojo a bit for the shed. you? Edited 2024-05-31 20:53 by CaptainBoing |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
I decided some time ago that it was too difficult, so I offered my picos to anybody in the shed. That resulted in me putting 3 picos plus a couple of other bits in a bag and sending them to Victoria. Unfortunately, the picos got lost so I ordered 3 more to be sent. That caused a lot of confusion, but it finished up OK. Then I got cross with myself for giving up so I ordered 3 more. I have played with them a bit but mostly they just sit on the desk and make me feel guilty. I still have other bits and there is a lot of work I could be doing for a mate but,,,,,,,,,,,,,,,,,,,,,,, Peter |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4223 |
Peter, PicoMite is great. But you should not feel guilty they are lying on the desk. Though I think the bare pico is not as attractive to pick up, as a pico in a unit. You may get more energy if you build one Pico into a unit. And that can easily be veroboard. On the forum I have recently seen boxes with a pico in it and nothing but flying wires, connectors. I have no idea what your "projects" are, but I always grab my PicoMiteVGA unit, even if I don't use VGA or PS2 keyboard, simply because it is a stable "brick" that I know works. It can survive a drop off the table, and has the basics (i.e. SD card) installed. My "work horse". And MMEdit. Without MMEdit I would not be able to write code longer than 10k. Volhout Edited 2024-05-31 22:34 by Volhout PicomiteVGA PETSCII ROBOTS |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
G"day Volhout I have always needed a "real project". Fortunately I have / had mates in farming and engineering and since I retired, these people have kept me busy with touch screens via Grogster and smaller screens to control things like bait spreaders. It has been fun but several things happened to cause complications. COVID stopped the supply of some chips and the pico meant a new learning curve. It is very frustrating because I have the time. I can afford it and the need is there. Sadly I am 86 and my brain is also. My wife passed away in 2022 after 3 years in a nursing home. I then got friendly with the girl (87) over the back fence but the pain in her arm turned out to be cancer but we had a good (short) year and so it goes. She passed away last November. The good thing is, it is raining |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2075 |
very sorry to hear that. having a project is always the kicker; the ability is there, the will is there, just nothing to aim it at. |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6098 |
I have far too many 'real' projects waiting to be started, and a few more waiting to be finished. My fingers no longer do what my brain tells them to do so I have had to abandoned many of the projects. I have decided to have a shed clean-out and the list of 'stuff' looking for a new home is growing exponentially. I will keep enough to keep me amused for a few years. Jim VK7JH MMedit MMBasic Help |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
Good morning Jim. My idea of a "real project" is when I am asked to make a device to do something. The first part is to determine what they really need and then modify it as we learn more. The final product almost never looks like the initial request and that is the good bit. I have never charged for my work so that has made me reasonably popular. The Bait Spreader project went on for years and is now in good shape but when I look back at where it started, WOW. I wouldn't have missed it for quids. Peter |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6098 |
Hi Peter, When I worked for CSIRO, the scientists would come to the workshop with a journal from another country and ask me to make the equipment they were using. The articles were never in English so it was interesting. I soon learnt that they didn't really want the 'same' just similar to do something different. After that, I would sit them down and demand to know what they were trying to achieve. Then I could make a piece of equipment that worked. Interesting times. A long time ago Jim VK7JH MMedit MMBasic Help |
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