Help me understand this NOR gate w/output tied high? - Page 1
Forum section: electronics / beginners

author: NickyDoes
author_rank: Contributor
timestamp: June 18, 2023, 05:19:23 pm
content: I'm learning about piezo ultrasonic transducers, and found a device to reverse engineer - to copy the transmit section. The output of a VCO goes to a dual 2-input positive NOR gate, but one gate output is tied to 5V and the piezo is an input. Some kind of hack/trick for this part? I'm totally confused. I even tried asking the resident Borg AI - but got a generic answer about logic gates.Can you shed some light on this so I can rest on this Father's Day?VCO/PLL part: CD74HC4046APWRdual gate part: SN74LVC2G02YZPR555 part: LMC555CTP

author: TimFox
author_rank: Super Contributor
timestamp: June 18, 2023, 06:06:56 pm
content: Can you re-draw it with explicit gate symbols?

author: wraper
author_rank: Unknown
timestamp: June 18, 2023, 06:52:11 pm
content: QuoteYour circuit makes not sense, most likely it's your failure to do reverse engineering properly. Besides not drawing the gate properly, why use quartz resonator symbol for transmitter?

author: wraper
author_rank: Unknown
timestamp: June 18, 2023, 07:00:17 pm
content: Maybe it's not SN74LVC2G02 to begin with?

author: NickyDoes
author_rank: Contributor
timestamp: June 18, 2023, 07:43:25 pm
content: I've reverse engineered properly.The IC in question is definitely the 74LVC2G02 - It's a CSP part, so I can read the markings on the die after removing from the board. I verified the location of U7 pin 1 three ways. It is correct.The 'transmitter' is an acoustic transmitter. The symbol is correct (mostly correct). I could change the REFDES to PZ? (from Y?), but it doesn't change the circuit.Note the pins going from U7 directly to Y2. Note the +5 (yellow) is at A2 and B2.

author: NickyDoes
author_rank: Contributor
timestamp: June 18, 2023, 07:45:43 pm
content: Quote from: TimFox on June 18, 2023, 06:06:56 pmI'm still new to KiCAD, and don't really know how to do that. While I can learn, then do it, it will take me hours. The circuit will still be the same, and it's two simple gates. That's why I fudged it in with rectangles for this question.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 18, 2023, 07:49:44 pm
content: The voodoo gate is apparently a crystal oscillator.I do not know if it is configured correctly but two nor gates can be used to make an oscillatorUsing the Xtal that is o your schematic

author: edavid
author_rank: Super Contributor
timestamp: June 18, 2023, 08:36:40 pm
content: Quote from: ndemarco on June 18, 2023, 07:43:25 pmNope. The gateoutputsmust be connected to the transducer.QuoteThen why are you calling it an XOR gate?Quote from: Wallace Gasiewicz on June 18, 2023, 07:49:44 pmAlso nope. The 74HC4046 is the oscillator. The gate package is just driving the transducer (probably push-pull drive).It must be an untuned transducer since it's driven with a poorly controlled chirp. Probably just a small tweeter.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 18, 2023, 08:48:49 pm
content: You sure U5 is not 3G17?The circuit would be much more sensible that way. Probably the usual 2 inverter push-pull piezo driver. Although I wonder why not use 2G14 instead.

author: Benta
author_rank: Super Contributor
timestamp: June 18, 2023, 08:58:00 pm
content: I think your reverse engineering failed on two points. That it's a 74xxx2G02 is reasonable.But I think your generated pinout is a bottom view; change that to top view and rotate it 180 degrees. Then it'll fit.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 18, 2023, 09:02:17 pm
content: edavid:So the Xtal type schematic is the transducer?Maybe even a Xtal transducer?I guess that makes more sense

author: amyk
author_rank: Super Contributor
timestamp: June 18, 2023, 09:04:41 pm
content: Quote from: Benta on June 18, 2023, 08:58:00 pmEven experienced designers have been caught by footprints that are top vs. bottom view.

author: Benta
author_rank: Super Contributor
timestamp: June 18, 2023, 09:17:11 pm
content: Quote from: Wallace Gasiewicz on June 18, 2023, 09:02:17 pmCrystals and resonators are also ultrasonic transducers. They're just not optimized for that function.

author: TimFox
author_rank: Super Contributor
timestamp: June 18, 2023, 09:35:55 pm
content: The 74LVC2G02 is a dual NOR gate, not a dual XOR gate."NOR" = OR followed by negation, or negation followed by AND"XOR" is only one input high gives high output, no inputs high or both inputs high gives low output.Datasheet (with top view)https://www.diodes.com/assets/Datasheets/74LVC2G02.pdf

author: NickyDoes
author_rank: Contributor
timestamp: June 19, 2023, 12:14:50 am
content: I fixed the post title to say NOR gate. The post body stated NOR gate. I'm at the point where I'm making errors.This drives a tiny piezo transducer at ~2 MHz.I'm confident I have the right part family - the part die is marked with (M) TI04 and "LVC2G". The part is marked CPN, which matches the datasheet.I can't get behind the 'flipped footprint' thinking. On the board layout, the pin next to +5V is also connected to +5V. GND is diagonally opposite to the +5V. See the attached Illustrator file. Maybe I'm not mentally 100% tonight??? I can't see how I can invert/rotate to make the +5V not be next to 1Y (output).Also, I looked at an example of the board with the part still soldered on. The Pin 1 (A1) mark is where I have my dot.Thanks for sticking with me on this. I know something is wrong, but can't figure out what it is - and I'm supposed to have this done for tomorrow morning.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 19, 2023, 12:56:02 am
content: TI marking for SN74LVC2G02 is CBN, not CPN.CPN seems to be a SN74LVC2G74.