7414 problem - Page 1
Forum section: electronics / projects

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 23, 2021, 04:06:57 pm
content: I have a 7414 going into a bucket brigade delay, its a SN74LS14N.If i scope the pulses they have rising edges, so the delay wont work.I have tried adding a resistor only it wont help much.If i take the signal from the PIC12 there is no problem, the pulses stay sharp.I thought this 7414 was with schmittrigger.Maybe if i take another 7414 with another package-name then SN74LS14N can it solve my problem ?Thanks.

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 23, 2021, 04:18:30 pm
content: What exactly is the problem? What you’ve posted makes no sense as to the actual problem, rather it talks about your proposed solution which makes this the classic x-y problem.xyproblem.info

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 23, 2021, 04:26:53 pm
content: Back when I experimented with BBDs, they had to be used with a two phase non overlapping clock.Panasonic had a specific IC for the purpose.@Whatsthat: thanks for the excellent XY problem definition.It is a very common occurrence in forum’s.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 23, 2021, 04:39:51 pm
content: I am trying to invert a PWM signal with the 7414.Taking the inverted signal from the MCU gives good results with sharp edges.The signal from the SN74LS14N gives no sharp edges, making the delay not working.

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 23, 2021, 05:01:02 pm
content: Makes zero sense. Sounds like a bad device or an incompatibility of some kind. All we know is 74LS and no sharp edges, whatever that means. You’re asking for too many assumptions.CMOS uP = works74LS14 = doesn’t workThat’s not enough information. A schematic? Waveforms? Anything other than words?

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 23, 2021, 05:17:13 pm
content: Use the 74HC14 version.The problem, without seeing any waveforms, is that a LS circuit has a weak totem pole pullup, and its Voh may not even go all the way to 5 volts.

author: Ian.M
author_rank: Super Contributor
timestamp: January 23, 2021, 05:19:09 pm
content: 74LS logic isn't CMOS and won't output rail-to-rail levels.Seehttps://www.onsemi.com/pub/Collateral/AN-314.pdf.pdfEither replace with 74HC14 or try adding a 3K3 pullup on its output.Edit: 'schmitt trigger' beat me to it!

author: HB9EVI
author_rank: Frequent Contributor
timestamp: January 23, 2021, 05:22:08 pm
content: No experience with bucket brigade delays here, but if rise/fall times don't match the needs with an LS, use an 74HC14 or even a 74AC14, should have a rise time of less than 4ns, the latter even under 2ns

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2021, 05:29:05 pm
content: The missed hint was to provide more supporting information --- What BBD?- Why does it need a sharp waveform? What tolerances does it require?- Why did you add the inverter? It sounds like it was working perfectly fine without it(!!?).- Why did you add a TTL inverter? Would a CMOS inverter not be more appropriate?FYI, most MCUs have output characteristics similar to 74HC or LVC family logic, and very different from 74LS.Or, even broader: why BBD at all? Why not a digital solution (ADC + DSP + RAM + DAC)? The result is far more flexible, and probably has better SNR or frequency response or whatever, I don't know.Tim

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 23, 2021, 05:31:50 pm
content: Quote from: WattsThat on January 23, 2021, 04:18:30 pmI see this assertion a lot and it isn't always true. The OP's problem could be that they MUST use a 7414. Sure, there may be other solutions to the overall goal but there could be valid reasons as to why a particular solution is being followed.If I ask the question "Why are the pulses coming out of the 7414 less sharp than the MCU?", that's the information I'm seeking, not someone else's interpretation of other ways to achieve the same effect. I'd be hoping for insight regarding short rise times, whether I've connected unused pins, have I followed the datasheet's layout recommendations etc. May be I have to use the particular IC because that's all I have available.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 23, 2021, 05:36:10 pm
content: I already solved this problem before with a AHCT, so stupid of me.Sorry for double post, only the problem is different.Sorry i always forgetting things.Hope i can find the correct part.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 23, 2021, 05:48:32 pm
content: Reason : i soldered a circuit and it dont works.Ofcourse i can rebuild and reconsider everything, i build this last year and it still not working, i wont do anything else then stressing or no electronics.If it works i can find out myself its crap, thanks for the advice.

author: TimFox
author_rank: Super Contributor
timestamp: January 23, 2021, 06:00:03 pm
content: As stated above, the 74LS14 and 7414 have normal TTL current-sinking outputs, which are only guaranteed to go to Voh > +2.4 V. The suggested pull-up resistor (to +5V) is probably needed to drive a CMOS input. If you don't want to unsolder the device, try tacking in a leaded resistor and see if that resolves the problem. You can calculate the resistor to source the maximum recommended Iol (at low output) from +5V. The CMOS versions of this part are a better choice, if you can remove the original part.Besides the voltage level problem when connecting 7400 outputs to CMOS inputs, the TTL pull-up output sources less current than the pull-down can sink, and may have problems driving capacitive loads in the positive direction.

author: markce
author_rank: Regular Contributor
timestamp: January 24, 2021, 03:34:44 pm
content: What BBD ic are you using?

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 24, 2021, 05:06:56 pm
content: MN3207from china

author: markce
author_rank: Regular Contributor
timestamp: January 24, 2021, 08:11:14 pm
content: App note shows a MN3102 as clock generator/driver. If you use a MCU as clock source on a different supply voltage than the BBD, you will need level conversion. This could be done with the MN3102 or CMOS logic.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 25, 2021, 02:59:56 pm
content: No they both run a seperate 5 volt, may vary 10 %