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| Do old 74-series logic NOT have Schmitt-Trigger inputs? |
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| jolshefsky:
--- Quote from: SiliconWizard on November 27, 2018, 04:08:55 pm ---Showing us your exact schematic and pictures of your setup would probably help. --- End quote --- Schematic attached. As it turned out, the chip was bad ... who knew, with it sitting in my junk bin. :palm: I finally went and tested all the gates and it seems the gates on pins 1-6 are not working right but the pins 8-13 gates are, so I tried it on gate 13-12 instead and it worked fine. D'oh! (Edit: for what it's worth, I just connected the gates to one another and tied the first to a power rail because it's quick to use those 1-pin jumpers on a breadboard.) |
| capt bullshot:
--- Quote from: jolshefsky on November 27, 2018, 08:46:28 pm ---breadboard --- End quote --- I don't think that half of the gates in the chip were bad and the others were OK. That would be a quite rare kind of failure mode. I rather believe that the power supply routing and the negative connection of the 1uF cap was different, depending on which gates you used. Some reasoning: These gates switch their outputs in maybe 1 ... 2ns from High to Low and vice versa. This causes a high current spike drawn through the power pins, this in turn causes a voltage drop along the supply wire. Depending on how long the wires are, and where exactly the 1uF cap is connected, this voltage drop causes enough feedback to the input of the gate the make it switch back, in turn causing another current spike, causing another voltage drop (same as before, but into the other direction), causing the gate to switch, ... You get the picture, I hope. This results in the 10Mhz oscillation of small amplitude. Keep your power wires short, use a decoupling cap close to the IC, connect the 1uF cap directly at the GND Pin if the IC - it should work better then. IMO the chip would work in a proper environment using a decoupling cap shortly tied to the power pins. Most probably some parasitic inductance and/or coupling caused the unwanted oscillations. |
| tggzzz:
--- Quote from: capt bullshot on November 27, 2018, 08:56:58 pm --- --- Quote from: jolshefsky on November 27, 2018, 08:46:28 pm ---breadboard --- End quote --- I don't think that half of the gates in the chip were bad and the others were OK. That would be a quite rare kind of failure mode. I rather believe that the power supply routing and the negative connection of the 1uF cap was different, depending on which gates you used. Some reasoning: These gates switch their outputs in maybe 1 ... 2ns from High to Low and vice versa. This causes a high current spike drawn through the power pins, this in turn causes a voltage drop along the supply wire. Depending on how long the wires are, and where exactly the 1uF cap is connected, this voltage drop causes enough feedback to the input of the gate the make it switch back, in turn causing another current spike, causing another voltage drop (same as before, but into the other direction), causing the gate to switch, ... You get the picture, I hope. This results in the 10Mhz oscillation of small amplitude. Keep your power wires short, use a decoupling cap close to the IC, connect the 1uF cap directly at the GND Pin if the IC - it should work better then. IMO the chip would work in a proper environment using a decoupling cap shortly tied to the power pins. Most probably some parasitic inductance and/or coupling caused the unwanted oscillations. --- End quote --- Just so. With solderless breadboards you spend more time debugging the breadboard than your circuit. For the reasons why, and cheap easy effective alternatives, see the links in https://entertaininghacks.wordpress.com/2020/07/22/prototyping-circuits-easy-cheap-fast-reliable-techniques/ |
| SiliconWizard:
True that breadboards can give you a few headaches. That said, building a simple 1 kHz RC oscillator with a CMOS Schmitt inverter should not be that tricky to build on a breadboard. I haven't seen any bypass cap in the OP's schematic, so adding one close to the IC's Vdd pin should do the trick (unless the IC is indeed toast). Haven't used a solderless breadboard in ages (I exclusively use SMD or through-hole prototyping boards when needed), but I remember using those long ago while learning and I had certainly built much more complex digital circuits on those than a simple low-freq oscillator, so I'd be curious to see a picture of the actual setup. And don't forget to add a bypass cap. ;D |
| tggzzz:
--- Quote from: SiliconWizard on November 28, 2018, 04:23:04 pm ---True that breadboards can give you a few headaches. That said, building a simple 1 kHz RC oscillator with a CMOS Schmitt inverter should not be that tricky to build on a breadboard. I haven't seen any bypass cap in the OP's schematic, so adding one close to the IC's Vdd pin should do the trick (unless the IC is indeed toast). Haven't used a solderless breadboard in ages (I exclusively use SMD or through-hole prototyping boards when needed), but I remember using those long ago while learning and I had certainly built much more complex digital circuits on those than a simple low-freq oscillator, so I'd be curious to see a picture of the actual setup. And don't forget to add a bypass cap. ;D --- End quote --- The frequency is, of course, of little importance. What matters is the rise/fall time. For measurements confirming basic fourier transform theory, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/ While the OP appears to have used ancient CMOS technology that would probably work on a solderless breadboard (with decoupling caps!), modern CMOS technology certainly wouldn't. For an example of 250ps risetimes, see https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941 That shows 5V/100ohm = 50mA switching in 0.25ns, i.e. 200MA/s (not mA/s). Plug that into v=Ldi/dt and the voltages are significant. |
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