| Electronics > Projects, Designs, and Technical Stuff |
| Do old 74-series logic NOT have Schmitt-Trigger inputs? |
| (1/4) > >> |
| jolshefsky:
I was working on a project that I think can be completed entirely with one quad NAND gate. I first wanted to just test that I could make a switchable-frequency ~1KHz oscillator, so I dug around and found I had a MC74HC14AN hex inverter with a 9630 date code—good enough for the oscillator. I wired up an RC low-pass filter from one inverter's output-to-input: output-resistor-input; input-capacitor-ground. I also grounded the remaining inputs. No matter what filter I set up, I was getting an oscillation in the >10MHz range. The input was not swinging more than a few millivolts away from VCC/2, basically ignoring the RC time constant. This implied the device did not have Schmitt-triggered inputs. 1996 seems like a time when all logic gates would have Schmitt-triggered inputs ... am I wrong? |
| MK14:
One thing that comes to mind is, did you put a decoupling capacitor on the power rails ? Ideally, close to the actual ICs power pins. |
| capt bullshot:
The HC14 definitely has a Schmitt-Trigger input, that is what it's supposed to. Others don't, even today they don't. Anyway, if your oscillator doesn't work with the '14, something else must be wrong - bad decoupling or breadboard comes to my mind. |
| spec:
Crossed posts :) Hi jolshefsky, The 74HC14 has Schmidt trigger inputs on all six of its not gates. Few other logic chips have Schmidt inputs though. The 74HC132 quad two input nand gate is the other common logic chip to have Schmidt inputs. Mk14 just beat me to it: you need a 100nf X7R ceramic capacitor between the supply pins. Also all the other 5 gates need to have their inputs connected to either logic 0 or logic 1, or the whole chip may get into a state- that may be happening. The chip may even destroy itself. Bear in mind that 74HC is very fast, so a good layout with short wires/ traces is essential. Are you using the integrating oscillator circuit or the differentiating oscillator circuit? The former is well behaved and the latter is not. Your best bet is to post a schematic and a picture of your layout so we can have a look. |
| eliocor:
Here it is: dual QUAD NAND SCHMITT TRIGGER port, the 7413... https://www.unicornelectronics.com/ftp/Data%20Sheets/7413.pdf connect together all inputs of one gate and you'll get an inverter for your oscillator. |
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