button Pull-up resistor / reset problem

Chicago Mike

I'm not sure if this belongs in the SX forum, or not.... This is an electrical problem I think.

I'm using a typical switch circuit with a 10k pull-up resistor attached, through a Hex Schmitt Invertor to a pin on an SX (I"m doing this to electrically debounce). on a bread board, this seems to have worked very well. No problems. I recent transposed the EXACT circuit onto a PCB, and I'm having some problems. When I close the switch it seems to reset the SX chip every 10 or 15 activations of the swtich. I've checked the PCB layout, and actually at one point even took the circuit, layed it out only with the switch portion of the circuit, and still see the problem sometimes. If I take the swtich OFF of the circuit board, and run the contacts off the board to a swtich, say a foot or so away, the probem seems to go down to 1 in 50 times.... Interesting. I put a 1k resistor inline with the switch and this seem to help, but not eliminate the problem. I've attached the schematic of the circuit I'm using. I'm at wits end and have been trying to trace this for a week now. There is never a dead short, so thats not doing it. All I can think is that I'm somehow sinking too much current on a pin maybe? Decoupling problems? I don't think there is creepage and clearance isses on the PCB, as when I re-laid out the board the exact same problem exists, and the switch inputs are the ONLY thing that cause this reset.

Attached is the drawing of the circuit. Any tips would be great.

Propability

Don't know if this is your situation but the breadboard would be using the DIP style of the chip. If your circuit board is using the surface mount then you should know that it has a slightly different pin layout( why -who knows).

PJAllen

Please see attached.

kwinn

If the pinout of the smt chip is correct you may want to try a smaller capacitor, say .1uf.

grasshopper

You could program a denouncing loop. I think that would solve your problem in a quick and simple manor.

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Leon

Have you decoupled the supply to the 74HC14 chip?

Leon

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Chicago Mike

Thanks everyone. PJ allens circuit did it (THANKS!). I'm guessing actually it was my placement of the Capacitor that did it.

As for decoupling, I've always been a little confused on when and how. I know its good practice, and I always do it on my rail voltage after the regulator (usually using a 1000uf), but as for decoupling the ICs... What is the method to calculate the correct cap value to put in parallel to the Vdd and Vss near to the IC? I've heard of 100uf 10uf, 10nf, 100nf.. These are huge differences, and I've never found a 'common excepted practice', or formula to follow. Placement is crucial too right? Decoupling should occur as near as possible to the IC pins? Or is this really not absolutely necessary? Any advice would be great!

Carl Hayes

No, it wasn't the placement of the capacitor. It was the fact that you were suddenly short-circuiting a charged capacitor with no current-limiting resistor on the short. This produced a LARGE pulse of current through the capacitor and switch and the interconnecting wires, which in turned produced a magnetic pulse, which in turn induced a voltage pulse in nearby conductors. That's why moving it a few feet away reduced the problem. PJ Allen saw this immediately and came up with a correct solution. Bravo to him.

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