I have an SPST momentary tactile switch and want the pulse, whatever length, to be extended to 500ms (unless of course the button is pressed for longer than 500ms but we can ignore that if we hold the pulse form the rising edge). I don't know if there is a single IC to hold a pulse like this or if I can do it with passive components like resistors and capacitors.
Pulse Extender – Designing a Long (500ms) Pulse Extender
integrated-circuitpulsesignalswitches
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I would offer a Conditional Yes.
Your suggested approach assumes nice, clean signals. If you were to pick up any noise on the trace you would run the risk of acting on faulty information.
For example: If there was a voltage spike(s) on the signal when you polled the input, you would read a 1 in your program. In your proposal, you would assume that the switch had be pressed (when it really had not) until you polled the input again after your "debounce interval". At which point you find out that, oh wait just kidding, the switch really wasn't pressed.
So, for the duration of the "debounce interval" you are continuing through your program presumably acting on the faulty information that the switch has been pressed.
Basically it comes down to:
What are you trying to protect against?
What's the worst that can happen if you're wrong?
If you were building a battery-operated kid's game in a plastic box, there probably won't be much electrical noise to screw up your inputs; so your simplified debouncing would be just fine.
However, if this part of life-support device in a hospital... you might want to go with a little more robust input debouncing logic.
I've done this on multiple products to give a short pulse on the reset line once, no matter how long the button is pressed. In my case VDDS is a 3.3V rail, and nRESET is the reset line to the MCU.
Start with power on, button not pressed (open).
a. nRESET is pulled high through R1.
b. C26 has 0V across it.
When button is pressed, the node of R4 and C26 is grounded.
a. Since the voltage across a capacitor can't change instantaneously, C26 will pull nRESET low as well.
b. C26 will begin to charge through R1. The time it takes C26 to charge will be dependent on the value of R1 and C26. This RC time constant as well as the IC's VIL threshold will determine the low pulse time. The voltage across C26 will eventually charge to VDDS as long as the button is held.
When the button is released, the node of R4 and C26 will be pulled up to VDDS through R4.
a. This will cause nRESET to try to jump up to VDDS + the charge across C26 (which is VDDS). This would be 2X VDDS, which may exceed the voltage rating of the IC. This is the reason for D1 to clamp the voltage at nRESET to a safe level.
C3 is present to provide high frequency decoupling of the nRESET line, and may or may not be needed, depending on the application.
As mentioned above, tuning of the timing of the reset pulse can be accomplished through changing the values of R1 and C26.
Best Answer
You can do this with CMOS inverter, a diode , cap and resistor to discharge the cap slowly.
It's called a monostable Multivibrator or 1 SHOT and there are many other ways to accomplish this.
But the typical IC's are state driven, while some are edge driven. They may be re-triggerable or non-retriggerable and the latter is what you wanted.
The LTC®6993-1 is a monostable multivibrator (also known as a “one-shot” pulse generator) with a programmable pulse width of 1µs to 33.6 seconds, with the non-retrig option and rising edge option.
Rev B
Easy 0.6s with 6 in one IC
simulate this circuit – Schematic created using CircuitLab
SIM LINK