Digital chip input handling

[carbonzit]

Couple questions regarding handling the inputs to digital ICs (the chips I'm using are all in the CD4xxx series).

1. I have a project where I'm using a 4013 as a latch, and I want it to come up in the unset state when I power it on. I'm using this arrangement, which seems to work OK:

Question is, what values of R and C would be best here? (I don't mean super-optimized, just what will accomplish what I want, which is a reliable reset.) The trade-off here seems to be between A) a small C giving a shorter pulse vs a larger C, and B) a small R giving a faster pulse but loading the input more.

Basically, are these values in the ballpark?

Yes, yes, I know there are probably better ways to do this, like a one-shot, a 555 or something, but I'd rather keep this as simple as possible and not have to add another chip. It's not a precision application.

2. In the same project I'm setting the flip-flop by using a pushbutton switch connected to the set input. Since there's nothing else connected (no other gate), do I need to add a resistor to ground so that this input doesn't float when the switch is open? Would the same (4.7K) value be good here?

(Anticipating replies: no, debouncing isn't needed here. Multiple pulses don't matter.)

 

[eTech]

Couple questions regarding handling the inputs to digital ICs (the chips I'm using are all in the CD4xxx series).

1. I have a project where I'm using a 4013 as a latch, and I want it to come up in the unset state when I power it on. I'm using this arrangement, which seems to work OK:

View attachment 146801

Question is, what values of R and C would be best here? (I don't mean super-optimized, just what will accomplish what I want, which is a reliable reset.) The trade-off here seems to be between A) a small C giving a shorter pulse vs a larger C, and B) a small R giving a faster pulse but loading the input more.

Basically, are these values in the ballpark?

Yes, yes, I know there are probably better ways to do this, like a one-shot, a 555 or something, but I'd rather keep this as simple as possible and not have to add another chip. It's not a precision application.

2. In the same project I'm setting the flip-flop by using a pushbutton switch connected to the set input. Since there's nothing else connected (no other gate), do I need to add a resistor to ground so that this input doesn't float when the switch is open? Would the same (4.7K) value be good here?

(Anticipating replies: no, debouncing isn't needed here. Multiple pulses don't matter.)

Q1 answer:
The reset pulse should be longer than the minimum pulse width at the operating supply voltage.
So V+ in your circuit should be applied to R for at least 180ns@5v.
Below was clipped from the CD4013B datasheet.

Q2 answer:

Yes, use a pullup or pulldown resistor as needed. CMOS Inputs should never be left floating.

 

[carbonzit]

OK, good answer, although I must admit I don't know off the top of my head how to calculate the width of the power-on pulse I'm getting from that cap and resistor; will have to look that up. I thought maybe someone would do that for me (heh).

But regarding the R value and how much current it draws, since there is another gate attached to that input (the output of a 555), at 5 volts that resistor should have ~1mA through it. Does that sound about right? (I'm by no means an experienced digital designer!)

 

[danadak]

Why using RC slow Trise, Tfall :

As you can see there is a lot of jitter in gate output. That can cause metastable states in
the downstream clocked logic.

Easy to use is single gate logic Schmidt trigger gate that cleans up this using hysteresis internal.

https://www.ti.com/lit/sg/scyt129g/scyt129g.pdf

https://www.ti.com/lit/wp/slla364a/slla364a.pdf

https://www.ti.com/lit/an/scba004d/scba004d.pdf

Regards, Dana.

 

[carbonzit]

I don't get it. Your simulation doesn't look anything like what I showed.

I really don't think I need a Schmitt trigger here.

 

[danadak]

Forget the sim, read the ap notes on slow Tr and Tf in CMOS.

Setup and hold times are affected by that input.....