Effect of parasit capacitor at the input of my digital circuit

[Emam]

Dear all,
I have a digital circuit (ASIC) designed for some specific purpuses. The circuit works well in most cases.
However sometimes, it does not work i.e. I dont see any output.
I am investigating if this problem could be related to parasit capacitor at the input of my circuit.

The input of my ASIC circuit should be connected to the gorund by the user of the circuit.in order that the circuit does its job.
For some reason the ground is at a very close distance to my input channel (following image)

When the user decides to connect the input to the ground, he should use two metalic pins (connected to each other at one side ) and place it between the ground the the input.

Could you please tell me that in this case:
- Do I have a parasit capacitors?
- If I have a parasit capacitor, could it affect the input signal ?
- How can I measure if I have capacitance?

Many thanks and bests regards.
S

 

[Arouse1973]

Do you need a pull up resistor on the input. A better circuit diagram would be more helpful for us.
Adam

 

[Emam]

Thank you very much for your response.
In fact, I dont understand a lot of these kind of electronic!!
Here is the image of my circuit:
Inputs are T1, T2, ..., T7
I think if I remember that before we had pull-up transisitrs and every things was OK, then we put some invertors or something! then it started to create problems!!
Many thanks

 

[Arouse1973]

All this for a touch sensor? Can you explain what all that's doing to save time and what all the control pins do, so we have an idea of how this works. Are the sensors in pairs or is it one T1 and GND T2 and GND etc....

Adam

 

[Emam]

No its not for a toch sensor. Its simply a circuit which permit to chosse an input between T1 and T7. These inputs correspond to different times (1s, 2s, etc...).
If the user want to select the T1, he sould connect it to a ground. For each input I have a graound. So 7 ground.

But, in général, can a capacitor at the input, change the state of the input (for. ex to 1 or to 0)?
Bests regards and thank

 

[Arouse1973]

A small capacitor on the input of the control lines will probably not cause you an issue because the invertors should be at a defined logic level. unless you have any of the inputs floating which could cause the gate to either switch when it shouldn't or the input float into no mans land causing excessive current and possible damage of the IC. All that said you shouldn't need a capacitor on the inputs T1 to T7, this will cause unnecessary switching transient currents from the invertor gates.
Adam

 

[Emam]

Thank you very much!
In this case, I will not investigate this problem any more.
Bests regards