Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

decoupling capacitor

Status
Not open for further replies.

meowth08

Member
Hi,

We have been tasked to make a simple circuit.

With this, I think noise would greatly affect the performance of the circuit.

I plan to include decoupling capacitors.

My question is: How would I know the values to use for the decoupling capacitors?

m8 :)
 
What is your circuit? Simple logic will suffice with 100nF across each chip. Something like a GSM modem need HQ tants in the hundreds of uF range.
 
Hi,

I'll come back when I have the schematics.

Thank you.
 
If it's normal digital logic, 0.1uF is the gold standard. Otherwise, the rule of thumb is big enough to filter out noise, but not so big that parts have to wait for the caps to fill. I wouldn't go over 10uF. If you have that much noise on your digital power supply you have other problems to deal with.
 
I still don't have the schematic. We are still working on it.

We are to make an address decoding circuit of a z80 microprocessor. The input will be using switches (retentive). A logic 0 will be connected to ground and a logic 1 will be from Vcc.

m8
 
Hummm... I should ask (quiz) you now... what's the right way to hook switches to digital logic? What things do you have to account for?
 
Hi sir,

The input will go to a 74ls244, an octal buffer.

It looks like this.

Vcc----resistor----(x)--->to input of 74ls244.

On (x), a switch is present that connects to ground when a logic zero is wanted.

I'll have to check on the datasheet Vih(high level input voltage), Vil(low level input voltage) etc. so I know what range the voltage should be to ensure a logic 1 or a logic 0. The other parameters are low level input current, or high level input current. This will allow me to choose what value of resistor to use.

m8
 
You have one part thought out right, but there is another thing you need to deal with.

Is max voltage and current all you have to worry about with a mechanical switch on logic circuits?
 
wait sir :)

I may be wrong about what I said earlier. This switch we are to use is a DIP switch...
 
Humm... It's less about the exact switch and more primitive and common to all switches... Take a look at this...

View attachment 66433

What do you think that is?

Edit: Let me know when you give up (^_^)
 
Last edited:
noise sir. :)

Added later:

It's already late here sir (37 minutes before midnight and I'm not yet home)and I'm on a public internet cafe.

I'll come back tomorrow hopefully with the schematic and answer to your question.

Thank you so much for the help.

I WON'T GIVE UP :) :) :)
 
Last edited:
Haha.

Thank you sir. So we should use use a debouncer circuit. And of course, decoupling capacitors.

Thanks again sir.

I'll let you know how the project will go. :D
 
As promised, here's the schematic of the circuit.
It is an address decoding circuit to be connected to a z80 emulator.
It inputs to the emulator whatever the status of the switches when the correct address (FF based on the circuit) is loaded.
Basically, this is input interfacing.
I hate this requirement! However, I learned about debouncing circuits and for that I'm thankful.

After all, the title of the thread does not match to this.
And I will need no decoupling caps or debouncing circuits.

What is important is, I learned. Special thanks to oblivion and to people in the chat room. :)
 
Hummm... I can't open your schematic.

If you don't mind, I would prefer an LTspice *.asc file, or an image of the circuit. An image would prolly be easier for you to do.
 
Hi,

I'd like to interject a quick note here...

De-bouncing is mainly needed on clock lines. For address decoders for example they usually are not needed because the address is set before the circuit is even turned on. This means there cant be any bouncing.

If you need to clock say a JK flip flop then yes you need de-bouncing. Any time any circuit has a problem with a switch input changing rapidly from 0 to 1 and back again before it settles needs de-bouncing.

Calculator switches for example also need de-bouncing if they are sampled fast. If they are sampled slow they might not need it however, but most calculator keypads would need de-bouncing so that the user typematic rate can be set high enough for normal human use. Without de-bouncing here we'd see multiply key strokes when only one is intended.

So first you have to ascertain whether or not you need de-bouncing and then go from there. Sometimes you can do it in software too.
 
Hi,

()blivion:
I'll show the image of the schematics on my next log in.
You can open the schematic using eagle 6.1. That's what I used to do it sir.

MrAl:
I have also entered the chat room and Boncuk suggested that I should try first without de-bouncing circuits.
He also emphasized that de-bouncing circuits are needed for circuits like counters etc.
Thank you for the information.

meowth08
 
An individual 10K pull up on each input for LS logic and 100nF per chip decoupler and you won't go far wrong with that. Just make sure the Z80 isn't driving against the 0v grounded switch. A circuit of your intentions would be useful!
 
We are nearly finished with the board. I don't know if we still can include those. We are to tap on the power supply of the emulator so I don't think we'll have a problem on noise. Just in case it fails, we are to borrow an oscilloscope to see what went wrong.

A circuit of your intentions would be useful!

I was complaining when I was making the layout of the board on EAGLE thinking that it is useless. It was so time consuming doing the board layout. But I needed to finish it since it is a requirement. Having a lack of knowledge almost killed me. :D Kindly educate me here sir :D Thank you.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top