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# help with digital design

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#### joepistritto

##### New Member
Hi-

I'm a newbie trying to get in to electronics. I have a technical background (I'm a software developer) but I don't know much about basic electronics. I was wondering if you guys can help with some newbie questions and give me pointers to where I can start....

I'm looking for good books to get me practical skills in building digital circuits. In particular, most electronics books I have found discuss the analog level stuff (like KVL theorems and RCL analysis and such). To be honest I never wade through this material enough to understand how this fits in with digital design.... This may sound like a stupid question, but how does the analog stuff fit in with putting together ICs? I understand some of the digital stuff (I even used VHDL to make a simple CPU in college -- but that was all simulated!), but I don't see how to translate a digital design to something physical (i.e., how do the capacitor, resistor, etc., fit in the picture?) I would like to be able to build my own circuits, stuff that I can interface with a computer......

Any help is appreciated.... I think I might be missing something because I always get bogged down by the math when I approach the basic electronics books. My math is rusty but I am sure it'll come back to me if I see how things fit in.

-j

I am not going to say that you *have* to understand analog to do any digital design, but the fundamental principles are often the same or very closely related. For instance, in high-speed circuits the instantaneous current changes and propagation delay are two concepts that are very analog in nature, but are carefullly considered in digital system design.

Another great example is that building audio processing circuits can be difficult in the analog domain because components age and drift, but audio signals are inherently analog so you have to move between domains. Understanding particulars of an analog-to-digital convert (ADC) then becomes important.

Having said that, a good book on basic digital is "Fundamentals of Digital Electronics, by Roth" and "Fundamentals of DC/AC circuits by Hazen"

Thanks for the reply... Actually, I think my question is more fundamental. I'm interested in putting together simple circuits using simple ICs. However, everywhere I look I see tutorials that example an IC's function and then they show you the schematic but with resistors or capacitors in the schematic with no explanation as to why they are there or why they are needed. So, my question is, why is the purpose of the resistors, capacitors, or inductors in digital circuits? I've heard they are necessary for smoothing out power or something, is that correct?

Also, what is the process to build a circuit? Let's say I got all of the ICs and I know how to hook them together because I know which AND gate needs to go to which OR gate and the flip-flops, etc... Once I get past that stage, is there something I need to do to actually realize it physically (as in wirewrapping it or breadboarding it?) Can I just hook up the ICs directly through wire-wrap? Or do I need to add in resistors and stuff in between (and if so, how do I determine which resistor values or capacitance values for capacitors?)

How does the propagation delay and stuff fit in? I think I'm just confused as to how the resistor/capacitor/etc. stuff fits in to the picture.

Thanks again for putting up with my dumb newbie questions...

-j

Get your hands on a copy of"the art of electronics" by Horowitz and Hill.

Its getting on a bit now, but it does go into a hell of a lot of stuff without using maths that would have Einstein reaching for textbooks.

It is difficult to know where your confusion is, but compatible logic ICs can be connected without any R-C, but sometimes CMOS logic can be used with R-Cs for analog functions. Some counters have provision for oscillator function with the addition of R-Cs. If you look up the data sheet, it should explain how to use external components.

I guess my confusion is in looking at some of the sample circuits out there. I see the IC chips and understand their function but then I don't get why there are capacitors and resistors in the diagrams...

I think I'm starting to understand now. I guess in most of these sample circuits, the capacitors are used to give good power delivery to the ICs to handle spikes. And the resistor is for interfacing like the LEDs to prevent it from burning out. Is that right?

Now, if this is so, how does one figure out how to put in the right capacitors (values) to make sure the ICs get good power?

Thanks again everyone!!!!

Capacitors are used to filter the power line and prevent voltage variations. In general, bigger is better, but if you know the frequency that is to be bypassed, the reactive impedance can be calculated from: Xc=1/2/pi/f/c where pi=3.1416, f is the frequency in Hz and c is the capacitance in farads. Capacitive reactance is measured in ohms. Sometimes the internal resistance and lead inductance of the capacitor is important.

A resistor in series with a LED is necessary to limit the current. The LED drops 1.5 Volts to 2 volts, depending on color. Ohms law applies: R=V/I where R=the resistor value in ohms, V is the voltage across the resistor, and I is the LED current. Normal LED current is in the 10 to 20 milliamp range.

take a basic electronics course. tis fun, and easy once u get the ahng of it. and it all ties in to digital logic gates, except you jsut need a few more wires to make sure stuff has a ground, rhater in logic all u see is a HIGH as a +5 v and you usualy dont see the ground, only on simulated IC's

Russlk, thanks for answering!!!

Can you explain more about the reactive impedence? I don't think I get this.... First of all, how do you find the frequency to be bypassed? Is this something I can get by reading the specs of ICs or do I get this from some type of measurement (with what tools? Like an oscillator?) Second, this equation returns the "reactive impedence", but I thought capacitors are chosen by Farads so how do I use this equation to choose the right capacitors to use?

Thank you very much...

The frequency is implicit in the circuit. If it is an audio amp, the frequency range is 30 to 20,000 Hz, if it is an RF amp, the RF is the frequency, if it is logic, the clock is the frequency. It is evident that you do not understand algebra, please read a book about it.

Apologizes in advance if my dumb questions make it sound like as if I don't even understand Algebra...... I assure you I have some knowledge of algebra. I am just very green in the field of electronics.

All I am asking is how to find the right capacitors to use given a digital circuit. I believe these are called decoupling capacitors? Now that I understand where the frequency component of the equation is coming from (the clock rate of my digital circuit), I still am asking about how to use your formula. There are still two free standing variables left, Xc and c. If I solve for c, which you state is the capacitance (measured in Farads, which I believe is the unit of measurement I need to pick a capacitor) how do I determine the value I plug in for Xc?

Thank you very much for your help... I feel really embarrased by my dumb questions but I really want to learn.

Sorry, Joe, but it seems, IMHO, you are trying to jump in in the middle and it just can't be done. You are asking questions like "what is a resistor for" or "what is a capacitor for," or even "what is a good value for C?" It all depends on how/why they are applied. A large value for C is not always right. Sometimes you even need to parallel large and small values of capacitors! Why? Don't capacitances in parallel just add? Why not just use a single larger capacitor?

Just because you want to work with digital doesn't mean you can ignore analog. You'll have to understand, for example, how to control current through an LED.

Seriously, get the AofE (Art of Electronics) and read the first couple chapters, and browse the rest of the book, with your breadboard in front of you. You'll feel like you've been working in the dark up until then.

I don't mean to be a downer... Electronics is fun.

j.

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