Module 4, Five Rules of Thumb. Welcome to Module 4. This is the last module in this whole course. In modules 1, 2 and 3, we had a pattern, we were laying a foundation and that foundation was being laid for the material here in Module 4. Let's recap super quick what we covered in modules 1, 2 and 3. So if you recall back Module 1, we looked at some technology trends of different communications protocols that are used commonly in electronics today. We meet a really clear distinction between serial and parallel communications and the advantages and disadvantages of each. And we really drilled in this point that every single digital bit is actually the symbol. It's a symbol expressed in an analog voltage going through some channel and how that analog voltage is produced and what the channel does to distort that symbol or that waveform and voltage. It affects the receiver's ability to decode the symbol back into the bits. We talked about, that whole process at a conceptual level two and three, Modules 2 and 3 built on that foundation. And now in Module 4, we're going to distill that down into rules of thumb that you can use to estimate this degradation in your circuits. On the hardware side, we talked about open drain drivers, and we're going to use those like we use it as examples previously. We're going to use those again and again here in Module 4 for our examples and how to use some of these rules of thumb. With Module 2 we got a little bit more theoretical, so there was nothing in Module 2 that was really specific to Raspberry PI's and doing work on Raspberry PI's. In fact, a lot of the topics in this whole course are generally applicable, but you're very, very likely to run into when using a Raspberry Pi. Because the distinction of why you're using a Raspberry Pi to do some project is that the Raspberry Pi has the compute side as a Linux operating system. And it also has this physical interaction with other electronics, the GPIO the whole 40 pin header being able to create signals that go through channels and get distorted. And this whole process is where these fundamentals and later these rules of thumb come in to help you make your Raspberry Pi projects work. That theoretical material in Module 2 was largely about capacitance. We talked about what capacitance is, how we would show up in your circuits and how you can estimate it based on physical structures. We talked about simulating using free and open source software when you have a circuit topology, like an RC time constant, that is rolling off or degrading a signal. Now, Module 3, we started talking about things in the frequency-domain. This may be a completely new way of thinking about signals to you, but that's good if it's opened your eyes to this, it's a helpful way to conceptualize how signals go down line. So basically you can look at it, a signal, in two ways, in the time-domain and in the frequency-domain. And if you have anything in the time-domain, it can be represented as a combination of sinus voids in the frequency-domain. And now when we think of it in the frequency-domain, we can talk about filtering effects of wires now that these are not actual purpose built filters, but nevertheless, even an RC time constant on a wire has a filtering effect. We also talked about some very simple models of the Harmonic content within your wave forms. So, if your wave form, as it was in our example, is just a clock signal, like a repeating pulse. We looked at an example of the Harmonic content of that and how the Harmonic energy decays away as you go up in frequency. Then in Module 3, we had two videos on background information on decibels, where they came from, how they work and how to use them. That was light on the mathematics, but was there to give you an intuitive feel of how to use decibels and why we do that. Now, the mathematics are pretty straightforward. You can look that up almost anywhere. Now, we did give the equation for a decibel in that video, you can also look up mathematical references for that elsewhere. But the point of the video was really to give you the intuitive feel of decibels, why they're used and how to use them had another video on filter terminology. So this is standard filter terminology that is applicable. Whether you're talking about purpose built intended filters such as for radio signals or analog signals, or if you're talking about the filtering effects of simple, even parasitic structures like an RC time constant on a data line. And now, after all of that modules 12 and three, we Can go on to module four and module four is taking bits of all the experience from modules 12 and three and distilling it into these five rules of thumb. Each of these rules of thumb will have a video or a set of videos and I'll describe them Briefly here, Rule # one is a relationship between rise time of a digital signal or a digital waveform in terms of the RC time constant. So this is a large reason of why we had to cover RC time constants and now it's time to distill that into a rule. So, this is a very simple mathematical relationship that comes from rise time definitions that we spoke about earlier and I think you'll find it very useful. Rule # two is also about rise time. This time it's a relationship with the three DB bandwidth of a filter. Now that might be an intended filter or an unintended filter. And by the way, this three dB bandwidth, If you go back to the filter terminology, video has to do with that corner frequency or the cut off frequency recall that the corner frequency or the cut off frequency two words for the same thing Are at the three DB bandwidth point. If that's fuzzy to you, you can go back and scan that video again to pick up or clarify any point on that. Well, talking about frequencies and the three DB bandwidth, what does that have to do with time domain? And the rise time rule number two is a really, really helpful rule that relates those two. Rule number three is also about rise time. Notice the top three, Rules 12 and three all relate Rise time to something else that you might know. and rule three Relates rise time two o'clock frequency. Now, Each rule has its own set of assumptions that go with it. And the assumption and rule number three is that the clock frequency and the electronics driving and receiving. That clock and the interconnect pathways. The channel is all sort of modern technology, right? There's nothing weird or unusual going on. We'll explain more about that later. But this relationship between the rise time and the clock frequency based on a set of assumptions, is a really useful tool to have in your back pocket to make some quick estimates when all the information that you have is the clock frequency of a signal. Now, rule number three and rule number four go together. Both of them together have this similarity. If you know the clock frequency then Rule number three can help you with the time domain information. The rise time Rule number four helps you with the frequency domain information. That is the bandwidth that a signal of a given clock frequency likely takes up in the frequency spectrum. So number four is also really useful but little teaser here and we're going to explain this later. Number four is actually derived out of combining number two and number three but it deserves its own place in this list as its own rule it's just that useful. And then number five, we gotta have number five. The # five is the one that tells you when not to use these rules. Just as important as knowing when to use these rules of thumb is knowing when not to use these rules of thumb. So this is the rule of thumb that talks about When the assumptions and rules 1, 2, 3 and four are violated. I'll give you a quick example just square this up in your mind of when something might be out of the range of assumptions. Everything that we've talked about here makes an assumption that the circuit elements are electrically small. We're going to talk more about that term later. But that is to me that the wave effects of traveling waves can be neglected. Everything is acting like lumped elements like resistors and capacitors that exists at a point a physical point in space. So when would you ever not have this? Imagine you had an abnormal situation where the wires were really long and they actually start to act like antennas or coupling elements. Rule # five is some ways to estimate when you are out of bounds and these rules do not apply and you need some other solution. So these set of five rules based on all the information for Modules 12 and three and previous courses in this coursera specialization are going to be really valuable tools. I use them all the time in my work. And I hope you will too in the future. So let's get started.