Through hardware security, it is important to understand how hardware is built first. In this set of slides, we will briefly reveal the fundamentals of digital logic design. We will talk about digital systems. And we'll talk about the design and the synthesis of such systems. We will start with how to specify such systems, how to implement digital systems, and how can we optimize such limitations. And we will use examples from both combinational logic and sequential circuit to demonstrate this procedures and concepts. And finally we'll talk about the don't care conditions. These are the conditions that have been played a crucial role in terms of the system synthesis and optimization. And don't care conditions you can also cause security vulnerabilities. However, when used properly, don't care conditions can also provide us a unique opportunity to build more secure, more trusted systems. A system can be viewed as a black box. Where we have the system here, it takes input and generates output. And the input output pair, what their relationship is, what we called the functionality. And the system is used to model the real world, and then trying to get reasoning behind all this phenomenas. So we know real world is analogue world. So which get captured by this figure here. Both the input and output signals, they are analog signals. And this captures precisely what happens in your real world and this x-axis is the time. However the computer doesn't like this continuous values. So what we have done is we, we digitalized all these signals. So this is a digital signal here, where the values take only discrete values. One level, two level, three level for this, this signal, and then two levels for the other signal. So this is a, becomes a digital signal. And also based on what these signals can change according to time t. We can put signal systems into two categories. The synchronized systems and asynchronized digital systems. In the synchronized systems the time, the signals can change its value only at the discrete time which is normally periodic which is what you call the clock. At each clock, they may or may not change. And then during the clock the signal stays at constant value. And in asynchronized version the value of input and output can change at anytime and Harvard they are restricted to these discreet values. The last accounts that I want to talk about is the difference between combinational logic and sequential logic. So in a combinational logic whenever you take an even input combination you always have the same output. No matter how many times you run this system and when you feed this input to the combinational logic. But for the sequential system, the input and output pair may not be unique, which means for the same input your output may not be the same, may be different. So why that is the case? That is because inside a sequential system, there is a unit called a memory element. The output depends both on the input, and the current value of the memory unit, which means the combinational logic inside the sequential circuit will take inputs from primary outputs, and also internally, from the memory units. And I use this to evaluate what will be the output. And then meanwhile, it's, it's going to update the memory units. So now we talk about how to specify a digital system. The specification of a system is a description of its functionality and other characteristics that is required for its, for its use. So, when people specify digital systems, normally they go from high level to low level, or binary-level. And we have mentioned about the systems. Normally they take inputs from real world. And real world is analog. And so the first step we need to do is we need to convert these analog signals to digital. And then for digital signals sometimes we still have to do encoding. So this is the input encoding and those are when the system output. An output will feed back to the real word. We also want to do encoding. So after the computation we got a binary output and then we need to convert it back to real word. It can be digital, can be analog. And in the middle, we need to determine how we can get output from the inputs. That is the one we call the Boolean function. This function determines, given inputs, what will be the outputs. And normally, these Boolean functions will be represented either as a truth table for combinational logic or as a finance state machine. Or sometimes we call the state transition table or state transition graph. These are used then to model syncrotion systems. So let's see a small example to see how we can specify a digital system. So we want to design a system that will tell you whether a month, a given month has 31 days or not. So we think about this. For this system, the input will be the 12 months of a year. So we write a high-level system spec. The input x can goes to from January to December. Which way will presented by decimal numbers from 1 to 12. Outputs will tell whether this month has 31 days or not. So we can represent this by a single bit z of 0 or 1, and 1 means it has 31 days and 0 means it doesn't have 31 days. And the functionality of this system can be represented by this step, step function. When x is 2, 4, 6, 9, or 11. This means when x is February, April, June, September, or November it won't, will, will not have 31 days. In other months, it will have 31 days. And we can easily draw a truce table to represent the same information. The input x goes from January all the way to December. And then the output z tells you that which months has 31 days. Which months doesn't has 31 days. And this is a high level system spec, and before we can design these systems, we need to convert a high-level system spec, to a low-level system specification. And by low level, we talk about the binary level. I mean, this system, we know that x input has 12 different values, 12 different nonces. And to represent 12 different objects we need four bits. That is why the input bits will be go from x3, x2, x1, to x0. And output has only two possibilities, 0 or 1. So there is only one bit for output. So now we can conv, convert the previous truth table to this truth table at the binary level. So this is x. At high level goes from January to December. This is output a z tells you whether it has 31 days or not 31 days. And in the middle we see the inputs encoding, where we used the Binary encoding. So January, which is 1 is encoded as 0, 0, 0, 1. And then April which is 4 is encoded as 0, 1, 0, 0 which is the binary 4. So the next step we need to do here is trying to figure out what will be the functionality. And remember before we write a functionality as a step function. And now since we have moved down to lower levels, or binary level description. So, we need to think about how to relax this function at the Boolean level, at the lower level. So, what we do is we take a look at the input, the four bit input and output. And this is the design phase. So we want to express the output as a Boolean function of the input. And by observation or by later we do any kind of optimization technique we figure out in this case the output z. This x3 complements, times x0, plus x3, times x0 complements, which is the same as x3, exclusive or, with x0. Which means that whenever x3 and x0, they are different the output z will be a 1. Whenever they're the same, the output will be a 0. For example 0 1 give you a 1, 0 0 give a 0, 0 1, give you a one, and the 1 0 give you a 1. Whenever they agree on each other, 1 and 1, it give you a 0. Kay?
