WEBVTT 00:00.400 --> 00:02.320 Welcome to this new section. 00:02.360 --> 00:08.720 I'm here and in our previous lectures we have explored the computer's most basic building block, which 00:08.720 --> 00:11.440 was the logic gates. 00:15.520 --> 00:22.360 Now we are ready to move on one step forward, which is logic circuits. 00:22.360 --> 00:27.400 So computers are not built from an individual gates alone, as you have seen so far. 00:27.600 --> 00:34.480 They are constructed from carefully organized groups of logic gates called logic circuits. 00:41.680 --> 00:52.160 Which makes up essential part of computer which works together to process digital information. 00:52.720 --> 00:58.920 And in this and the next lectures, we will dive into how to build some of the fundamental logic circuits 00:59.080 --> 01:05.870 that make up essential parts of a computer like CPU, memory units and other devices. 01:06.190 --> 01:09.630 However, we will not attempt to describe these complex systems in full. 01:09.670 --> 01:13.430 Instead, we will carefully examine a few small but crucial parts of. 01:14.470 --> 01:18.470 Logic circuits and focus on understanding the concepts behind them. 01:18.630 --> 01:23.070 And our main objective is to provide you with a clear and approachable introduction to the principles 01:23.070 --> 01:26.150 that underlie these important circuits. 01:42.310 --> 01:50.110 Logic circuits can be broadly classified into two types the combinational. 01:55.350 --> 01:56.950 And the sequential. 02:04.140 --> 02:06.700 Now let's focus on understanding the difference here. 02:07.460 --> 02:13.620 A combinational logic circuit is one where the output depends only the current inputs at that specific 02:13.620 --> 02:19.460 moment, so there's no memory or dependency on what happened before. 02:19.620 --> 02:22.940 So this system forgets any previous inputs. 02:23.460 --> 02:27.260 In contrast the sequential logic circuit is different. 02:27.580 --> 02:32.300 Here the output depends both on the current input and the past inputs. 02:32.340 --> 02:36.940 And this means the circuit has some form of memory to remember previous states. 02:37.420 --> 02:44.460 Now let's use a similar or familiar example your TV remote control. 02:44.500 --> 02:48.100 Now suppose you enter a number directly like five. 02:49.020 --> 02:52.540 The television immediately switches to channel five. 02:52.940 --> 02:58.260 The output, which is the selected channel, depends only on your current input, which is the number 02:58.260 --> 02:58.940 you typed. 02:59.380 --> 03:03.380 And it does not matter which channel you are watching before you're watching. 03:03.420 --> 03:05.760 You may maybe watching the, uh, 10th. 03:05.800 --> 03:07.120 Channel 15th. 03:07.160 --> 03:10.000 Channel 500th channel on your television. 03:10.440 --> 03:11.520 No matter where. 03:11.560 --> 03:12.520 What are you watching? 03:12.520 --> 03:17.080 If you press the five button, the channel will switch to fifth channel. 03:17.120 --> 03:17.600 Right. 03:17.840 --> 03:26.080 But the sequential logic circuits does not work like that when you press the channel up or channel down 03:26.080 --> 03:26.680 button. 03:29.280 --> 03:33.480 The television needs to know the current channel to decide the next one. 03:33.640 --> 03:43.120 So if we are on the channel fifth, if you press the channel down button, the television needs to know 03:43.120 --> 03:46.680 which channel we are in and add one. 03:46.680 --> 03:49.840 So it will move us to sixth channel. 03:49.840 --> 03:58.880 And if we press the channel up button, the television again needs to know which channel we are in. 03:58.880 --> 04:06.310 So it will, uh, add one on our current channel and switch to channel six. 04:06.310 --> 04:07.470 But yeah. 04:08.390 --> 04:08.990 Or five. 04:09.470 --> 04:10.710 Yeah, it doesn't matter. 04:10.710 --> 04:12.390 You get the logic here. 04:13.390 --> 04:21.430 Now in this, uh, section we will work with the combinational logic circuits in next lectures however. 04:24.750 --> 04:29.430 We will explore sequential logic circuits in depth. 04:30.510 --> 04:36.510 Now, before we get into designing logic circuits, it is very important to understand how digital zeros 04:36.550 --> 04:41.230 and ones are physically represented in electronic circuits. 04:41.630 --> 04:46.710 Now, digital circuits use two different voltage levels to represent the two binary states, as you 04:46.870 --> 04:49.950 have learned so far in this previous lectures. 04:50.310 --> 04:59.150 So a high voltage represents state one and the low voltage represents another state, which is in this 04:59.150 --> 05:00.230 case state law. 05:00.390 --> 05:11.940 So usually uh the higher voltage represents a Presents binary one and the lower voltage represents binary 05:12.140 --> 05:12.820 zero. 05:13.020 --> 05:16.340 But sometimes it can be the other way around, depending on the circuit design. 05:16.340 --> 05:23.740 But in the 99%, 99.99% of the time you will see this same. 05:23.740 --> 05:25.900 So upper voltage represents one. 05:26.780 --> 05:30.260 The zero or lower voltage represents basically zero. 05:31.940 --> 05:36.420 So terminology you must know is in an active high signal. 05:36.700 --> 05:42.540 The one is represented by a high voltage and the zero is represented by the low voltage. 05:44.300 --> 05:52.980 But the actor in an active low signal which is very rare, one is represented by a low voltage and the 05:52.980 --> 05:56.500 zero is represented by the high voltage. 05:56.500 --> 05:58.700 So this one is very rare. 05:59.060 --> 06:01.940 So I'm just explaining it. 06:01.940 --> 06:08.930 So you know that it is not an obligation to use one as a higher volt. 06:08.970 --> 06:09.370 Right. 06:09.890 --> 06:12.250 Or zero as a volt. 06:16.850 --> 06:17.330 So. 06:19.490 --> 06:22.850 An active high input can be connected to active low input. 06:22.850 --> 06:26.210 But the designer must make sure to adapt the signal correctly. 06:27.130 --> 06:35.330 So imagine a device expects a logical one to be a low voltage in this case active volt. 06:35.970 --> 06:41.770 But your signal is active high where one is high voltage. 06:43.290 --> 06:45.290 In this case, you need to invert. 06:49.450 --> 06:55.770 Which also also you can call that complement the signal before connecting these each other to it. 06:56.010 --> 07:01.930 Now this is important in real world electronics because misunderstanding active high low connections 07:01.930 --> 07:05.370 can cause logic errors in circuits. 07:06.650 --> 07:09.880 And the key point for this course is for simplicity. 07:09.880 --> 07:17.440 In this course we will always discuss only in terms of logic levels, zeros and ones without mentioning 07:17.480 --> 07:19.240 their actual values. 07:19.240 --> 07:24.880 But in real design work or data sheets you will often encounter this voltage level terms. 07:24.920 --> 07:32.920 Knowing them helps you communicate professionally with hardware engineers and read technical documents. 07:37.400 --> 07:41.680 Now here we will draw a quick diagrams. 07:42.280 --> 07:51.160 Um, you can on your draw, on your notepad, on your board during this lecture to practice on your 07:51.360 --> 07:51.840 own. 07:54.760 --> 08:01.720 And you may ask here how we can defer defer this combinational logic circuits in a real circuit design 08:01.720 --> 08:07.000 is basically combinational logic circuits gets the outputs. 08:11.510 --> 08:19.110 Gets the inputs, which is basically in n vars and which is variables. 08:21.350 --> 08:23.830 And gives us the basically outputs. 08:26.150 --> 08:32.510 So this is what this is basically a combinational logic circuit. 08:47.470 --> 08:49.470 But here we will have one. 08:49.790 --> 08:56.950 It will have the sequential logic circuit where it will have an input. 08:56.990 --> 09:04.350 Yes it will also have an output but it will also have the memory elements. 09:04.350 --> 09:09.730 So how the memory elements work is you will basically here just will get here. 09:10.170 --> 09:16.410 And yeah that is memory elements. 09:19.370 --> 09:21.170 And here this is basically. 09:23.770 --> 09:26.330 The combinational logic circuit. 09:26.770 --> 09:29.650 But since it has. 09:38.370 --> 09:42.850 So as I said the main difference is here between the combinational and sequential. 09:42.890 --> 09:45.810 The combinational logic circuits is also a logic circuit. 09:45.810 --> 09:54.330 But it it is it also has the memory elements which will practice in this lecture. 09:57.890 --> 10:04.490 So the summary of this lectures is computers are built from logic circuits. 10:04.690 --> 10:12.600 Logic circuits are two types the combinational and sequential Which combination means current inputs 10:12.600 --> 10:20.720 only and the sequential is current plus past inputs plus past inputs. 10:20.960 --> 10:25.560 And in this section we will focus on combinational circuits. 10:27.560 --> 10:34.120 And also you learned that binary ones and zeros are physically represented using different voltage levels. 10:35.600 --> 10:39.920 And it is crucial to understand active high and active low signaling. 10:40.080 --> 10:46.880 And in the next lecture we'll start designing and analyzing basic combinational logic circuits such 10:46.880 --> 10:51.000 as adders multiplexers, decoders. 10:51.640 --> 10:55.040 So prepare your notes and ready to sketch diagrams. 10:55.040 --> 11:00.720 Because visual thinking is a powerful tool when working with low level digital systems. 11:00.880 --> 11:02.520 Now, thank you for watching. 11:02.560 --> 11:03.400 I'm typhoon. 11:09.160 --> 11:10.880 And I'm waiting you in next lecture.