WEBVTT 00:12.740 --> 00:18.830 Welcome to the for loop Wildblue lecture in this lecture I'm going to introduce and explain how the 00:18.830 --> 00:23.170 for loop and while loop can be used in VHDL. 00:23.180 --> 00:29.240 First let's talk about the for loop for loop is commonly used to generate multiple instances of the 00:29.240 --> 00:33.100 same logic in VHDL for loops. 00:33.110 --> 00:35.090 They go away after synthesis. 00:35.090 --> 00:41.150 So we commonly use these to construct circuits for you know if we want to generate something over and 00:41.150 --> 00:43.260 over a for loop is a great way to do this. 00:43.280 --> 00:45.580 But after synthesis the four loops go away. 00:45.590 --> 00:52.280 They're just used to help make our code a lot cleaner is a lot less lines and a little more easy to 00:52.280 --> 00:53.190 understand. 00:53.360 --> 01:01.480 The key words in a for loop bar you have four in two loop and end loop. 01:01.680 --> 01:10.080 So an example of a for loop would be we have two signals who have signaled a b and c and their standard 01:10.080 --> 01:15.520 logic vectors from four down to zero so we know that there are five bits wide. 01:15.900 --> 01:20.910 And now we have our four loop statement where we're going to use this to generate R and gates. 01:20.910 --> 01:26.560 So we have our four I mean zero 2 for loop. 01:26.580 --> 01:32.850 So what that's going to do is take this value by and it's going to iterate from 0 to 4. 01:32.850 --> 01:38.260 So we'll get five iterations of 0 1 2 3 and 4. 01:38.460 --> 01:43.640 And inside of that loop whatever statements we replace is going to be iterated five times. 01:43.650 --> 01:45.630 So we have the statement. 01:45.640 --> 01:48.750 See why is equal to. 01:48.930 --> 01:51.460 I ended with be-I. 01:51.600 --> 01:58.860 So this is going to generate 5 different and gates where we have the input signal A and B will be added 01:58.860 --> 02:02.000 together and create the signal C.. 02:02.010 --> 02:09.270 So if you look we have the five different and gates where our age zero is added with a zero and the 02:09.270 --> 02:10.770 output is zero. 02:10.860 --> 02:17.730 And this goes on for a one be one all the way down to a four before we get our output see. 02:17.790 --> 02:20.250 And this really is a quick and easy way. 02:20.310 --> 02:32.310 Instead of writing C I yield to AI and B or C zeroes equals to a 1 or 0 0 9 C one is equal to a 1 and 02:32.310 --> 02:37.160 B 1 all the way down to a 4 or C 4 is equal to a 4. 02:37.170 --> 02:40.910 And before this is a lot cleaner and quicker way of doing this. 02:40.920 --> 02:43.530 Now here we only have five and gates. 02:43.710 --> 02:49.950 But if you have a lot more complex circuit where you're working with say 100 and Gates or however many 02:49.950 --> 02:52.720 you have this is a quick and easy way to do that. 02:52.800 --> 02:58.320 And you can also do more complex things where you can implement flip flops or component instantiations 02:58.320 --> 02:59.550 of different circuits. 02:59.670 --> 03:05.820 But this is the general idea after synthesis this for loop example the for the for loops going to go 03:05.820 --> 03:06.470 away. 03:06.510 --> 03:12.320 We're just going to generate for endgames Now we'll talk about a while. 03:12.410 --> 03:15.420 These are very similar in nature to a for loop. 03:15.440 --> 03:17.300 However there are several differences. 03:17.370 --> 03:22.970 And the big one being that the condition is checked before the loop is entered in a for loop. 03:22.970 --> 03:28.100 We specifically tell the loop How many times we're going to iterate like with our example we said we're 03:28.100 --> 03:29.610 going from zero to four. 03:29.690 --> 03:33.320 So we knew right off the bat we're going to be iterating five times. 03:33.320 --> 03:39.370 However in a while loop we have a condition and this condition is checked before we go into the loop. 03:39.440 --> 03:42.990 And then every time we iterate through that loop we check that condition. 03:43.100 --> 03:47.840 And if that condition evaluates to true then we get out of the loop. 03:48.020 --> 03:53.300 So gives it that loop continues running until the condition evaluates to false. 03:53.300 --> 03:57.710 So it's important to know that the condition eventually evaluates to false. 03:57.740 --> 04:04.820 So if we look at the syntax of how the while loop would take place we have the loop name while condition 04:05.180 --> 04:08.690 and the condition is whatever you set out to be. 04:08.690 --> 04:14.340 While that's true we're going to be executing the statements and then we end our loop. 04:14.480 --> 04:19.080 And so you could do the same exact thing you do in a while versus a for loop. 04:19.220 --> 04:24.230 However you just have to make sure at some point in time that your condition will evaluate to a false 04:26.220 --> 04:31.290 so for example we have a process of calling it P2. 04:31.440 --> 04:38.160 We have it evaluates on the Ellen underscores the variable the one we're saying that to evaluate one 04:38.670 --> 04:39.750 and then we begin. 04:39.840 --> 04:42.810 Now we have our while loop for a while. 04:42.810 --> 04:48.290 The one is equal to less than or equal to ninety nine. 04:48.300 --> 04:49.630 We want to loop through. 04:49.770 --> 04:57.510 So our out Z is being set to Alan underscore z z 1 plus 8. 04:57.600 --> 05:03.250 And really the important thing to note here that is the one is constantly being incremented the values 05:03.300 --> 05:06.350 being set to Z One plus one. 05:06.360 --> 05:13.800 So we're going to increment through this loop 100 times or 99 times because you will start with z one 05:13.800 --> 05:19.830 starts at 1 and we'll go through nine nine times until the while the one is less than or equal to ninety 05:19.830 --> 05:20.190 nine. 05:20.200 --> 05:21.720 We'll just keep looping through. 05:21.990 --> 05:27.470 And so once we hit the one hundredth time we'll get out of the loop and the process and get out. 05:27.540 --> 05:33.400 Now if we would take out the statement z one is equal to Z One plus one. 05:33.920 --> 05:39.680 We would create a condition we call an infinite loop if that while loop will never go to falls. 05:39.690 --> 05:41.720 Then you'll sit in that loop and spin forever. 05:41.850 --> 05:47.580 And this can be a problem either your synthesizer will catch this and spit on a warning or if it doesn't 05:47.940 --> 05:52.850 hear program or your code and be HGL will never get out of this process. 05:52.890 --> 05:54.500 And so that can cause an issue. 05:54.500 --> 05:57.310 So that's just something have to be careful with when you're working with a while loop. 05:57.330 --> 06:00.010 Just be very cautious of infinite loops. 06:01.340 --> 06:06.260 So the main difference if we can bear for a loop while loop for loop will iterate a specific amount 06:06.260 --> 06:13.100 of times where a while loop will iterate based upon the code you tell it until the condition is false. 06:13.340 --> 06:18.880 And when you're working with a while loop just be very very cautious of an infinite loop. 06:19.010 --> 06:21.780 Now that you have the knowledge of for loops and while loops. 06:21.800 --> 06:27.740 Next we're going to learn about when else statements and with select when statements.