WEBVTT 00:12.140 --> 00:18.500 Welcome to when else statements with select When statement lecture and this lecture I'm going to talk 00:18.500 --> 00:25.990 about the when else with select and with select when statements in the VHDL language. 00:26.000 --> 00:31.910 So first let's talk about the when else statement A Well statement allows a signal to be assigned a 00:31.910 --> 00:34.890 value based on a set of conditions. 00:34.970 --> 00:38.810 The statement is considered a concurrent signal assignment. 00:38.810 --> 00:44.690 This is directly placed into your architecture and it is when all statements are very similar to a case 00:44.690 --> 00:45.640 statement. 00:45.710 --> 00:53.120 However a case statement when you create them you must place them inside a process and your micel design 00:53.510 --> 00:58.640 where when else statement does not need to happen inside of a process. 00:58.940 --> 01:03.870 So an example of a one else statement we have two signals A and B. 01:03.980 --> 01:09.250 They're both standard logic vectors or signal B is from 3 and 0. 01:09.260 --> 01:17.690 So it's 4 bits wide or signal A is from 1 down to zero so two bits wide and we'll use or when else statement 01:18.160 --> 01:26.630 and or when a statement is going to assign a value to B when depending upon the value of a sort of value 01:26.630 --> 01:27.230 of b. 01:27.260 --> 01:31.790 For example the first line will be equal to 1 000. 01:31.790 --> 01:34.280 When a is equal to zero zero. 01:34.550 --> 01:43.410 Otherwise B will be equal to 0 1 0 0 when they is equal to 0 1 and just on down the line or B will be 01:43.410 --> 01:45.200 equal to 0 0 1 0. 01:45.200 --> 01:53.090 When a is equal to 1 0 and also be equal to 0 0 0 1 when a is equal to one one. 01:53.090 --> 01:58.040 Now you'll know if you look at this statement you could say hey I can implement this in a case statement 01:58.460 --> 02:01.210 and me personally I would do that. 02:01.220 --> 02:06.470 These are just kind of it's more of a personal preference in VHDL as the lot of other languages you 02:06.470 --> 02:12.980 can do a lot of the same things just using different coding style structures or different statements. 02:12.980 --> 02:18.500 So in this case if you wanted something to put it directly in the architecture and you want it to happen 02:18.500 --> 02:21.990 before clock edge you would use a When statement. 02:22.010 --> 02:28.190 However if you need it to happen on the rising or falling edge of a clock or in Iowa every time a signal 02:28.220 --> 02:32.520 changes state you would want to use a case statement and place in a process. 02:32.600 --> 02:38.000 And so it's more like a personal preference or depending upon the specific design application. 02:39.110 --> 02:45.740 Now if we have a with select When statement This allows one of several possible values to be assigned 02:46.130 --> 02:52.940 to a signal based on a select expression the choice is slight and must be determined when being compiled. 02:53.210 --> 02:55.160 This is similar to a case statement as well. 02:55.160 --> 03:01.040 So you'll notice that with the select statement as well as the with select When statement. 03:01.100 --> 03:03.830 They're very similar they do the same exact thing. 03:03.850 --> 03:06.310 It's once again just kind of a preference. 03:06.350 --> 03:11.280 I've been doing this for multiple years and I almost always go with a case statement. 03:11.300 --> 03:15.860 I just think they look cleaner nicer and it's easier for me to understand but it's a personal preference 03:15.860 --> 03:16.390 thing. 03:17.730 --> 03:23.020 When when you're working with with Slyke When statement we have an example. 03:23.050 --> 03:35.770 So we have our with a select Y is equal to zero when 000 or C 1 when 0 0 1 and a comma. 03:36.070 --> 03:39.550 And we keep going down line like are C to when 0 1 0. 03:39.580 --> 03:45.150 Or C 3 when 0 1 1 and then we have our zero win others. 03:45.330 --> 03:50.960 And so it's a slight difference from the with select statement or with statement. 03:51.040 --> 03:55.180 This is kind of a a little difference but once again like I said it's just kind of a personal preference 03:55.180 --> 03:59.400 whichever you prefer on what looks cleaner to you what's easier for you remember. 03:59.590 --> 04:05.100 And so you kind of just make that decision now that we have covered all the different statements and 04:05.100 --> 04:05.610 loops. 04:05.610 --> 04:10.110 Next we're going to discuss the HGL processes and concurrent statements.