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Welcome to when else statements with select When statement lecture and this lecture I'm going to talk

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about the when else with select and with select when statements in the VHDL language.

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So first let's talk about the when else statement A Well statement allows a signal to be assigned a

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value based on a set of conditions.

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The statement is considered a concurrent signal assignment.

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This is directly placed into your architecture and it is when all statements are very similar to a case

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statement.

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However a case statement when you create them you must place them inside a process and your micel design

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where when else statement does not need to happen inside of a process.

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So an example of a one else statement we have two signals A and B.

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They're both standard logic vectors or signal B is from 3 and 0.

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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

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and or when a statement is going to assign a value to B when depending upon the value of a sort of value

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of b.

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For example the first line will be equal to 1 000.

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When a is equal to zero zero.

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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

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equal to 0 0 1 0.

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When a is equal to 1 0 and also be equal to 0 0 0 1 when a is equal to one one.

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Now you'll know if you look at this statement you could say hey I can implement this in a case statement

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and me personally I would do that.

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These are just kind of it's more of a personal preference in VHDL as the lot of other languages you

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can do a lot of the same things just using different coding style structures or different statements.

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So in this case if you wanted something to put it directly in the architecture and you want it to happen

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before clock edge you would use a When statement.

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However if you need it to happen on the rising or falling edge of a clock or in Iowa every time a signal

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changes state you would want to use a case statement and place in a process.

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And so it's more like a personal preference or depending upon the specific design application.

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Now if we have a with select When statement This allows one of several possible values to be assigned

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to a signal based on a select expression the choice is slight and must be determined when being compiled.

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This is similar to a case statement as well.

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So you'll notice that with the select statement as well as the with select When statement.

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They're very similar they do the same exact thing.

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It's once again just kind of a preference.

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I've been doing this for multiple years and I almost always go with a case statement.

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I just think they look cleaner nicer and it's easier for me to understand but it's a personal preference

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thing.

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When when you're working with with Slyke When statement we have an example.

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So we have our with a select Y is equal to zero when 000 or C 1 when 0 0 1 and a comma.

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And we keep going down line like are C to when 0 1 0.

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Or C 3 when 0 1 1 and then we have our zero win others.

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And so it's a slight difference from the with select statement or with statement.

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This is kind of a a little difference but once again like I said it's just kind of a personal preference

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whichever you prefer on what looks cleaner to you what's easier for you remember.

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And so you kind of just make that decision now that we have covered all the different statements and

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loops.

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Next we're going to discuss the HGL processes and concurrent statements.