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Welcome to the for loop Wildblue lecture in this lecture I'm going to introduce and explain how the

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for loop and while loop can be used in VHDL.

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First let's talk about the for loop for loop is commonly used to generate multiple instances of the

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same logic in VHDL for loops.

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They go away after synthesis.

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So we commonly use these to construct circuits for you know if we want to generate something over and

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over a for loop is a great way to do this.

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But after synthesis the four loops go away.

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They're just used to help make our code a lot cleaner is a lot less lines and a little more easy to

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understand.

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The key words in a for loop bar you have four in two loop and end loop.

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So an example of a for loop would be we have two signals who have signaled a b and c and their standard

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logic vectors from four down to zero so we know that there are five bits wide.

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And now we have our four loop statement where we're going to use this to generate R and gates.

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So we have our four I mean zero 2 for loop.

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So what that's going to do is take this value by and it's going to iterate from 0 to 4.

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So we'll get five iterations of 0 1 2 3 and 4.

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And inside of that loop whatever statements we replace is going to be iterated five times.

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So we have the statement.

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See why is equal to.

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I ended with be-I.

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So this is going to generate 5 different and gates where we have the input signal A and B will be added

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together and create the signal C..

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So if you look we have the five different and gates where our age zero is added with a zero and the

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output is zero.

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And this goes on for a one be one all the way down to a four before we get our output see.

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And this really is a quick and easy way.

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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

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B 1 all the way down to a 4 or C 4 is equal to a 4.

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And before this is a lot cleaner and quicker way of doing this.

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Now here we only have five and gates.

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But if you have a lot more complex circuit where you're working with say 100 and Gates or however many

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you have this is a quick and easy way to do that.

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And you can also do more complex things where you can implement flip flops or component instantiations

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of different circuits.

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But this is the general idea after synthesis this for loop example the for the for loops going to go

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away.

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We're just going to generate for endgames Now we'll talk about a while.

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These are very similar in nature to a for loop.

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However there are several differences.

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And the big one being that the condition is checked before the loop is entered in a for loop.

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We specifically tell the loop How many times we're going to iterate like with our example we said we're

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going from zero to four.

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So we knew right off the bat we're going to be iterating five times.

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However in a while loop we have a condition and this condition is checked before we go into the loop.

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And then every time we iterate through that loop we check that condition.

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And if that condition evaluates to true then we get out of the loop.

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So gives it that loop continues running until the condition evaluates to false.

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So it's important to know that the condition eventually evaluates to false.

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So if we look at the syntax of how the while loop would take place we have the loop name while condition

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and the condition is whatever you set out to be.

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While that's true we're going to be executing the statements and then we end our loop.

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And so you could do the same exact thing you do in a while versus a for loop.

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However you just have to make sure at some point in time that your condition will evaluate to a false

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so for example we have a process of calling it P2.

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We have it evaluates on the Ellen underscores the variable the one we're saying that to evaluate one

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and then we begin.

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Now we have our while loop for a while.

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The one is equal to less than or equal to ninety nine.

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We want to loop through.

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So our out Z is being set to Alan underscore z z 1 plus 8.

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And really the important thing to note here that is the one is constantly being incremented the values

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being set to Z One plus one.

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So we're going to increment through this loop 100 times or 99 times because you will start with z one

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starts at 1 and we'll go through nine nine times until the while the one is less than or equal to ninety

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nine.

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We'll just keep looping through.

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And so once we hit the one hundredth time we'll get out of the loop and the process and get out.

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Now if we would take out the statement z one is equal to Z One plus one.

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We would create a condition we call an infinite loop if that while loop will never go to falls.

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Then you'll sit in that loop and spin forever.

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And this can be a problem either your synthesizer will catch this and spit on a warning or if it doesn't

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hear program or your code and be HGL will never get out of this process.

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And so that can cause an issue.

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So that's just something have to be careful with when you're working with a while loop.

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Just be very cautious of infinite loops.

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So the main difference if we can bear for a loop while loop for loop will iterate a specific amount

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of times where a while loop will iterate based upon the code you tell it until the condition is false.

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And when you're working with a while loop just be very very cautious of an infinite loop.

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Now that you have the knowledge of for loops and while loops.

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Next we're going to learn about when else statements and with select when statements.