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Default bit file


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Hello, I am new here,  I was just wondering about two questions

How would I use VHDL code as well as ucf to change the LED light intensity?? I know the up voltage is 3.3V and I cant seem to give it any other voltage aside from 3.3 using LVCMOS33 on the nexys 4 DDR.

However I noticed that the built in default bit file in the board was able to make the light seem deemer, so I am also wondering,where do I download that default bit file so I can look at the code??? or how would I extract it back out from the board?


Thank You

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Hi Megaxoplasma,

To control the LED intensity you only switch it on part of the time, using a technique called Pulse Width Modulation (a.k.a. PWM)

If you switch the LED on and off quick enough, you are only able to see the average intensity. To avoid flicker you need to switch it at least on and off at least 50 times per second.


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On the first embedded circuit board I ever worked on, the vendor provided a demo by which the LED grew from off to on and back again gently.  They did this by using a sine wave, and then mapping the output to the PWM controller mentioned above.  At the time, I thought this was just awesome!

Since then, I've discovered that a similar effect can easily be created from a triangle wave and that the sine function isn't needed at all.

Now, I think any beginner should have fun creating a project similar to the "Knight Rider" effect from years ago:

It's an easy and a fun project.



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Hello megaxoplasma,

To answer your other question about the default bit file, the factory program that is loaded onto the Nexys 4 DDR is (according to another applications engineer) actually a complex MicroBlaze project, so you would not be able to easily observe how the LEDs are working on that default project, nor do we have that project posted to our Wiki (although I'm not sure why).

If you would like an additional resource in addition to the advice that hamster and Dan kindly provided, I would recommend looking at the GPIO demo on the Nexys 4 Resource Center on our Wiki (link), which cycles through different colors on the Tri-color LED. 


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