Digital Electronics, in this context, refers to the study of the basic building blocks of digital logic. The digital chips that we use on a daily basis, from the simple 8 bit AVR to the latest quad core monster have one thing in common. They all use logic gates as the basic building blocks that define their digital logic.
If you want to be successful in defining digital logic then it is important to have some understanding of the basic building blocks. Digital Electronics refers to AND, OR, and NOT gates and all of the digital logic structures that can be derived from them. Its not crucial to have a deep understanding but it is important to know what all the basic gates are and what can be derived from them. Examples of structures that can be derived are:
7400 series logic chips are probably the most familiar example of digital logic. FPGA's can easily implement, on the fly, any functionality contained in 7400 series logic chips.
It's good to take a look at some of these resources to get a better understanding of what Digital Electronics encompasses.
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