Reverse Polarity: Understanding and Application

[EvLer]

So, we are just starting ABEL... and I don't quite understand reverse polarity usefullness or application. Here's the extract from my textbook (Wakerly, 3rd ed.):
"In this example, the reverse-polarity equation has one less product term than the normal polarity equation (whatever...the example), so the compiler will select this equation if the targeted device has selectable output inversion."

could someone explain or give an example how this concept is used...
ok, so i understand that compiler will go for a more optimal option, but what does not make sense is that it would select let's say F' over F if F' is more optimal, but the circuit should operate on F not F'! how is this making sense?
Maybe I am asking the wrong question...anyway, I'm confused.

thanks much.

 

[berkeman]

I believe it refers to the fact that you can write an equation using OR or AND functions to gather up terms. Like:

A & B = !(!A + !B)

In some equations, it may take fewer terms when you use one form or the other, which may help you fit in a smaller CPLD, for example.

 

[ravenprp]

Reverse polarity is a concept used in digital logic design to optimize the use of logic gates and reduce the complexity of a circuit. It involves using the complement or inverse of a logical expression to simplify the design and reduce the number of gates needed. This can result in a more efficient and cost-effective circuit.

In the example given from your textbook, the reverse-polarity equation has one less product term than the normal polarity equation. This means that the reverse-polarity equation has been simplified by using the complement of the original equation. The compiler, which is a software tool used to convert a logic design into a physical circuit layout, will select this equation if the targeted device has the capability to perform output inversion. This means that the device can output both the original expression and its complement.

An example of using reverse polarity in a circuit design would be in a digital clock. The clock has two outputs, one for the hour and one for the minute. Instead of using separate logic gates for each output, the designer can use reverse polarity to simplify the design. By using the complement of the hour output, the designer can use the same gates for both the hour and minute outputs, reducing the number of gates needed and making the circuit more efficient.

In regards to your question about the circuit operating on F instead of F', it is important to note that in digital logic, the complement of a logical expression is equivalent to the original expression. This means that the circuit will still operate on the same logical expression, whether it is in its original form or its complement. The difference is in the implementation of the circuit, where using reverse polarity can reduce the complexity and improve efficiency.

I hope this helps to clarify the concept of reverse polarity and its application in digital logic design. It is a useful tool for simplifying designs and optimizing the use of logic gates. If you have any further questions or need more clarification, please don't hesitate to ask.