what does active low mean

[preethi19]

i understand what active low means. the pin gets enabled when the logic is low (0) and so the pin works at logic (0). but my doubt is why do we need active low logic to set something to work. I mean if we want that pin to work why cant we just set it in such a way wer the pin works when it is high... wat difference does it actually make... either way we just want the pin to be enabled. why does it matter if we enable it with high or low logic... can someone pls explain...

 

[ted]

First of all, what is active is kind of arbitrary decision, as there is no reason to prefer one for another. However, there are practical reasons and just plain old conventions.

One is the practical transistor technology, which is based on physical properties of semiconductors. It just happens, that NPN transistors and N-cannel MosFets are "better" or more "efficient". Either you believe me, or dig about the subject in text books or Google, as I wouldn't like to write a whole book here about the semiconductor physics.

Using NPN / N-MosFet leads to a topology, which is often using less power, when the output is high (especially notable in open-collector/open-drain configurations). Therefore the "idle" is often better to be the high level, and active to be the low level.

The other aspect is related to a tradition: Enable signals are often active-low, because "once upon time" the above was very much more critical, even if today the difference may be smaller.

However, the most important aspect is, that you shouldn't care at all, if the high or low is active. The only requirement is that you KNOW which way it is in the circuit at hand. Then the question is quite irrelevant and definitely on the background. And when you understand some basic Boolean algebra and wired-or / wired-and techniques, you can even do some nice tricks available, when the "sense of true" is inverted way - such as the so-called negative logic.

 

[betwixt]

Active low will work regardless of the line type. For example, in some systems there may be several different logic '1' levels. It is also consistent on lines that use a pull-up resistor (wire-OR). Basically, zero is zero, higher than zero could be anything.

Brian.