Active low pins and circles

[ingamedeo]

I know that circles, when normally used in logic circuits, mean "not", therefore they invert the signal. (e.g. on the output of an AND port).

However, what is their meaning when they are present on the edge of a Flip-Flop, for example, with the CLEAR signal?
In order to CLEAR the FF, do I need to send a low ("0") value on the wire before the circle or a high ("1") value?

My understanding is that the circle simply means that the CLEAR signal is active-low, so I would send a "0" signal which would get inverted into a "1" and fed into the FF, activating the CLEAR function. Is that right? Or shall I send a "1" signal, which then would be flipped into a "0" and fed to the FF?

Thank you

[rstofer]

The signal is active low.  When you want to clear the flop, set the pin to logic 0.

[ingamedeo]

The signal is active low.  When you want to clear the flop, set the pin to logic 0.

Thank you! Grand!

[T3sl4co1l]

The circle means "active low".  It's sometimes used to show active-low signals, like if you see inverted inputs connected to inverted outputs.  Example, sometimes you'll see a chain of inverters (usually for buffering or delaying signals) with inverted outputs connected to inverted inputs.

Of course, an inverter with active-low input and active-high output is just the same thing as the other way around.  Or an active-low-input OR is identical to an active-low-output AND (de Morgan's theorem).

Tim

[glarsson]

Of course, an inverter with active-low input and active-high output is just the same thing as the other way around.  Or an active-low-input OR is identical to an active-low-output AND (de Morgan's theorem).

This is important to know when looking at older schematics. The way I, and many more, drew schematics was to use rings on all "active low" inputs and outputs. A line would always connect a ringed output with a ringed input, or non-ringed output to a non-ringed input. If the intention was to express "OR function between two active low input signals" the symbol would be drawn as an OR/NOR gate with ringed inputs, not as an AND/NAND gate as shown in the data sheet for the part used. E.g. a 7400 NAND gate could be drawn as an AND gate with ringed output or as and OR gate with ringed inputs.