RS FlipFlop

A flipflop is a digital circuit that can be used to store a single bit of information. They come in a variety of types that all work slightly differently. Flipflops will be discussed in detail in a later guide. For this guide we just need to know about the RS flipflop.

The "RS" stands for Reset-Set.  The way this flipflop works is that it can be set (making its state high) and reset (making it's state low) by using its two inputs: S and R, respectively. It also has an output that reflects its internal state. Flipflops typically also have an inverted output which will be the logical negation of the output (i.e. low if the output is high, high if it's low).

We won't worry about the internal details at this point. A later guide dedicated to flip flops will develop the idea of a flipflop from the inside-out. R is the reset input, S is the set input, and Q is the output. It's something of a standard to use Q to denote the output. This was tradition even when I got started. The Q with a bar over it is the inverted output. We'll call this Q-bar. If Q is high, Q-bar will be low, and the opposite. On the flipflop this is denoted by the small circle at the Q-bar output. A small circle like this in digital logic diagrams means inversion.

If both R and S are low, then the flipflop is stable and nothing changes. If S goes high briefly, Q becomes high, regardless what it was previously. If R goes high briefly, Q becomes low. Both R and S being high at the same time is not usually an interesting case, but if it happens, Q will go high.

In practice both R and S will normally be low, one of them can be made high for some reason to cause Q to go high or low and stay that way until R or S goes high again.  As such, an RS flipflop is a basic unit of memory, able to store a single bit of information.

This guide was first published on Mar 13, 2018. It was last updated on Mar 13, 2018. This page (RS FlipFlop) was last updated on Nov 13, 2019.