Preset and Clear in Digital Electronics

We discussed Flip Flops in detail in one of our previous post. In this post, we will discuss about two additional I/Ps – Clear (Cr) and Preset (Pr) available in most of the flip flops.

As flip flop may come up in random states when power is switched on, it is necessary to use some method where all the flip flops can be set or reset at the same time by a single switch – it is inconvenient to set or reset each flip flop individually. This is achieved by incorporating PRESET and CLEAR inputs in flip flops which can over-write all other inputs. These I/Ps are asynchronous and independent of the clock. While they are present, all other operations are inhibited. They force the output to a certain state irrespective of the I/Ps and clock.

Active High

Active High means that Preset and Clear I/Ps are inactive when low.

Pr = 1, Q = 1, Q’ = 0

i.e. if the Preset I/P is 1, the output will be forced to SET state irrespective of other I/Ps.

Cr = 1, Q = 0, Q’ = 1

i.e. if the Clear I/P is 1, the output will be forced to RESET state irrespective of other I/Ps.

It is not possible to Preset and Clear a flip flop at the same instant of time as Q can’t be 0 and 1 at the same time.

i.e. Pr = Cr ≠ 1 at any instant

Active Low

Active Low means that Preset and Clear I/Ps are inactive when high.

Pr’ = 0, Q = 1, Q’ = 0

i.e. if the Preset I/P’ is 0, the output will be forced to SET state irrespective of other I/Ps.

Cr’ = 0, Q = 0, Q’ = 1

i.e. if the Clear I/P’ is 0, the output will be forced to RESET state irrespective of other I/Ps.

It is not possible to Preset and Clear a flip flop at the same instant of time as Q can’t be 0 and 1 at the same time.

i.e. Pr’ = Cr’ ≠ 0 at any instant

 

Leave a Reply

Your email address will not be published. Required fields are marked *

*

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code class="" title="" data-url=""> <del datetime=""> <em> <i> <q cite=""> <strike> <strong> <pre class="" title="" data-url=""> <span class="" title="" data-url="">