# What is the difference between clock and pulse?

What is the difference between clock and pulse ? For example:

• Is the CLK-signal a pulse ?

By the way, why do they invented the term frequence ? In my point of view, they could have said 20 times that period in a second?

• Why did they invent the term speed? They could have said "20 times that distance in a second" ... May 25, 2015 at 12:22
• You're going to have to find the English word for what you're calling "tact". It's not a recognised term in electronics (us engineers can be rather tactless) May 25, 2015 at 13:05
• Don't you mean "beat" instead of "tact"?
– user59864
May 25, 2015 at 13:07
• The author just corrected tact > clock. I think this is a translation barrier - for example the czech word takt sometimes represents clock speed. May 25, 2015 at 13:42
• English also has many words which use the "tach" prefix to mean "rate": tachometer, tachycardia, tachypnea May 25, 2015 at 18:32

1.) Generally, when referring to a "clock", the signal in question is a never-ending pulse train with known frequency, amplitude, and edge rates.

However, a single pulse used, for instance, to initiate the propagation of a data signal through a "D" type flip-flop is often referred to as a "clock pulse" and, in fact, many logic chip diagrams label the clock input "CP".

2.) The term isn't "frequence", it's "frequency", and it was invented in order to indicate the number of generally regular occurrences of an event in a particular unit of time. In the scientific community, the frequency of an occurrence is measured in hertz, with one hertz being equal to one cycle per second.

By the way, why do they invented the term frequenc[y] ? In my point of view, they could have said 20 times that period in a second?

In fact if you look at old writing, we did used to say "50 cycles per second" or "50 cps" to describe a frequency. You would see radio frequencies described in terms of "kilocycles" and "megacycles" as a shorthand for thousands or millions of cycles per second.

Only relatively recently did it become very common to say "50 Hz" instead. According to Wikipedia, the unit of hertz was introduced in 1930, but only became widely adopted after 1960.

Of course the quantity that these units measure has been called frequency for a very long time. According to Etymonline, the sense of the word meaning "rate of recurrence" dates form 1831. We use frequency rather than "rate of recurrence" or "times the period occurs in one second" because it's much shorter to say, and having shorter terms for frequently used concepts helps us to study and think about physics.

By the way, why do they invented the term frequence (sic)? In my point of view, they could have said 20 times that period in a second?

Well, that would be the definition of frequency (with a "y", not an "e" at the end). Frequency is not a human invention, it exists everywhere in nature and has existed long before humans.

Frequency is the rate at which an event occurs. The rotation frequency of earth around its axis is 1 rotation a day, or 0.0000116 Hz. Talking about clocks, frequency will normally be defined as the number of clock pulses per second. Your microcontroller may have a 16 MHz clock, then the clock generates 16 million pulses per second. So a clock signal consists of an indefinite series of pulses.

A pulse is sudden change in signal level, in a digital signal from low to high or vice versa, and after some time a return to the original level. A pulse is most often relatively short, but your digital signal may consist of a 1 second pulse, so that's not a requirement.

In a digital clock signal the pulses are often half the period wide, what is called 50% duty cycle. A 1 MHz signal has a 1 µs period, then the clock pulses will often be 0.5 µs wide.

Note that a repetitive signal with a given frequency consists of pulses, but you can also have pulses in a non-repetitive signal. A single pulse will have zero frequency.

This question has some good answers explaining frequency.