# what to input to CLK for 74 series IC?

Discussion in 'Electronic Basics' started by [email protected], Mar 19, 2007.

1. ### Guest

hi there, i'm doing a IC tester and it is able to only input high and
low voltages to the Input pin of the IC. For example, for a Inverted
NOT IC, when i input high, it will output low. However, what input
should i set for the CLK input pin of 74 series IC such as 74LS73A
(flip-flop)? An additional osscilator?And what does it meant by
"negative going edge of pulse"?. It is not high or low voltage,but the
transition from high to low with time equivalent with one high or low
signal, am i right?

2. ### JeffMGuest

[email protected] gmail.com wrote:
(was: what to input to CLK for 74 series IC?)
You
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teach.

3. ### Michael BlackGuest

I would have thought the datasheet would explain it, with some sort of table
showing the states of the device. I just checked, the TI book does have
such a table.

Or, one could get an actual 7473 flip flop and put it in a breadboard. Put
some LEDs on the output, and then play around with different combinations on
the input. Do it enough times, and the poster(s) will build up such a table.

Michael

4. ### Jon SlaughterGuest

The clock is used to syncronize. The clock is used so that when the clock
goes on then the circuit will "compute" its output. That is, when the clock
is in the "off" state it gives time for the device to do the computation and
have a stable output. You basically trigger the circuitry inside the
component to start the "computation" and it takes so long. You know the
maximum time it takes and this is the smallest time the clock can then turn
back off(or on depending how you look at it).

So in a clock you basically have 2 states. When the clock is "off" this
means that the computation is taking effect and the output is not ready.
When the clock goes on this means the output is ready and you can get the
information off the output and it will be correct.

Now this can happen in two ways. You can use voltage state to tell you the
state of the clock or the edges. (Sometimes its easier with the edges for
specific reasons)

Suppose you want to do your flip flop. You put your signal on the input. Now
if its a synchronous ff it will not have a correct output. You have to
trigger the clock so that the "computation" will take place. In this case
it doesn't matter which type of clock you use and doesn't matter how many
pulses but you need to make sure you sample the clock when it is on.

I'll leave you to figure out how to convert a synchronous flip flop to an
asynchronous flip flop. Obviously you have to somehow use the clock pin when
the signal changes. If you have a seperate digital control line you can
just use that on the clock and control the clock directly. Its best to play
around with the thing and see how it works before you try and build
something that actually uses it.

5. ### Randy DayGuest

'Negative edge' is when the signal goes from high to low.

'Positive edge' is when the signal goes from low to high.

The pulse below has a positive edge followed by a negative
edge:

+5 __
0__| |__

HTH