Preparation of the
material |
Use of a synchronous
Rocker D like meter modulo 2 |
Use of a Meter like
Divider of Frequency |
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Created it, 06/10/19
Update it, 06/10/28
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During this practice, you will check in experiments the operation of the meters and the dividers.
A meter makes it possible to know the number of events which have occurred for a given period. A divider reduced in a report/ratio given the number of events which arise on the entry of the divider.
The two functions, counting and division, are often fulfilled by the same circuit. An analogy can be given by the clock of a bell-tower, represented figure 1. Indeed, the number of the past seconds is divided by 3 600.
Every 3.600 seconds the chimes announce that one hour has been just passed.
Into numerical electronics, the events are translated into impulses applied to entry CLOCK of the circuits concerned as illustrated on figure 2.
The frequency of the output signal of a divider is always lower than that of the input signal. The relationship between the frequencies of the exit and input signals is an integer N.
In the case of a meter, the logical state exits evolves/moves with each clock pulse.
Thus, the exits inform about the pulse repetition frequency applied to the entry of clock. In this way, the exits make it possible to know the pulse repetition frequency arrived at the entry.
The principal characteristics of a meter are :
its capacity
its module
The capacity is the greatest number of events which a meter can add up; a mileage odometer of car for example has a capacity of 99 999 kilometers.
The module, on the other hand, is the number of the possible states of the meter, including the initial state. It is thus equal to the capacity increased by one.
Thus, a mileage odometer of car has a module of : 99 999 + 1 = 100 000.
1. - PREPARATION
OF THE MATERIAL
To carry out the experiments of this practice, you will have to use the following integrated circuits :
MM 74C74 or equivalent
MM 74C175 or equivalent
MM 74C86 or equivalent
MM 74C08 or equivalent
MM 74C163 or equivalent
MM 74C00 or equivalent
MM 74C193 or equivalent
MM 74C85 or equivalent
MM 74C32 or equivalent
2. - FIRST EXPERIMENT : USE OF A SYNCHRONOUS ROCKER
D LIKE METER MODULO 2
In this experiment, you will realize simplest of the meters by using a rocker D cabled out of divider by 2.
The capacity of counting of this very simple circuit is limited; indeed, it counts only up to 1. It thus has a capacity of 1 and it is of module 2.
While being very simple, this circuit is however important because it constitutes the basic element of the meters of higher capacity.
2. 1. - REALIZATION OF THE CIRCUIT
a) Remove matrix and contacts of the group of connectors all the connections and components relating to the preceding experiment.
b) Take the integrated circuit MM 74C74 (double rocker D), introduce it on the matrix into the position indicated to the figure 3-a and carry out connections indicated.
The figure 3-b shows the electric diagram of the circuit carried out.
2. 2. - OPERATIONAL TESTS
a) Place SW0 and SW1 on position 0, then introduce the card into the socket-outlet and energize the digilab. You note that the LED L0 and L1 are extinct.
b) Put now SW1
on position 1 :
thus, entry PRESET becomes inactive. The
rocker is with state 0 (Q
= 0 and 
= 1) since entry CLEAR
is active. This is confirmed by the fact that L0
is extinct and L1 lit.
c) Place SW0 on position 1, entry CLEAR becomes inactive like entry PRESET. The rocker thus is forced any more in no state and it can thus transfer at exit Q the logical level present on the entry D at each positive transition from the clock signal.
d) Press then on the button P0, the level H present on the entry D is thus transferred at exit Q, which you notice by observing the LED L0 lit and the extinct LED L1.
Indeed, the exit Q
passed on the level H and the exit
on the level L.
Thus, the circuit took into account a clock pulse and announced by a change of state at exit Q.
e) Support once again on P0 ; L1 ignites and L0 dies out.
Indeed, the level L
present on the entry D was transferred at
exit Q. the rocker returned to the state 0
(Q = 0 and
= 1.
One can interpret this by saying that the meter exceeded its capacity and that it is turned over in its initial state.
f) Commutate the switch of the food on “OFF”.
In short, the examined circuit proves to be a meter of capacity 1.
Indeed, put at zero initially by means of entry CLEAR (left Q with state 0), it is able to announce only one impulse applied to its entry CLOCK.
This event was indicated by the exit Q which passed from the level L on the level H. the meter thus went to state 1.
By supporting one second time on P0, therefore by applying one second clock pulse, you exceeded the capacity of the meter and this last returned to the state 0 initial.
Since two impulses are enough so that the meter returns in its initial state, the meter is of module 2.
The succession of states 0 and 1 of the meter can be represented by the diagram of the states of figure 4. This one shows well that there are only two possible states.
The passage from one state to another of the meter occurs only on arrival of an active face on entry CLOCK.
3. - SECOND EXPERIMENT : USE OF A METER LIKE DIVIDER
OF FREQUENCY
In this handling, you will continue the examination of the same circuit as that used in the preceding experiment. This circuit will be examined not like meter, but like divider.
Previously, the rocker was used as meter of module 2. Now, this one will be used to divide by 2 the frequency of the signal applied to entry CLOCK.
3. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the connection
which connects pin 3 of the integrated circuit MM
74C74 into the P0
contact.
b) Connect pin 3 to contact CP1 and the L2 contact, as indicated in the figure 5-a.
c) Lay out the first generator of clock of the digilab on frequency 1 Hz by connecting the contact COM 1 to the contact located by inscription 1 Hz as indicated in the figure 5-c.
The electric diagram of the circuit carried out is represented with the figure 5-b.
3. 2. - OPERATIONAL TESTS
a) Place SW0 on position 0 and SW1 on position 1.
b) energize the digilab, you note that L2 flickers at the frequency of 1 Hz while L0 is extinct and L1 lit.
c) Put SW0 on position 1, you note that L0 ignites and dies out periodically. It remains lit during one second, then remains extinct the second following one.
The period of the signal on the exit Q is worth thus 2 seconds and the frequency of this signal is 1 / 2 Hz. L0 thus flickers at a frequency 2 times smaller than that of L2.
In other words, the circuit divides by 2 the frequency of the signal present on entry CLOCK. Since the input signal at a frequency of 1 Hz, the output signal thus has a frequency of 0,5 Hz.
Figure 6 represents the succession of states 0 and 1 in entry and the corresponding changes of the state of the exit.
As you can observe it, the exit Q of the rocker changes state only with the rising face of the clock signal. To obtain a complete cycle at exit, one thus needs two complete cycles of the clock signal. The frequency of this signal is thus reduced half.
The mileage odometer of car could be regarded as a divider if it generated for example an aural signal (or luminous) each time it indicates 00 000. Indeed, such a signal (by supposing that the speed of the car is constant) would have a frequency 100 000 times smaller than that of the signal which advances the meter of one kilometer.
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