Operational tests of the
logical circuits synchronous MM 74C175 |
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Created it, 06/10/19
Update it, 06/10/27
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7. - FIFTH EXPERIMENT : STUDY OF THE OPERATION OF AN ASYMMETRICAL OSCILLATOR
During the third experiment, you could note that the time of extinction and the time of illumination of the LED were identical.
Indeed, the output signal of the oscillator is a rectangular signal as represented on figure 6. Its period of oscillation is 1 second. This rectangular signal is symmetrical, being at the high state 1 / 2 second and at the low state 1 / 2 second also.
It can prove to be useful to obtain an asymmetrical signal, i.e. from which the durations of the high state and low state are different.
The time of illumination of the LED will be, for example, higher than the time of extinction.
In this experiment, the circuit carried out enables you to obtain such an asymmetrical signal.
7. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food and remove all the components and the connections relating to the circuit carried out previously.
b) Introduce on the matrix an integrated circuit MM 74C14, an electrolytique capacitor with the tantalum of 1 µF - 10 V, by respecting the polarities, a resistance of 2,2 MW (red - red - green - but) and one of 470 kW (yellow - purple - yellow - but), in the positions indicated to the figure 14-a.
Lastly, carry out the connections indicated.
The figure 14-b represents the electric diagram of the circuit carried out.
7. 2. - OPERATIONAL TEST
a) Connect the food. The circuit starts to oscillate with a frequency from approximately 1 Hz. Indeed, the LED L0 ignites and dies out at this same frequency. Times of extinction and illumination are about identical, i.e. approximately 1 / 2 second. The output signal east thus symmetrical.
b) Disconnect the food. Insert on the matrix a resistance of 100 kW (maroon - black - yellow - but) and two D1 diodes and D2 of the type 1N 4148. You thus modify the circuit like illustrated with the figure 15-a. The electric diagram correspondent is located at the figure 15-b.
c) Connect the food. The time of illumination of the LED L0 is longer than the time of extinction.
Indeed, the LED is lit when the exit of the circuit is on a level H. Ainsi, only the D1 diode leads while the D2 diode is polarized in direction reverses and behaves like an open switch.
The operation of the circuit is then equivalent to that represented with the figure 16-a: its period is approximately 1 second, the exit being with the state H approximately 1 / 2 second.
On the other hand, when the exit pass on the level L, led the D2 diode and D1 the polarized diode in opposite direction behaves like an open switch. The circuit becomes equivalent to that represented with the figure 16-b. During this interval of time, the period of oscillation is worth approximately 1 / 4 second.
The duration of extinction of the LED is worth half of this period of oscillation, that is to say 1 / 8 second.
In this assembly, the sum of times of discharge and load of the condenser is different according to whether the exit is with the state H or with the state L because resistances in series with the two diodes are different.
d) Disconnect the food and interchange two resistances.
e) Connect the food. The durations of lighting and extinction of the LED are reversed compared to those observed previously.
f) You can test several times this assembly by modifying the values of resistances Ra and Rb. You will observe the variations over times of lighting and extinction, like their periodicity.
This type of circuit generating a dissymmetrical signal can be very useful.
You can calculate the period of oscillation in the following way :
time during which the exit is with the state H = 1,1 x Ra x C
time during which the exit is with the state L = 1,1 x Rb x C
total period T = 1,1 (Ra x C) + 1,1 (Rb x C) = 1,1 x (Ra + Rb) x C
Frequency of the signal :
8. - SIXTH EXPERIMENT : OPERATIONAL
TESTS OF THE SYNCHRONOUS
LOGICAL CIRCUITS
You studied the concept of logic synchronous and the need for having a clock signal to order at the same time several circuits.
You will carry out a simple experiment which will help you with better assimilating this concept of synchronous logic.
To carry out this experiment, you will use a new integrated circuit : the MM 74C175 whose diagram is deferred to the figure 17-a.
This circuit includes/understands four rockers D whose entries CLOCK and CLEAR are common.
The figure 17-b presents to you the
table of operation for each rockers D. These
rockers are activated by a face going up indicated 
and applied to entry CLOCK.
Entry CLEAR
is active on the level L. With this
condition, Q is with the state L
and 
is with the state H, independently of the
logical state of the other entries (symbol X in the columns CLOCK
and D).
8. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food, remove all the components and the connections relating to the preceding experiment.
b) Introduce on the matrix the integrated circuit MM 74C175 by positioning it as indicated in the figure 18-a then carry out the connections indicated.
Note in particular the connection between contacts COM1 and 1 Hz which allows the startup of the first generator of clock of the digilab.
The circuit carried out is represented with the figure 18-b.
The entry of clock common to the four rockers is connected to contact CP1 where arrives the clock signal of 1 Hz.
The LED L7 makes it possible to visualize this clock signal.
The entries D of the rockers are connected to switches SW0, SW1, SW2, SW3 and the exits corresponding to the LED L0, L1, L2, L3.
8. 2. - OPERATIONAL TEST
a) Put the switches on position 0.
b) Connect the food.
The generator of clock functions and delivers a signal of frequency 1 Hz announced by the LED L7. The LED L0, L1, L2 and L3 are extinct.
c) Observe the LED L3 and L7 and commutate switch SW3 on position 1.
The exit Q of the fourth rocker passes to the state H and the LED L3 ignites. The latter ignites only with the face going up of clock, announced by the lighting of L7.
In other words, you observe that the LED L3 and L7 ignite at the same moment. The LED L3 remains then lit because switch SW3 is always in position 1.
d) Test the various realizable combinations with the four switches.
The LED corresponding to the switches in position 1 ignite, those corresponding to the switches in position 0 die out, always at the moment when the LED L7 ignites.
This is an example of synchronous circuit. I.e. the four rockers take into account simultaneously the four data present on their entry only at the moment determined by the ascending face of the clock.
Thus, the operation of the four rockers is synchronized by a clock signal.
Moreover, the changes of logical state occurring constantly at the entry of the rockers are finally taken into account only at one well defined moment.
This type of circuit makes it possible to synchronize various events which can arrive on four logical entries. It is the case with four switches.
In the next practice, you will equip it with a controlled food and will make new increasingly interesting experiments.
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