Achievements
practice of Triggers de Schmitt |
Trigger
carried out with doors NAND |
Applications
of the rockers of Schmitt |
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Created it, 06/09/09
Update it, 06/09/19
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2. - ROCKERS OF SCHMITT
2. 1. - DEFINITION AND FUNCTION
A transition from a logical level L on a logical level H, applied to the entry of a reverser, can be schematized as indicated in figure 28.
Schematized thus, it appears that the signals present at the entry and the outlet side of the reverser present quite right faces, i.e. the tension varies instantaneously logical state in a complementary logical state.
However, this is a purely theoretical vision. The real signals move away from this representation theoretical and applied to the same logical circuit, would have the form represented on figure 29.
It thus appears that a logical signal spends a certain time (here t2 - t1) to pass from a logical state to another.
A second remark is essential. If one refers to the figure 30-a, it appears that the tension presents variations due to the parasites or the “noises”.
The latter are defined like disturbances or variations of tension on a small scale on an electric signal.
A buffer can be defined like an amplifier of current, i.e. a circuit preserving the shape of the signal and increasing the power available to its exit.
This buffer presents, for example, a threshold of swing equal to Vcc / 2, as represented with the figure 30-a. However, the input signal has disturbances. At exit, the logical signal is not thus stable but present oscillations as not represented at the figure 30-b.
Indeed, the undesirable oscillations at the entry on several occasions cross the threshold of swing of the buffer.
It was thus necessary to design logical circuits which can stage these two types of disadvantages.
They are the rockers of Schmitt or triggers of Schmitt. The fundamental idea is to create two thresholds of swing, one on the rising face of a signal, the other on the downward face of this signal. This is represented on figure 31.
At moment t1, the tension present at the entry reaches the threshold of swing VT+, the exit very quickly passes from the logical level L at the logical level H, although threshold VT+ is crossed several times during oscillations present at the entry of the trigger.
During the face going down, it is at the moment t2 that the input signal crosses the threshold of swing VT-. The exit then passes very quickly from the logical level H at the logical level L.
The two moments of swing are the two moments when the signal crosses for the first time the threshold considered. It is obvious that the more difference (VT+) - (VT-) is important, the more insensitive this circuit will be reliable and with the parasitic fluctuations superimposed on the original signal. This variation of tension between the two thresholds is called hysteresis. It is a characteristic suitable for a trigger of Schmitt. The hysteresis loop is represented on figure 32.
The arrows on this diagram indicate the direction of course of the input voltages and at the exit of the trigger.
It appears clearly that the exit passes from the level L on the level H as soon as threshold VT+ is crossed at the entry of the rocker (blue arrow). In the same way, it is necessary that the tension of entry goes down with VT- so that the exit passes from the level H on the level L (red arrow).
The difference (VT+) - (VT-) also the “margin of noise constitutes” which is the variation of tension that a signal can have without involving of particular incident on the operation of a circuit. Figure 33 takes the form of a signal present at the entry of a rocker of Schmitt.
At a given time, the entry crossed threshold VT+, the exit is thus on the level H.
One sees the disturbances of the input signal, but this signal never reaches threshold VT-, therefore the entry is considered permanently with the state H.
The following symbol (
)
indicates that a logical circuit has a hysteresis loop.
Examples are given on figure 34.
2. 2. - ACHIEVEMENTS PRACTICE TRIGGERS OF SCHMITT
2. 2. 1. - BASIC TRIGGER
In the trigger of figure 35, two R1 resistances and R2 are associated a buffer.
Two resistances are assembled in
tension divider bridge. The entry of the buffer has a very high resistance,
about a few tens of MW
(in technology CMOS). The effect of this
buffer will thus be neglected on the tension divider bridge. For that, R1
and R2 will have rather large values. For
example, R1 = 22 kW
and R2 = 100 kW.
In this case, we have the following
relation:
Let us apply to the entry E the signal indicated on figure 36.
At the beginning, V1 = Vu = 0 volt. As V1 increases, the tension of entry of the Vo buffer also increases and Considering remainder null. Indeed, it is necessary that Vo reaches Vcc / 2 so that the exit S rocks on the level H.
The V1 tension necessary to the swing of the buffer is the tension of higher threshold VT +.
From the preceding
relation, let us express this V1 tension of
swing.
Right before the swing, the Vo
tension is thus equal to Vcc / 2 and the
output voltage Considering is still null.
Let us replace Vo and Vu
by their value in the equation .
The V1
tension of swing which one calls VT+ is thus
given by the relation 
.
If one replaces in this case R1 and R2 by their value and knowing that the supply voltage is 5 volts, one obtains a tension of swing of :
This is thus the value of the higher threshold.
As long as the V1 tension will remain higher than the tension of threshold lower VT-, the exit S will remain on the level H (thus with tension Vcc).
When the tension of V1 entry goes down again, the buffer rocks on the level L for Vo = Vcc / 2.
Thus let us
calculate VT- using the
equation
by replacing Vo by Vcc
/ 2 and Vu by Vcc.
from where :
The relation is obtained 
:
Let us replace R1, R2 and Vcc by their numerical value :
The lower threshold is thus 1,95 volt.
Hysteresis is worth (VT+) - (VT-) = 3,05 - 1,95 = 1,1 volt.
It would be in addition possible to increase the value of hysteresis by taking a value for R1 higher than 22 kW.
2. 2. 2. - TRIGGER CARRIED OUT WITH DOORS NAND
Here, we do not use resistances. This trigger is represented on figure 37. It includes/understands three doors NAND at three entries carried out in technology CMOS. The operation of this trigger uses the following property : the tension of the threshold of swing is a function of the number of entries connected together to which is applied the control signal. This threshold will be all the more high as it will have entries connected there together.
In the at-rest state, the entry E and the exit S are at the logical level L. When the input voltage increases and reaches VT+, door 1 commutates, the entry SET passes on the level L and the exit S on the level H.
When the input voltage E goes down again and crossed threshold VT-, door 3 commutates and its exit passes on the level H. the exit S also commutates and Thus passes by again on the level L. this assembly is well a trigger having two thresholds of swing VT+ and VT-. Hysteresis (VT+) - (VT-) is worth approximately 1 / 3 of Vcc is 1,66 volt for Vcc = 5 volts.
If one wants to reduce hysteresis to 1 / 6 of Vcc, it is necessary to join together only two entries of door 1. This is indicated on figure 38.
Thus, threshold VT+ is decreased.
This particular circuit is often used as rocks of Schmitt available in the form of integrated circuit of family CMOS.
2. 3. - APPLICATIONS
OF THE ROCKERS OF SCHMITT
The applications of the rockers of Schmitt are numerous and some were already treated. It is the case when it is a question of removing certain rectangular signals from parasites or of improving of the rising faces or descendants which vary too slowly.
In chapter 3, the trigger will be presented in an assembly astable.
2. 3. 1. - TRANSFORMATION OF A SINUSOID INTO A RECTANGULAR SIGNAL
The assembly is that indicated on figure 39. With the entry a sinusoidal signal of frequency F is applied. To the exit, one obtains a rectangular signal of identical frequency F. two resistances R1 and R2 constitutes a tension divider bridge and C is a condenser which is used to uncouple the input signal compared to the entry from the trigger as Schmitt.
If one wants to obtain a square signal at the exit, one will choose to fix a V1 tension which is equal to (VT+) - (VT-) / 2. This appears clearly on figure 40.
This assembly can be used to convert a sinusoidal tension produced by a tachometric generator into a wave train having a frequency proportional to the number of revolutions of the generator.
2. 3. 2. - DEBOUNCING CIRCUIT
In the assembly presented at figure 41, it is a question of delivering an impulse of tension without a phenomenon of rebound to the closing of the contact appearing.
With the closing of the switch, there is bounce of the contacts, but the condenser C limits the variations of potential to item Vc and the hysteresis of the trigger makes it possible to preserve the logical level H at exit.
2. 3. 3. - DETECTOR OF LIGHT
The assembly of figure 42 makes it possible to detect a certain threshold of light to order, for example, the extinction of a lamp.
F is a photosensitive resistance whose value decreases when the light increases.
Arrived at a certain threshold of illumination, point A exceeds threshold VT+ of the trigger of Schmitt and the exit rocks at the logical level L.
Even if the light intensity undergoes light fluctuations, the exit remains on the level L.
This assembly also functions in the opposite direction. When the light intensity decreases, point A crosses potential VT- and the exit passes by again on the level H.
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