Meters
of Module higher than four |
Presentation
of 2 integrated Meters 7493 and HEF 4024B |
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Created it, 06/09/09
Update it, 06/09/23
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This theory presents the whole of the digital circuits used for the functions of counting and countdown.
1. - DEFINITION AND FUNCTION OF A METER
A meter (or discounting machine) is an electronic circuit primarily made up by a unit of rockers and generally of a combinative network.
This meter (or discounting machine) makes it possible to enter the number of events which occur during a given time.
Each event is translated into pulse.
These circuits generally have an entry (sometimes two entries) on which manage the impulses to count or to deduct.
Information available is located on the whole of the exits of the rockers.
There are many applications of the meters.
We can quote the counting of objects (figure 1), the measurement of time (figure 2), the division of time for obtaining clock signals allowing the order of the synchronized systems (figure 3).
Note :
A meter whose contents increase by a unit increments.
A discounting machine whose contents decrease by a unit décrémente.
There is a large variety of meters which you will discover during next chapters.
2. - ASYNCHRONOUS BINARY COUNTERS
The asynchronous binary counters use the pure binary code to count (or to deduct).
These meters are asynchronous, because only the first rocker receives the clock signal.
All the rockers which follow this one are ordered by the preceding rocker as indicated in figure 4.
2. 1. - THE DIVIDING METER BY TWO
The assembly located on figure 5 is the simplest meter since it uses that a rocker of the type D and that it is able to count one event.
The exit 
is rebouclée on entry D.
The chronogram of figure 6 makes it possible to follow the
evolution of the clock signals and the exits Q
and .
Let us suppose that the exit Q
is on the level L at the moment t0,
therefore
and D on the level H.
At moment t1 occurs the first active face. The exit Q rocks and passes on the level H since the entry D is on the level H.
Between moments t1 and t2, the entry D is on the level L. Thus, at the moment t2, Q returns on the level L and D on the level H.A the moment t3, Q passes by again on the level H and so on.
The period of the signal which is present on the exit Q is thus the double of that of the clock signal.
In other words, the frequency of the output signal is half of that of the clock signal. For this reason this assembly is a divider by 2. It is the basic element of the majority of the meters.
This meter has two states, which are 0 and 1, the state of a meter being defined by a particular combination of the logical states of the various exits. This meter can detect only one impulse, with the proviso of fixing the initial state of the rocker.
We will see that there is a problem involved in the travel time inside the rocker.
Indeed, if you look at figure 7, you notice that there is a transitory state between moments t1 and t2 and between moments t3 and t4.
We will reconsider this problem during this theory.
A divider by two can also be obtained with a rocker JK as represented figure 8.
This rocker functions in mode TOGGLE. The chronogram is same as that relating to the rocker D located on figure 6. This operating mode TOGGLE was presented during theory 5.
2. 2. - A METER MODULO 4
The assembly located on figure 9 is a meter made up starting from two rockers D.
This assembly is well an asynchronous meter since the clock signal H is not applied that to entry CLOCK of first rocker (CLOCK1).
Exit 1
is connected to entry CLOCK of second rocker
(CLOCK2).
Each rocker is cabled out of divider by two.
The chronogram of figure 10 makes it possible to follow the evolution of the meter in the course of time.
At the moment t0, the two Q1 exits and Q2 are on the level L.
With the first active face of clock (moment
t1),
the Q1 exit commutates and passes on the
level H. 1
passes on the level L.
At the moment t2, Q1
passes by again on the level L and 1
on the level H, therefore an active face is
applied to the entry of clock of the second rocker. Q2
thus passes on the level H.
At the moment t3, Q1 passes by again on the level H and Q2 remains on the level H.
At the moment t4, Q1 returns on the level L and Q2 also. The two exits returned in their initial state. One thus needed four clock pulses to find the initial state of the two rockers.
The truth table of figure 11 makes it possible to summarize the evolution of the meter and the divider by 4.
This meter is of module 4. The module is the number of logical states formed by the whole of the exits of the meter.
In this case, it is about a meter having four logical states (00, 01, 10, 11) in binary code or 0, 1, 2 and 3 in decimal code).
The capacity of this meter is 3. The capacity is the maximum number of events which a meter can enter. It is always equal to the module minus one since during the initial state (here 00) no event was still taken into account.
The divided Q1 exit by two the frequency of the clock H and the divided Q2 exit by four this same frequency of the clock H. On figure 10, it appears well that the period of the signal at Q1 exit is worth the period of the clock twice and at Q2 exit the period of the signal is worth four times the period of the clock.
Generally, it is always possible to use one or more exit of a meter to have a division of the frequency of the clock. In the clock of figure 2, this property is used to count the time which passes.
Indeed, the clock signal of frequency 1 Hz is divided by 60 and makes it possible to obtain a signal of period 1 minute. This second signal is in its turn divided by 60 in order to obtain the signal of period 1 hour. Then, it is enough to count the hours up to 24 so that one day was passed.
2. 3. - METERS OF MODULE HIGHER A FOUR
(Return to
the theory 9TS)
By connecting three rockers D as indicated in figure 12, one obtains a meter of module 8.
Three rockers D cabled out of divider by 2 are used.
The chronogram of figure 13 makes it possible to include/understand the operation of this meter. The principle of operation is always the same one.
Each stage makes it possible to divide by 2 the signal applied to its entry of clock. On the Q3 exit, the signal which one can take is thus at a frequency 8 times smaller than the clock signal.
Generally, it is thus possible to increase the module of an asynchronous meter by increasing the number of rockers. With a new rocker, the double module.
If a meter has n rockers, its maximum module is worth 2n. For n = 4, the module is worth 16, for n = 5, it is worth 32,…
It is possible to obtain a meter of odd module (3, 5, 7…) by using the same types of assemblies as those seen previously. That will be presented to you later on.
In addition, it is possible to replace each rocker D by a rocker JK cabled in mode TOGGLE (the entries J and K are cabled to “1”). Figure 14 represents a meter modulo 4 carried out with two rockers JK.
We will reconsider the problem of the transitory states. Figure 15 represents part of the chronogram of operation of a meter modulo 8.
On this chronogram, it appears that the duration of the unstable periods (transitory states) is a function of the number of rockers. This duration is worth to the maximum t4 - t1 = 3q in this case.
For this unstable period (t4 - t1), instead of passing directly from state 3 to state 4, the meter passes successively by the transitory states 2 and 0. It is considered here that the travel time of each rocker is appreciably the same one (q). Actually, these three travel times can be different.
It is obvious that if the number of rockers increases, the duration of the unstable period also increases. This is due to the asynchronous working of the meter since the rockers react the ones on the others in cascade.
For this reason, one uses an impulse of taking away which makes it possible “to read” the state of the meter. This impulse will be shifted compared to the clock signal one duration old higher than that of the transitory states. This impulse could be generated using monostable.
Figures 16 and 17 present two diagrams of taking away of the contents of a meter.
In the case of figure 17, it is also possible to store the contents of the meter in the register for the period that one wishes.
These transitory states are thus one of the principal factors which will limit the frequency of clock of the meter.
In technology MOS, with q » 100 ns, and 4 rockers D, the period of the transitory states is worth approximately 400 ns. If one holds approximately additional 100 ns to take the contents of the meter, the maximum frequency of operation will be 2 MHz :
If one wants to work at a relatively high frequency and to use a meter of great capacity, will thus have to be used a synchronous meter.
2. 4. - PRESENTATION OF TWO INTEGRATED
METERS
2. 4. 1. - THE METER INTEGRATED 7493
Figure 18 represents the general diagram of the meter integrated 7493 produced in technology TTL like its stitching.
The general diagram is the same one as that of figure 14. The entries J and K of the rockers are cabled internally to “1”.
A master clear asynchronous of the meter is possible thanks to the entries R0 and R1. For that the two entries R0 and R1 must be simultaneously to “1”.
This meter can function out of divider by 8 by presenting the clock on the entry B or out of divider by 16 by presenting the clock on entry A and by connecting exit QA to the entry B.
2. 4. 2. - THE METER INTEGRATED 4024
Its functional diagram and its stitching are given on figure 19.
This circuit is carried out in technology MOS. Symbol “N.C.” mean “off-line”.
It is a binary counter on 7 floors in cascade. Its logic diagram is given on figure 20.
is the entry of clock. MR. is the entry of master
clear asynchronous priority. The presence of a level H
on MR. gives all the stages of the meter to
zero independently of .
This meter is incremented on the downward face of
and can count up to 27 - 1 = 127 impulses.
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