Created it, 06/09/09
Update it, 06/09/28
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4. - MEMORIES CCD
Memories CCD are used than the preceding memories. The abbreviation is formed of initial of Load-Coupled-Device (device with couple of load).
They are devices in technology MOS made up of many capacitive cells connected in series so as to form registers with shift of the dynamic type, similar to those described in theory 8.
Figure 51 presents the synoptic diagram of a memory CCD of a capacity of 16 384 bits.
The memory is composed of 64 registers rebouclés on themselves, each one formed of 256 cells, therefore able to contain 256 bits.
The amplifiers which precede and which follow each register have as a task to regenerate the loads of the cells.
One can reach only one register at the same time and the selection of the desired register is carried out by sending an address on 6 bits (26 = 64).
The stages of the registers contain the data in the form of loads, matérialisation already met in the RAM dynamic.
In this case also, the loads must be periodically refreshed and the data are unceasingly moving.
That is ensured by the four clock signals indicated by f1, f2, f3 and f4.
The advantages offered by memories CCD are as follows : great capacity and relatively low cost.
The access time of these memories is high ; because, because of their structure it is necessary, for reading or to write a bit for example, to select the register which contains it then to wait until the bit while circulating, arrives to the exit of the register.
You point out that the RAM, like the ROM, are random access memories, i.e. time necessary for the operation of reading or writing is independent of the physical position, in the matrix of the memory, the cell which one wants to reach.
Memories CCD are on the other hand with access series, the time put by the bits to be transferred from one cell to following is about that of a cycle in a random access memory.
Consequently, the access time of a memory series with 256 bits per register (as in the preceding example) is 256 times the cycle time of a random access memory.
For this reason, memories CCD are not able to compete with memories RAM, but can be regarded as memories of replacement of great capacity for computers; in this case, they are called mass memories.
5. - MAGNETIC STORAGES
They are of two types : memories with tori, old, and bubble memories which are destined for brilliant future.
5. 1. - FERRITE CORE STORAGES
These memories RAM not birds were very much used ; but the complexity of their manufacture, their cost and their obstruction gradually made them give up.
5. 1. 1. - PRINCIPLE
Certain manganese and copper ferrites (ferroxcube) show two very interesting characteristics. Their coercitive force (Hc), i.e. the field for which magnetizing is cancelled, is very low. In addition, the hysteresis loop of these ferrites is quasi rectangular (figure 52-a).
It is thus enough to take logical conventions :
Writing : State 0 will be given by the direction of the magnetic field generated by a current circulating in the direction of the arrow drawn on L1 rolling up of figure 53 and consequently of remanent induction + Br resulting from this field. The torus will be with state 1 when the direction of induction is the reverse of that necessary to produce state 0.
Reading : Let us make traverse L1 rolling up by a current I. This current produces a magnetic field. Depending on the initial state of magnetizing of the torus, two things can occur :
If the torus were at the origin in the remanent state of induction +
Br (state 0) its permeability
weak and, if the hysteresis loop of ferrite is perfectly rectangular, like is
represented figure 52-b, one does not collect any terminal voltage of L2
rolling up.
If on the contrary, the initial state of the torus were with state
1, the induced field, if it has a value at least equal to Hm,
the swing of the magnetic state of this torus in state 0
causes. By doing this, the point of operation will cross a zone to strong
permeability of the characteristic of ferrite, from where it will result a flux
variation which will generate at the boundaries of L2
rolling up an electromotive force.
5. 1. 2. - CONNECTIONS AND USE
We saw that to make rock a magnetic core with rectangular hysteresis loop of state 0 (+ Br) with state 1 (- Br), one needs a field or a magnetomotive force at least equal to Hm.
If to produce a Hm field, one needs a current Im traversing a wire, it will be necessary if two rollings up are used, two currents Im / 2 of the same directions. One will obtain the swing of the torus thus only if two rollings up are fed simultaneously by an impulse of current Im / 2.
This principle of coincidence of the currents is that used in the magnetic storages.
The diagram of figure 54 shows the structure of a ferrite core storage. One sees a matrix with 9 ferrite cores there. In reality, these memories comprise thousands of them.
This system makes it possible to memorize 9 bits.
The tori are laid out according to lines and columns. To reach the one of these tori for reading or writing, one sends simultaneously an impulse on the wire of the lines (Y) and on that of the columns (X).
Let us take for example the torus located at the intersection of the line b and column 2. Let us suppose that in the beginning all the tori of the matrix are with state 0. If we send at the same time in two wire an impulse of current at least equal to Im, the torus rocks with state 1. So now we make traverse two same wire by an opposite current of direction, the torus goes rebasculer in state 0 and the wire of test which crosses all the tori of the matrix will be the seat of an impulse resulting from the reading of the information contained in memory.
We see that the reading of the memory is destructive, i.e. information is lost after each reading ; it is thus necessary in this technology, to rewrite information after each reading.
In the systems suggested by the manufacturers, instead of reversing the direction of the impulses in a pair of wire for the reading and the writing of information, one uses two vertical wire and two horizontal wire fed in opposite direction. A pair is specialized for the writing and the other for the reading. To rewrite information after reading, it is thus enough to reinject on wire of writing an impulse slightly delayed compared to that of reading.
Figure 55 represents the synoptic diagram of such a device of rewriting.
Figure 56 gives an example of control circuit of tori by transistors such as it was used.
5. 2. - MAGNETIC BUBBLE MEMORIES
The magnetic bubble memories are, just like memories CCD, of the memories with access series.
5. 2. 1. - MAGNETIC BUBBLE
In a magnetic material in thin layer, the magnetized zones take unspecified forms and are distributed in a random way ; however, if an external magnetic field is applied, perpendicular growing, these zones narrow until forming tiny cylinders (figure 57).
These cylinders, which one breaks by a magnetic field reverses, constitute “magnetic bubbles” and can be used to memorize information.
5. 2. 2. - DESCRIPTION OF BUBBLE MEMORIES
Figure 58 represents a bubble memory cross-section.
The bubbles of 2 µm diameter circulate in the thin magnetic layer (2 to 3 µm) deposited on a nonmagnetic substrate (generally a garnet of yttrium gadolinium or glass).
The external permanent magnetic field, perpendicular to the plan of the memory, is essential to the existence of the bubbles. It is provided by a set of permanent magnets.
A magnetic field turning is provided by two small orthogonal rollings up out of aluminum wire. This magnetic field very weak turning compared to the magnetic field permanent, known as of stabilization, makes it possible to make circulate the bubbles (figure 59).
The bubbles are guided in their displacements by a Permalloy guide whose reasons vary according to the manufacturers (figure 60).
5. 2. 3. - OPERATION OF BUBBLE MEMORIES
A bubble memory consists of a kind of register with shift rebouclé on itself in which a bubble represents a bit 1 and its absence a bit 0.
To write in memory, bubbles will thus have to be created.
For reading, it will be necessary to detect the presence of the bubbles. But it will have to be waited until the bubbles corresponding to the selected address ravel in the detector of bubbles because of sequential access of the memory, from where the need for making them circulate.
One carries out all that while organizing various ways out of rings or loops thanks to the Permalloy guides.
Figure 61 shows the organization of a bubble memory.
In order to increase the capacity of these memories, one adds to the principal ring of the secondary rings which are small registers out of ring in which the bubbles circulate.
To read the memory, it will be necessary to select the secondary ring, to transfer the data of this one in the principal ring then to read the data in series.
We see on figure 61 that it is necessary to read information to duplicate the bubble.
Indeed, the reading being destructive, one manufactures thanks to an earth phantom circuit a double of this one and it is this new bubble which passes in the read-out circuit whereas that of origin continues to turn in the principal loop.
The detection of the bubble itself is carried out by means of a bridge of magneto-resistances. The bubble, while passing on a magneto-resistance, produces an impulse of tension then.
The transfer of the bubble of a secondary loop to the principal loop is carried out thanks to an one-way circuit called loading and unloading post and whose principle resembles that of the circuit of duplication.
5. 2. 4. - PROPERTIES OF BUBBLE MEMORIES
The magnetic bubble memories have the advantage of having a great capacity (about a million bits) under a low volume but have a high access time (10 to 50 ms).
They have to replace the their restricted volume and their weak weight, mass memories to magnetic disks because of their great reliability.
The memory given in example on figure 61 includes/understands secondary 157 loops of 641 bubbles each one, that is to say 100 637 bits.
Some memories include/understand several principal loops, one says that they have several pages.
The bubble memories available in the trade are conditioned in cassettes including/understanding the whole of the circuits necessary to their operation. Their capacity varies 60K bytes with 4M bytes, their access time is a few tens of ms.
With the bubble memories this panorama of the memories is completed. In next theory 13, you will be able to see the programmable logic networks which, in certain cases, can advantageously replace the ROM memories because they are less expensive.
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