Created it, 06/09/09
Update it, 06/09/28
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3. 3. - PROGRAMMABLE ROM MEMORIES
As we saw in the preceding chapter, the ROM cannot be used apart from great series, taking into account the requirements of manufacture.
This is why, in order to obviate this disadvantage, the manufacturers developed of the programmable ROM by the user himself and not requiring of this fact not special orders, therefore also no delivery period.
Several different types are available.
3. 3. 1. - PROM (PROGRAMMABLE READ ONLY MEMORY)
Those have, in the place of the usual electric connections of the fuses which one can dissolve by means of a programmer of PROM.
These fuses can be made in nickel-chromium, polycrystalline silicon, or titanium tungsten.
Figure 43 represents the diagram of a bipolar cell of PROM with its fuse.
In the PROM, the one-way element diode or transistor is assembled in series with the fuse.
When the fuse is intact, all occurs as in the preceding case and columns and lines are connected.
When one wishes to remove a fuse, it is enough to dissolve it (it is said that it is burned). One carries out this operation by sending an impulse of current which is obtained by increasing the supply voltage temporarily.
Figure 44 shows a fuse which is not other than a contracting in the driver, therefore a point of weakness.
Figure 45 shows a fuse titanium tungsten been based at the time of the operation of writing and programming.
A ROM can, thanks to a programmer, being programmed in a few minutes.
The major part of the PROM is directly compatible with the ROM from the stitching point of view. This is why it is convenient, in making of a prototype, to use a PROM which will be replaced in the final version by the ROM equivalent.
It is not necessary to program the totality of the PROM in only once. Generally, one programs of it a part at the same time progressively with the needs. Each time new programs are introduced, one can then use the remaining part of the PROM.
One will be able to also correct certain errors and to dissolve a forgotten fuse, but one will not be able of course to act in the other direction. Because a burnt-out fuse is irremediably!
A PROM is thus programmable only once.
3. 3. 2. - DEAD MEMORIES REPROGRAMMABLE EPROM OR REPROM
Once programmed, a ROM or a PROM could not be programmed once again.
This is why it was developed a new kind of ROM, the EPROM (Electrically Programmable Read Only Memory) still called REPROM (Reprogrammable Read Only Memory).
This news memories can be reprogrammed several times. This advantage makes it possible to make evolve/move the systems while making improvements in the programs. Indeed, no program is perfect as of the beginning and often it will be necessary to make turn a program under the real conditions of use to detect certain defects.
To carry out REPROM, there are several possibilities. We will describe here only most largely widespread.
All the one-way elements defining the points reports are consisted transistors MOS special called to insulated grid or FAMOS (Flotting spoils Avalanche injection MOS) of which one is schematized figure 46.
a) Programming
At the beginning, the transistor does not lead and columns and lines are isolated.
On the other hand, if the control electrode at the same time is polarized as one sends a strong impulse of tension between the drain and the source, the function NP of the drain leaves in avalanche, i.e. the charge carriers multiply and acquire sufficient energy to cross oxide insulating and to be made trap by the floating grid.
From this moment and even after disappearance of the impulse of tension of programming, these loads remain captive and the floating electrode remains permanently charged, which makes the MOS conducting.
The phenomenon remains stable during ten years, the pressure loss reaching 30 % then. But by safety, it is advised to refresh these memories every five years.
Columns and lines are thus durably in short-circuit. We see in this case, that we established a contact (and not removed a contact like previously).
b) Obliteration
To erase a memory REPROM, one exposes the crystal during approximately ten to thirty minutes to a “bombardment” of ultraviolet rays.
In order to be able to carry out this operation, the cases of memories REPROM are equipped with a quartz window. This one protects them from the solar radiation rich in infra-red, which cannot thus cause an accidental obliteration, but lets pass the artificial ultraviolet radiation used during obliteration.
Figure 47 represents such a case.
This ultraviolet radiation must have a very short wavelength of 2 537 Ĺ (10-10 m) very exactly, with a very high intensity (six Watt-second per square centimetre). It is of this fact recommended not to expose itself to this radiation, which can cause serious burns with the eyes. This radiation produces ozone in addition.
During the “bombardment” by the ultraviolet radiation, one polarizes the control electrode so as to drive out the loads. The loads previously trapped then receive a sufficient energy which again enables them to cross the insulating barrier which separates them from the substrate.
The table of figure 48 gives the principal characteristics of some current EPROM.
| Designation | A number of bits | Organization | A number of pins | Access time (ns) | Dissipated power (mW) | Vcc |
| 2 716 | 16 384 | 2 048 x 8 | 24 | 450 | 525 | 5V ± 5% |
| 2 732 | 32 768 | 4 096 x 8 | 24 | 450 | 790 | 5V ± 5% |
| 2 764 | 65 536 | 8 192 x 8 | 28 | 250 | 790 | 5V ± 5% |
c) Disadvantages and advantages of the REPROM
The REPROM have however two disadvantages.
First of all, it is not possible to modify or to erase only one storage position
because only a general deleting is possible.
A disassembling of the component is necessary for deleting.
Also it was created a new category of memories : The EAROM (Liable to deterioration Electrically PROM) still called EEPROM (Electrically erasable PROM) which are erasable PROM electrically.
Deleting and the writing byte by byte are possible in these memories. One uses for that an impulse of a score of volts.
d) Various technologies used for the manufacture of the RAM and the ROM.
Figure 49 shows the various cases used for the conditioning of the memories.
Figure 50 shows the various technologies used for the realization of the memories.
3. 4. - MEMORIES NOVRAM
The EEPROM called also E2 PROM have the advantage of preserving permanently their contents (nonvolatile memories). The number of deletings and writings however is limited. Their time of obliteration is relatively long (10 ms).
This is why it appeared recently on the market of the called hybrid circuits NOVRAM, which are consisted of association in the same case of a static RAM and E2 PROM.
When the system is fed, only the RAM functions, and one profits then from the access time court of the RAM.
When a breakdown of food occurs, one then safeguards the contents of the RAM in the EEPROM. This safeguard requiring only a few tens of milliseconds, one puts large condensers in the food so that they provide energy necessary to this safeguard.
With the powering, the RAM is rewritten with the contents of the EEPROM.
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