This type of memory is similar to the PROM with the difference that the information can be erased and re-record multiple times. Its name comes from the English acronym Erasable Read Only Memory.
Programming is carried out by applying a voltage between 10 and 25 Volts to a special memory pin for approximately 50 ms, depending on the device, at the same time the memory location is addressed and the information is put into the data inputs. This process may take several minutes depending on memory capacity.
The EPROM memory, like the memories seen above, is made up of an array of isolated gate N-Channel MOSFET transistors.
Each transistor has a floating SiO2 gate (without electrical connection) that in normal state is off and stores a logic 1. During programming, by applying a voltage (10 to 25V) the gate region becomes electrically charged, causing the transistor to turn on, thus storing a logical 0. This data is stored permanently, without the need to maintain the gate voltage since the electrical charge in the gate can remain for an approximate period of 10 years.
On the other hand, the erasure of the memory is carried out by exposing the device to ultraviolet rays for an approximate time of 10 to 30 minutes. This time depends on the type of manufacturer and to carry out the erasure, the integrated circuit has a transparent quartz window, which allows ultraviolet rays to reach the photoconductive material present in the isolated gates and in this way achieve that the charge is dissipate through this material by turning off the transistor, in which case all memory cells are left at logic 1. Generally, this quartz window is located on the surface of the encapsulation and is covered with an adhesive to prevent the entry of ambient light that can erase the information, due to its UV component.
According to Abbreviationfinder, EEPROM memory is programmable and electrically erasable and its name comes from the acronym in English Electrical Erasable Programmable Read Only Memory. Currently these memories are built with MOS (Metal Oxide Silice) and MNOS (Metal Nitride-Oxide Silicon) technology transistors.
Memory cells in EEPROMs are similar to EPROM cells and the basic difference is in the insulating layer around each floating composite, which is thinner and not photosensitive.
The programming of these memories is similar to the programming of the EPROM, which is carried out by applying a voltage of 21 Volts to the isolated MOSFET gate of each transistor, thus leaving an electrical charge, which is sufficient to turn on the transistors. and store the information. On the other hand, memory erasure is carried out by applying negative voltages on the gates to release the electrical charge stored in them.
This memory has some advantages over EPROM Memory, of which the following can be listed:
- Words stored in memory can be erased individually.
- No ultraviolet light is required to erase the information.
- EEPROM memories do not require a programmer.
- To rewrite it is not necessary to do a previous erase.
- They can be rewritten approximately 1000 times without problems storing the information.
Note: The information storage time is similar to that of EPROMs, that is, approximately 10 years.
EPROM flash memory
FLASH memory is similar to EEPROM, that is, it can be programmed and erased
electrically. However, it gathers some of the properties of the memories previously seen, and is characterized by having a high capacity to store information and is simple to manufacture, which allows to manufacture models of equivalent capacity to EPROMs at a lower cost than EEPROMs.
The memory cells are made up of a stacked gate MOS transistor, which is made up of a control gate and an isolated gate, as indicated in figure 4.1. The insulated gate stores electrical charge when a high enough voltage is applied to the control gate. In the same way as EPROM memory, when there is electrical charge on the isolated gate, a 0 is stored, otherwise a 1 is stored.
The basic operations of a Flash memory are programming, reading and erasing.
As already mentioned, programming is carried out by applying a voltage (generally 12V or 12.75 V) to each of the control gates, corresponding to the cells in which 0’s are to be stored. To store 1’s it is not necessary to apply voltage to the gates because the default state of the memory cells is 1.
The reading is made by applying a positive voltage to the control gate of the control cell.
memory, in which case the stored logic state is deduced based on the change of state of the transistor:
- If there is a 1 stored, the applied voltage will be enough to turn the transistor on and flow current from the drain to the source.
- If there is a stored 0, the applied voltage will not turn the transistor on due to the stored electrical charge in the isolated gate.
To determine if the data stored in the cell is a 1 or a 0, the current flowing through the transistor at the moment the voltage is applied to the control gate is detected.
The erasure consists of the liberation of the electrical charges stored in the isolated gates of the transistors. This process consists of applying a sufficiently negative voltage that displaces the charges as indicated in figure 4.2.