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ROM or Read Only Memory is a kind of IC which comes with a package of data and instructions already embedded by the manufacturer. It is attached with all the electronic devices that we find around ourselves like microwave ovens, digital irons, digital washing machines, digital telephones, stereos, LCD TV and almost everything that starts with the word Digital or has anything to do with electronics. A ROM is also called a Firmware. Its core purpose in any electronic device is to control all mechanical parts if any by already stored instructions or in computers to help load bootstrap programs and it can also keep a little data to make things interesting and less boring for humans to operate.
Basically, a ROM chip is a non-volatile IC i.e. it doesn't require power to keep the data in place. Since as its name suggests it is read only, the data inside the ROM is permanently unchangeable and there is nothing one can do to alter or remove it.
The data in a ROM is embedded in a large batch and if any of the data is corrupted or erroneous, the whole batch is worth throwing away in garbage but once it is successfully embedded and working, the cost may reduce down to a few cents (well of course in few PKR if you are so stereotype).
Unlike a RAM, which requires a transistor (and a capacitor if Dynamic, or a Logic if Static) to hold data and perform read and write operations, ROM requires a complete diode to hold data and become readable. The diodes are inserted on a grid of equal Rows and Columns intersection points on the ROM chip on a very microscopic level. A diode is a kind of logical component that allows voltage to run only in one direction and has a very subtle threshold that is called "Forward Breakover " which tells how much power is required to break through a certain circuit to turn it on (i.e. "1" in binary btw) and read it like 1. If there is no diode embedded on intersection, there is a 0 by default(This is so neat). So basically while manufacturing a ROM, these tiny diodes or no-diodes are printed on the silicon chip which will be there forever.
Diodes exist to show that there is a value 1 which will be readable once threshold is reached.
No diodes to show that there is a default 0.http://www.electronics.dit.ie/staff/tscarff/memory/rom.gif
Kinds of ROMs
PROM (Programmable ROM)
Well, not all the ROM chips are required pre-programmed or if any particular little task is required to use a ROM, manufacturing of single or little amount of ROM may become very expensive. For this reason, Programmable ROMs were invented. They are type of ROMs which can be programmed only once and can be read many times.
Initially, they are blank in a manner that they don't contain any data or instruction (well, that's not all true because they contain healthy fuses all around the chip so default value in a PROM for each intersection point is "1" and it makes the PROM downright full) but as ROMs are designed by using grids of rows and columns where each intersection point contains a diode, the same grid based designed is used to manufactured a PROM with a minor difference which is they contain a "Fuse" (a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit) on each grid row x column intersection point.
A PROM fuse on the grid intersection line can be made of nickel-chromium, polycrystalline silicon, or titanium tungsten.When a plotter is set to program a PROM, for each binary 1, it leaves a fuse intact and for each binary 0, it blows up a fuse (Genius!). Another little piece of information that lies hidden here is, all the fuses are assembled after a one-way diode in series (duh, Of course you cannot just "burn" a fuse on such microscopic level by amplifying voltage as much as you desire, so keeping things smooth and in control, a one-way diode with a traditional and convenient threshold would be nice to keep in the same neighborhood).
Another very important point is the reusability of a PROM chip in different sessions, though they are only once burnt and therefore are irremediable, a PROM chip can be burnt in parts to keep the other space intact for further burning (let's say, after correcting a program or any addition in instruction and data of the existing program, BTW, I wish you read my paper thoroughly so you could read this conflicting point once again that in a CD-R, more data can be added up in parts).
Diodes are placed before a fuse
Fuse will blow if 0 and will be intact if 1, right now in initial state where all are 1http://www.electronics.dit.ie/staff/tscarff/memory/rom.htmhttp://www.electronics.dit.ie/staff/tscarff/memory/prom.gif
EPROM (Erasable Programmable ROM)
Not all the programs are compiled perfectly in just one attempt. Programmers have to compile a program, burn it on the chip and assemble it with the machine to check out whether it is working okay or not. In a long haul, programming with ROM or PROM specifically can be very uneconomical in terms of time and resources. A chip that may be programmed a number of times would be a nicer solution in this case.
An EPROM (Erasable Programmable ROM) is quite the solution that may address the issue. EPROMs are also called a REPROM or a Reprogrammable ROM. These memory chips use Floating Gate Technology (though there are a number of ways to construct an EPROM, but FGT is the most common one).The architecture of an EPROM contains grids of columns and rows with each intersection point containing two transistors. Between the two transistors exists a thin oxide layer. The first transistor connected with bit line (column) is called a Floating Gate and other connected with the word line (row) is called a Control Gate transistor. Before a Floating Gate transistor, a capacitor is in place to hold charge to release when required with a specific charge amount (approx. 10 to 13 volt).
When a link is established by the means of thin oxide layer between Floating gate and Control gate, it will be a binary 1. The hard part here is to ionize the thin oxide layer between Floating Gate and Control Gate to hold a binary 0. For this purpose, more voltage is provided to the oxide layer till it is full enough with electrons to break the connection between the bit line and word line. This release of charges onto a transistor creates a Quantum Effect (yes you read it right) called Fowler-Nordheim tunneling. This can also be stated that when the oxide layer is fully or more than 50% charged, it becomes a binary 0 and for the contrary case, becomes a binary 1.
Now the interesting part is to obliterate the chip to bring it to initial condition (i.e. FYI binary 1 for every cell of ROM). UV (Ultraviolet) waves are applied over the chip through a window (a QUARTZ window, to be more specific) which is attached right on the top of the chip. Almost enough light is shined over the chip with a very fine frequency (approx. 2537 angstroms) which may not supposedly penetrate the glass and plastic of the box it is covered with. The light must also be enough to break through the oxide layer's negative charges so to set it to binary 1 and let bit line and word line connect. The time that it requires to perform this operation is also very subtle i.e. around 20 to 30 minutes. If an EPROM chip is exposed to UV light more than enough, the Floating gates will be open forever and won't be able to trap any further electrons forever.
Shows a Fowler-Nordheim tunneling process where electrons exist to disconnect the Word line from Bit Line to hold 1 and don't exist to link Bit line with Word line to hold 0.
EEPROM (Electrically Erasable Programmable ROM)
OK, EPROMs are cool but they sure have their disadvantages. An EPROM may be usable for programming for many times but to reprogram it or to edit an existing program in an EPROM, we have to first remove or obliterate the entire chip (i.e. to reset it completely) and to burn the whole program again. If any particular program takes 5 to 6 times to compile successfully (i.e. as we desire it to work), we might have to erase and burn the complete program this so many times. Now consider the timing that it takes to obliterate the whole chip that is almost 20 to 30 minutes (at max.), one must be crazy enough to wait again and again until finally it's good to go.
Another problem may occur when you just want a particular part of the program to edit, there is no way you can do it without erasing the whole chip and to burn it completely again with the modified part (now this must suck). Another drawback is the use of different equipment (plotter) to burn EPROM and another equipment (UV light container) to erase it.
The above told drawbacks (not all but mostly) were pretty neatly addressed in the EEPROM (e-e-p-ROM or double E-P-ROM) chip which is an Electrically Erasable Programmable ROM chip with everything almost like its predecessor EPROM. It also has one drawback i.e. it was slow on erasing but for other problems like editing just a single part of a program and need another instrument for the erasing was not required for these kinds of chips. BTW there are two kinds of EEPROM (as I must write it for no reason, because the other type is obsolete now and no one cares);
Well, what I will be discussing in my paper here is Serial EEPROM, Parallel EEPROM are now vanishing from the surface of earth due to the existence of so many legs (like a centipede of course, the larger seems the uglier). But as serial circuits have some of their own notorious cons like consumption of more voltage and dissipation of heat which in addition makes it slower to work, is seriously a pain, still it wins because it is not ugly looking chip like parallel EEPROM and its small size makes it cuter to install in a large circuitry (design wins over performance).
So how it works, well quite simple, just provide with more electricity in terms of voltages to the oxide layer that may generate electric flux and since we already have field effect transistors in place (Floating gate and Control gate are electric field effect transistors), they will simply nullify the negative ion on the oxide lay and the two gates shall merged which will eventually create a binary 1. Since each transistor is a field effect transistor in the EEPROM, it is a bit wide operational transistor. Programmers for EEPROM carry an inbox utility to perform a bit by bit erasure for EEPROM chip. This is also very obvious that like EPROM they are not very dense due to carrying two transistor, an oxide layer and a capacitor for containing only 1 bit and of course they are still slow in erasing mechanism because they are only bit wide operational but the good thing is we can program it, edit it or do anything we like with it in real time. They also don't need any quartz window attached over their tops which is the only major difference between EPROM and EEPROM.
EEPROMs are the predecessor of a new kind of ROM which almost addresses all the possible short-comings of the PROM, EPROM and EEPROM.
The most updated form of the ROM today is in shape of flash memory. Flash memory as it predecessors EPROM and double-EPROM are manufactured using the same fundamental architecture of Floating Gate and Tunneling. It possesses all the same difference from EPROM like EEPROM does, the only feature that it possesses other EEPROM is that it can erase or obliterate the data within it in chunks and sectors. So unlike a bit wide operation, it is a chunk wide operational chip. The size of the chip may vary as how it is formatted to keep the data.
Since it can erase data in chunks and not in bits, lesser circuitry is required to manufacture a flash memory which makes it denser than EEPROM and less expensive than EEPROM. This feature also gives it a head above than HDD which contains a spinning media and a larger size than a flash memory but still the cost of flash memory per bit is much higher than a HDD which uses magnetic disk and electric flux to contain data but not any logic gates. The cost of flash memory is getting cheaper each day which makes it available in terms of a technology called flash based Solid State Drive that has started replacing the current HDD market by a good factor. Apart from that, Flash memory is slower than all kind of ROMs and RAMs due to lesser transistors installed and more populated for each bit.
EEPROM are essentially a NOR based technology which contains 2 transistor to hold one bit which makes it less denser in serial circuit but flash memory can be manufactured using either NAND or NOR based technology. When a NOR based technology is used to manufactured a flash memory, in a serial circuit, only a single transistor is required to hold 1 bit which eventually gives it an extra level of addressing and makes it denser in terms of capacity but slows it down too when it comes to writing. Erasing on the contrary is much faster since all it has to do is to reset a whole chuck of bits or a complete memory using the built-in Wiring within the chip circuit.
The most common kind of a flash memory available today is a USB Flash drive, a Smart Drive (Solid State Floppy Disk Card) in mobile phones and cameras, SSDs and of course the BIOS chip in our PCs.
NOR Based Flash Memory architecture, as we can see that only on transistor exists for each Floating gate per memory cell and one Control gate per row which is sufficient for the whole column it is attached with. The whole row will decide how big a complete chuck of data it can contain. To erase the data, simply the bit line is "flashed" which ultimately gives the complete chuck a default 1.File:Nand flash structure.svg