ROM at work

Similar to RAM, ROM chips contain a grid of columns and rows. But where the columns and rows intersect, ROM chips are fundamentally different from RAM chips. While RAM uses transistors to turn on or off access to a capacitor at each intersection, ROM intersection, ROM uses a diode to connect the lines if the value is 1. If the value is 0, then the lines are not connected at all.

A diode normally allows current to flow in only one direction and has a certain threshold, known as

The forward breakover, that determines how much current is required before the diode will pass it on. In silicon-based items such as processors and memory chips, the forward breakover voltage is approximately 0.6 volts. By taking advantage of the unique properties of a diode ROM chip can send a charge that is above the forward breakover down the appropriate column with the selection row grounded to connect at a specific cell. If a diode is present at the cell, the charge, will be conducted through to the ground, and, under the binary system, the cell will be read as being “on”. The neat part of RAM is that if the cell’s value is 0,thereis no diode at that intersection to connect te column and row. So the charge on the column does not get transferred to row.

As you can see, the way a ROM chip works necessitates the programming of perfect and complete data when the chip is created. You cannot reprogram or rewrite a standard ROM chip. If it is incorrect or the data needs to be updated, you have to throw it away and start over. Creating the original template for a ROM chip is often a laborious process full of trial and error. But the benefits of ROM chips outwrigh the drawbacks. Once the template is completed, the actual chips can cost as the as a few cents each. They use very little power, are extremely reliable and, in the case of most small electronic devices, contain all the necessary programming to control the device. A great example is the small chip in the singing fish toy. This chip, about the size of your fingernail, contains the 30, second song clips in Rom and the control codes to synchronize the motors to the music.

PROM

Creating ROM chips totally from scratch is time-consuming and very expensive in s mall quantities. For this reason, mainly, developers created a type of ROM known as programmable read-only memory (PROM). Blank PROM chis can be bought inexpensively and coded by anyone with a special tool called a programmer.

PROM chips have a grid of columns and rows just as ordinary ROMs do. This difference is that every intersection of a column and row in a PROM chip has a fuse connecting them. A charge sent through a column will pass through the fuse in a cell to a grounded row indicating a value of 1. Since all the cells have a fuse, the initial (blank) state of a PROM chip is all 1s. To change the value of a cell to 0, you use a programmer to send a specific amount of current to the cell. The higher voltage break the connection between the column and row by burning out the fuse. This process is known as burning the PROM.

EPROM

Working with ROMs and PROMs can be a wasteful business. Even though they are inexpensive per chip, the cost can add up over time. Erasable programmable read-only memory(EPROM) addresses this issue. EPROM chips can be rewritten many times. Erasing an EPROMs are configured using an EPROM programmer that providevoltage at specified levels depending on the type of EPROM used.

This charge causes the floating gate transistor to act like an electron gun. The excited eletron and pushed through and trapped on the other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, it has a value of 1. When the charge passing through drops below the 50 percent threshold the value changes to 0. A blank EPROM has all of the gates fully open, given each cell a value of 1.