Digital Logic Design Lecture # 13 University of Tehran

Outline Rom

Rom Each of our design isn’t necessarily a function that can be realized with packages such as MUX’s, adders etc. Sometimes a function has to be realized from scratch using gates. To do this kind of design, and do the wiring, testing and reliability checks of a circuit like this can be time consuming and costly.

Rom (continued…) Consider a box like the one below and its truth table:

Rom (continued…) Building all the minterms we have:

Rom (continued…) To realize any function with four inputs, all we need is the 16 minterms shown in the last slide. The idea of a programmable device is actually to put such a structure in an IC, and let the user connect the minterms he/she needs to realize the particular function.

Rom (continued…) The structure shown in the previous slides for a programmable device is not practical nor efficient because of the large gates it is using. What we need to do here is to distribute these gates in our structure. For instance a distributed AND gate for a minterm would look like:

Rom (continued…) This NMOS structure with ratio logic is different from the CMOS logic we have seen so far. The shown structure can be redrawn as:

Rom (continued…) The OR gates of our structure can also be distributed through out. Combination of OR gates and AND gates to implement a function is shown in the following figure:

Rom (continued…) Now we are near to a real programmable device structure. What we need in a programmable device is the ability to realize any function we need. To have this we use ‘fusible logic’ in the OR plane of our structure, giving the use to diffuse any transistor, and thus reach any wanted design, whereas the AND plane that makes our minterms stays untouched.

Rom (continued…) From now on we will be using the following notation instead of the transistors notation using ‘.’s where connections are permanent and ‘x’s where they are programmed.

Rom (continued…) Consider the following structure. We can look at this structure as a memory component that gives us a specified content (output) for every address space (input) we ask for, but this memory can not be written to and is thus called a Read Only Memory (ROM). The ROMs we saw were structured as follows: AND plane  fixed OR plane  programmable

Rom (continued…) ROM structures have always been changing from when they started to form. After ROMs that were only programmed by the factory as asked for by the user came PROMs (Programmable ROMs). The user could program PROMs as he wished but only once and any mistake could not be undone. Then came EPROMs (Erasable Programmable ROMs) that could be erased using ultra violet light and were again programmed by the user through diffusing of unwanted transistors in the OR plane.

Rom (continued…) And last of all were the EEPROMs (Electrical Erasable Programmable ROMs) that no longer needed ultra violet light to be erased and could be erased in special voltage and timing conditions.