Single Chip microcomputer memory structure

How the Memory works:

1. Memory structure

Memory is the place where data is stored. It is to use the level of the high and low to store data, that is, it is actually stored in the level of higher and lower, rather than we are accustomed to think of 1234 such numbers

Figure 2. Monolithic computer has such a memory, this is a memory: a memory like a small drawer, a small drawer has eight small lattice, each small lattice is used to store "charge", the electric charge through the wire connected with it came in or released, each small drawer called a "unit."

With such a construct, we can start to store the data, we want to put in a data 12, that is, 00001100, we just put the second and third small lattice in charge, and the other small lattice in the charge to let go of the line. The problem comes out, a memory has a lot of units, the line is in parallel, when the charge will be put into all the units, and the release of the charge in each unit will be released, so that no matter how many units of memory, can only put the same number, this is not what we want, So to make a slight change in the structure, look at Figure 2, there is a control line on each unit, I want to put the data into which unit, give a signal to the unit control line, the control line to open the switch, so that the charge can flow freely, and the other unit control line no signal, so the switch does not open, Will not be affected, so that, as long as the control line of the unit is controlled, you can write different data to each unit, similarly, if you want to take data from a cell, just open the corresponding control switch on the line.

2. Memory decoding

So, how do we control the control floss of each unit? This is not simple, the control line of each element is led to the outside of the integrated circuit is not OK? Things are not so simple, a piece of 27512 memory has 65,536 units, each line is cited out, this integrated circuit has to have more than 60,000 feet? No, what? To think about reducing the number of lines. We have a way to call this decoding, a brief introduction: a line can represent 2 kinds of states, 2 lines can represent 4 states, 3 lines can represent several, 256 states and need a few lines to represent? 8 Kinds, 8 lines, so 65536 states we only need 16 lines to represent.

3, the memory of the selection and the concept of the bus

At this point, the decoding problem solved, let us pay attention to another issue. Where does the eight lines of each unit come from? It is from the computer, generally, the eight lines in addition to a memory, but also to pick up other devices, so the problem is out, these eight lines are not dedicated between the memory and the computer, if an individual unit is connected to the eight lines, it is not good, such as this memory The number in the cell is 0FFH and the other memory unit is 00H, so is the line in the end high -level or low? So we're going to make them separate. Method of course is very simple, when the outside of the wire to the SCM pin came in, not directly to each unit, the middle of a set of switches on the line . Usually we let the switch off, if it is really to write to this memory, or to read the data from memory, and then let the switch on the line. This set of switches is selected by three leads: Read control, write control, and chip selector . To write the data to the slice, first select the slice, then send out a write signal, switch clash, and the transmitted data (charge) into the film. If you want to read, select the slice first, then send out a read signal, the switch is closed, the data is sent out. Note in Figure 4, the read and write letters are also connected to another memory, but because the chip selection is not the same, so although there is a reading or letter number, but there is no chip selection signal, so the other memory will not "misunderstanding" and open the door, causing conflict.

From the above introduction we have seen that the eight lines used to pass the data is not dedicated, but a lot of devices are common use, so we call it a data bus, buses in English is called bus, is the bus lane, who can walk. and 16 address lines are also linked together, called address bus.

Classification of semiconductor memory

According to function can be divided into read-only and random access memory two major categories.

• Read only memory of the English abbreviation for ROM (read only memory), literally understand is only from the inside to read, cannot write in, it is similar to our books, sent to our hands back, we can only read the contents of the content, we can not arbitrarily change the contents of the book.
• Random memory of the English abbreviation for RAM (random Access memory), that can be rewritten at any time, but also can read the data inside, it is similar to our blackboard, I can write things up at any time, can also use the blackboard wipe erase rewrite.

Explain a few common concepts:

• PROM, called a programmable memory. This is like our exercise books, bought when is blank, can write things up, but once written up, can not wipe off, so it can only write once, if written wrong, on the reimbursement. (has now been eliminated)
• EPROM, called a programmable read-only memory for UV-erase . The contents of it after writing, if it is not satisfied with a special method can be removed after rewriting, which is irradiated with ultraviolet rays, ultraviolet light is like "disambiguation spirit", can remove the word, and then rewrite. Of course, the number of times, it will not be brilliant, so this chip can erase the number of times is limited-hundreds of times it. (has now been eliminated)
• EEPROM, also known as E2prom called Electric erasable Programmable read-only memory, it is similar to the EEPROM, write things can also erase rewrite, but it is convenient, do not need lighting, as long as the use of electricity can erase or re-rewrite data, so convenient many, It also has a long lifespan (ranging from tens of thousands of to hundreds of thousands of times).
• Flash, known as flash memory, is an improved product of EEPROM, its biggest feature is that it must be erased by block (the size of each block varies, the product of different manufacturers has different specifications), and the EEPROM can erase only one byte at a time (byte). Flash is now commonly used for mass storage, such as U-disk

Single Chip microcomputer memory structure