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How does SCM work?

2022-07-22 15:53:00 【an520_】

One 、 Analysis of the internal structure of single chip microcomputer

Let's think about a problem , When we write an instruction into the MCU in the programmer , Then remove the MCU , The single chip microcomputer can execute this instruction , Then this instruction must be stored somewhere in the MCU , And this place can still keep this instruction from being lost after the MCU is powered down , What is this place ？ This place is the read only memory inside the single chip microcomputer, that is ROM（READ ONLY MEMORY）. Why call it read only memory ？ Didn't we write in two numbers just now ？

Originally in 89C51 Medium ROM Is an electrically erasable ROM, be called FLASH ROM, The programmer we used just now , Under special conditions, by external equipment ROM Write operation , Under the normal working conditions of single chip microcomputer , You can only read from that side , You can't write data in , So we still call it ROM.

Two 、 Some basic concepts

1、 The nature of numbers and physical phenomena

We know , The computer can perform mathematical operations , This makes it very difficult for us to understand , Computer , Although we don't know its composition , But they are just some electronic components , How can we do mathematical operations ？ We do math problems such as 37+45 Here's how it works , First write on paper 37, Then write below 45, Then the brain calculates , Finally, write the results , Raw materials for computing ：37、45 And result ：82 It's all written on paper , Where is the computer placed ？

To solve this problem , Let's do an experiment first ： Here is a lamp , We know the lights are either on , Or it doesn't light up , There are two states , We can use ’0’ and ’1’ To replace these two states , The specified light is ’1’, If it doesn't light up, it means ’0’. Now put on two lights , There are several States ？ Let's take a look at the list ：

Please write down 3 Condition of lamp 000 001 010 011 100 101 110 111 Let's see , This 000,001,101 Isn't it the binary number we learned ？ Originally , The light on and off is just a physical phenomenon , But when we put them in a certain order , The light on and off represents the number . Let's take another Abstract step , Why is the light on ？ Because the output circuit outputs high level , Energize the light . therefore , When the light is on or off, it can be replaced by whether the output of the circuit is high level or low level . such , Numbers and levels are high 、 Low contact .（ Please think about it , What other similar examples have we seen ？（ Naval ） Lantern language 、 Flag language , The telegraph , Even red 、 A green light ）. Articles that help to understand the level ：CPU How to identify the code we write ？

2、 Bit means

Through the above experiment, we already know ： A light is on, or the level of a line , Can represent two states ：0 and 1. In fact, this is a binary bit , So we call a thread a “ position ”, use BIT Express .

3、 The meaning of bytes

A line can represent 0 and 1, Two lines can express 00,01,10,11 Four kinds of state , That is, it can express 0 To 3, And three roots can express 0~7, Commonly used in computers 8 Put the wires together , Simultaneous counting , I can represent 0-255 altogether 256 States . this 8 A thread or 8 A bit is called a byte （BYTE）.

How memory works

1、 Memory structure

Memory is the place where data is stored . It uses the level to store data , in other words , What it stores is actually the high level 、 low , Not what we're used to thinking 1234 Numbers like this , such , One of our mysteries was solved , There's nothing mysterious about computers .

As shown on the left above ： A memory is like a small drawer , There are eight small squares in a small drawer , Each small cell is used to store “ Electric charge ” Of , Charge is transferred in or released through the wire connected to it , As for how the charge is stored in the small lattice , We don't have to worry about it , You can think of wires as water pipes , The charge in the small lattice is like water , That's easy to understand . Each small drawer in the memory is a place for data , We call it a “ unit ”.

With such a structure , We can start storing data , Want to put in a data 12, That is to say 00001100, We just need to fill the second and third cells with electric charge , Just let go of the charge in other small cells （ Look at the picture above, right ）. But the problem came out , Look at the picture above, right , A memory has many units , The wires are connected in parallel , When you put an electric charge , Will put the charge into all the cells , And when the charge is released , Will discharge the charge in each unit , In this case , No matter how many memory units there are , Can only put the same number , This, of course, is not what we want , therefore , A slight change in structure , Look at the picture above, right , There is a control line on each unit , Which unit do I want to put the data into , Just send a signal to the control line of this unit , This control line turns the switch on , So the charge can flow freely , There is no signal on the control line of other units , So the switch doesn't open , Not affected , such , As long as the control lines of different units are controlled , You can write different data to each unit , Again , If you want to get data from a unit , Just turn on the corresponding control switch .

2、 Memory decoding

that , How do we control the control lines of each unit ？ This is not easy , Just lead the control line of each unit to the outside of the integrated circuit ？ It's not that simple , a slice 27512 There are... In the memory 65536 A unit , Lead out every line , This integrated circuit must have 6 More than ten thousand feet ？ no way , What do I do ？ Try to reduce the number of lines . We have a method called decoding , A brief introduction ： A line can represent 2 States ,2 The root line can represent 4 States ,3 The root line can represent several ,256 How many lines are needed to represent ？8 Kind of ,8 Root line , therefore 65536 In this state, we just need 16 One line can represent .

3、 Memory chip selection and bus concept

thus , The decoding problem is solved , Let's focus on another problem . Where do the eight wires fed into each unit come from ？ It's connected from the computer , In a general way , These eight wires are connected to a memory , And other devices . So the problem comes out , Since these eight wires are not dedicated between memory and computer , If you always connect a unit to these eight lines , It's not good , For example, the value in this memory unit is 0FFH Another memory unit is 00H, So this line is at a high level , Still low ？ Don't you have to fight? It depends on who has hurt ？ So we need to separate them . Of course, the solution is simple , When the outside wire is connected to the pin of the integrated circuit, come in , Not directly connected to each unit , Just add another set of switches in the middle . Usually we leave the switch on , If you really want to write data to this memory , Or to read data from memory , Just turn the switch on again . This set of switches is selected by three leads ： Read control end 、 Write control terminal and chip selection terminal . To write data to the chip , Select this piece first , Then send a write signal , The switch is closed , And send the data （ Electric charge ） Write on chip . If you want to read , Select this piece first , Then send a read signal , The switch is closed , The data is sent out . The read and write signals are also connected to another memory at the same time , However, due to the different selection ends , So although there are read or write signals , But there is no chip selection signal , So another memory won't “ misunderstanding ” And open the door , Cause conflict . How about choosing two chips at the same time ？ As long as it is a well-designed system, it will not , Because it is controlled by calculation , Not controlled by us , If there is a case that two pieces are selected at the same time , There is something wrong with the circuit , This is not in our discussion .

We have seen from the above introduction , The eight wires used to transmit data are not dedicated , But many devices are shared by everyone , So we call it data bus , The English name of the bus is BUS, It's always the bus lane , Anyone can go . And sixteen address lines are also connected , Call it address bus .

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