The protection circuit of IO port inside the single chip microcomputer and the electrical characteristics of IO port, and why is there a resistor in series between IO ports with different voltages?

Upper figure , This is one of Huada 32 Bit MCU IO Oral diagram , Let's look directly at the secondary tube in the frame , See how they protect  

Suppose we supply electricity VDD = 3.3V

So when I type in 5V When , What's the status ; When the input 5V when , If the diode voltage drop is 0.7V, that IO The voltage must be embedded in 4V, When you type 4V when ,IO The voltage will be embedded into 3.3V

Suppose when you input a negative voltage , Assume that the input -5V when ,IO Will be embedded in -0.7V, Here's the picture

Then read the above explanation , We are thinking whether it is embedded in 4V Or embedded in -0.7V, Who bears the excess voltage ？

For example, the actual 5V The signal input is embedded into 4V, that 1V Who is bearing the voltage ？ We all know that input has internal resistance , Then the rest 1V Voltage pain is added to the internal resistance ; If I want to say that I am ideal 5V There is no internal resistance , If so 5V Type in , It is very likely that the diode will be burned

So after the above explanation, we know why all single-chip computers IO Basically, there are two diode protection circuits

Now let's take a look at the electrical polarity marked in the normal data manual

We first saw VIN voltage , What it requires most is VCC+0.3 , If it is 3.3 Power supply , So the biggest 3.6V, Then we can calculate that the voltage drop of his diode is 0.3V No 0.7V;

Of course, the reality is cruel , Sometimes there will be voltage 4V perhaps 5V such , Therefore, the manufacturer's designers are also very considerate , If it exceeds the acceptable voltage, there is also a chance to retain , The input current is a little smaller , Give me face , From here, we can understand why there are many chips with different voltages IO A resistor in series should be added to communicate between , Brothers, at this time, we know the above parameters , Then does the current also know that the maximum cannot exceed 5mA, Can this resistance also be selected

eg: One 5V Measurement chip , Now? 3.3v The SCM that supplies power should read IO Data acquisition data , So we're going to DATA Connect a resistor in series , Avoid burning our MCU IO mouth

Then we can calculate R > (1.4v/5mA) = R > 280 ohm

So I'm basically happy to know why there are so many IO Both strings 1K perhaps 500 Ohm