How to connect 5V serial port to 3.3V MCU serial port?

Write it at the front ： Due to the different levels of the two microcontrollers , Serial communication may fail , At this time, it needs to be solved by level conversion circuit , This paper gives two methods , One is built by triode , The other is MOS Pipe construction , Such questions often appear in the written examination of hardware engineers .

3.3V SCM and 5V The idea of SCM communication

3.3V Single chip microcomputer	Direction of communication	5V Single chip microcomputer
Send logic 1（ Corresponding voltage 3.3V）	→	Acceptance logic 1（ Corresponding voltage 5V）
Send logic 0（ Corresponding voltage 0V）	→	Acceptance logic 0（ Corresponding voltage 0V）
Acceptance logic 1（ Corresponding voltage 3.3V）	←	Send logic 1（ Corresponding voltage 5V）
Acceptance logic 0（ Corresponding voltage 0V）	←	Send logic 0（ Corresponding voltage 0V）

MOS Tube conversion circuit

working principle ：

1、 When 3V3 SCM sending logic 1, namely 3V3_TX=3.3V,Ugs=0V,MOS Pipe cut off ,5V_RX adopt R2 Pull up to +5V,5V_RX=5V;

2、 When 3V3 SCM sending logic 0, namely 3V3_TX=0V,Ugs=3V,MOS The pipe is open ,5V_RX Will be pulled down ,5V_RX=0V;

3V3_TX Send to 5V_RX

Then someone said ,5V How to send the data to 3V3 Single chip microcomputer ？ Is it OK to change the signal direction and power supply , Let's take a look .

working principle ：

1、 When 5V SCM sending logic 1, namely 5V_TX=5V,Ugs=0V,MOS Pipe cut off ,3V3_RX adopt R1 Pull up to 3V3,3V3_RX=3.3V;

2、 When 5V SCM sending logic 0, namely 5V_TX=0V,Ugs=5V,MOS The pipe is open ,3V3_RX Pulled down , therefore 3V3_RX=0;

The above analysis seems reasonable , In fact, the following circuits are not available , The above first 2 There is no problem with dot , It's mainly the first 1 spot , When MOS When the pipe is cut off ,5V_TX Of 5V The voltage will pass MOS The body diode of the tube reaches 3V3_RX, send 3V3_RX The voltage of is higher than 3.3V（4V many , Depends on the conduction voltage drop of the body diode ）. In this case , One side 4V More voltage and 3.3V There is a differential pressure , after R1 The resistance will consume electricity ; On the other hand ,4V Too much voltage can also damage 3.3V Single chip microcomputer RX Pin .

This circuit is not available

Use simulation software to simulate , You can see MOS When the pipe is cut off , The output is 4.44V, Significantly higher than 3.3V, It verifies the above viewpoint .

Simulate the above unavailable circuit

How to design 5V Send to 3.3V Single chip microcomputer ？ Actually, it's simple , Two devices are done , The following is the circuit diagram .

1、5V_TX=5V when , diode D1 end ,3V3_RX=3.3V;

2、5V_TX=0V when , diode D1 Conduction ,3V3_RX≈0.6V; actual 3V3_RX How much is the , Depending on D1 Forward conduction voltage drop , Because to get a lower voltage , commonly D1 Select Schottky diode , Schottky has the advantage of small conduction voltage drop .

5V_TX Send to 3V3_RX

Triode conversion circuit

working principle ：

1、 When 3V3 SCM sending logic 1, namely 3V3_TX=3.3V,NPN Triode cut off ,5V_RX adopt R2 Pull up to +5V,5V_RX=5V;

2、 When 3V3 SCM sending logic 0, namely 3V3_TX=0V,NPN Triode conduction ,5V_RX Will be pulled down , therefore 5V_RX=0V;

3V3_TX Send to 5V_RX

Using a triode ,5V SCM sends to 3.3V Single chip microcomputer , Whether the power supply and signal can be interchanged , and MOS Tube circuit is the same , The following circuits also cannot be used .

The reason is that 5V_TX by 5V when , Emitter reverse bias , however 5V Through resistance R1 From the base of the triode to the collector of the triode , Cause positive deviation of collector , and MOS Tube circuit is the same , send 3V3_RX The voltage is higher than 3.3V（4V many ）, You can simulate , I won't simulate here .

This circuit is not available

as follows , given 5V MCU to 3.3V Circuit diagram sent by MCU , With two NPN Triode construction .

working principle ：

1、 When 5V SCM sending logic 1, namely 5V_TX=5V,Q1 Conduction ,Q2 The base of is pulled down ,Q2 end , therefore 3V3_RX=3.3V;

2、 When 5V SCM sending logic 0, namely 5V_TX=0V,Q1 end ,Q2 Conduction , therefore 3V3_RX=0V;

5V_TX Send to 3V3_RX

You can see , I didn't draw two NMOS Pipe construction 5V MCU to 3.3V The circuit sent by MCU , Actually, replace the triode in the figure above with NMOS tube , Can achieve , In the actual circuit design , To reduce costs , Try to use less devices to connect the circuit , Therefore, the diode scheme is generally used .

This is the end of today's article ...

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Oh no , The above question is not over

The reason why I forward this article is that I discussed this problem with my friends in the evening , We also held a Tencent meeting because of this problem .

If you want to connect 3.3V MCU serial port , We should think of it at the first time that we also use 3.3V The single chip computer of , Two 3.3V There must be no problem with the power supply of single chip microcomputer . And now there are a lot of ARM chip , All are 3.3V Of GPIO Port voltage ,5V Single chip microcomputer as a primary entry level , Has been slowly replaced , Of course , And after 1.8V The single chip computer of .

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