Table of contents
1.2 Unrecognized - not a driver problem
1.3 Unrecognizable - not the problem of bootloader burning
1.3 How to solve unrecognized problems?
1.3.1 Most likely the chip has a problem
1.3.2 Check circuit connection problems
1. Device descriptor problem
1.1 Device Descriptor
The definition given by Baidu Encyclopedia is quite complicated and not suitable for novices to learn and understand. Here I cut out the picture directly, just look at this picture to understand. 
(Image source: Station B)
1.2 Unrecognized - not a driver problem
First of all, it must be clear: the device descriptor cannot be recognized, not because the driver is installed or not , it has nothing to do with the driver.
There are too few answers to similar questions on the Internet, and most of them ask you to update and install the driver. I will explain it clearly for you here:
If this is the case:
The problem is that the driver is not installed. The solution is to download the CH341SER driver (that is, the CH340 driver) from the Qinheng official website.
The detailed installation process is here: CH340/CH341 Driver Installation Tutorial_哔哩哔哩_bilibili
But if this is the case:
Then don't think about the driver. Because at least the computer can recognize and communicate with the former, and the latter cannot be recognized by the computer, so it must not be a driver problem.
1.3 Unrecognizable - not the problem of bootloader burning
If the CH340 chip works normally, but the self-made development board has not burned the Bootloader for the time being, the normal phenomenon should be as follows:
As shown in the figure, in the "Port" column, you can see that the CH340 serial port can be recognized normally.
But it will be stuck in the upload code link. That is, the upload progress bar is stuck at 99%.
1.3 How to solve unrecognized problems?
1.3.1 Most likely the chip has a problem
1. The soldering time is too long, resulting in chip damage. (You can solve it by searching for "drag welding" at station B, which is a frequent offender for newcomers)
In addition, the soldering time should not exceed 5s at a time, just drag soldering. It is not necessary to fill each pin with tin, as long as the pins and pads have some tin to ensure the connection.
2. The static electricity on the body was not dealt with before soldering, which caused the static electricity to break down the chip.
A common phenomenon is that an abnormal sound is heard during the soldering process but the source is unknown. It is likely that the chip has been broken down. (It is recommended to wash your hands before starting work)
3. Insufficient welding causes the chip to fail to communicate normally. It is recommended to check several pins of the chip for soldering integrity.
4. The chip itself has quality problems, it is recommended to solder a few more boards.
If the problem still exists, it is recommended to re-purchase the CH340 chip.
There are also many CH340 fake chips on Taobao, and you can't tell whether it is good or bad just by looking at it. Here, the author does not recommend any Taobao stores. The only advice is to read the store reviews before buying.
(Attachment: The market price of Atmega3287P-AU fluctuates between 20-30 yuan/pcs, and the price of CH340C fluctuates between 7-15 yuan/pcs)
1.3.2 Check circuit connection problems
1. Check if there is any problem with the circuit connection:
This is the schematic of CH340C.
This is the schematic diagram of CH340G. (Pay attention to switch RTS# , external crystal oscillator)
And: Check whether RX and TX are reversed
RX：Receiver RX（CH340）->TX(Atmega328P-AU)
TX：Transfer TX（CH340）->RX(Atmega328P-AU)
2. (CH340G) Check whether the crystal oscillator has started to vibrate: connect one of the multimeters to GND and the other to the two poles of the crystal oscillator. It should be 1.67V on one side and 0.7V-1.2V on the other side, which is a successful start-up. (The difference between one side and the other side is about 1-2V, and the specific voltage varies slightly due to different components in the hands of individuals)
3. PCB design issues
Check the design of the circuit board itself: D+, D-; RX, TX whether there is differential wiring, whether there is a filter circuit, etc.
Check whether VCC is wiring type or copper laying type? It is recommended to lay copper on the VCC power port, and set VCC on Net.
Check for fuses and anti-backflow diodes
Check the PCB design for circuit problems such as short circuit, AD shortcut key TDR
2. Bootloader burning failed
Burning Tutorial: Arduino Nano Development Board - EasyEDA Open Source Hardware Platform
1. Check the contact problem: soldering integrity, whether the jumper is connected properly?
False soldering may be in the various pins of the chip (D13, D12, D11, RESET), the connection pins (RX, TX) between CH340 and Atmega328P, etc., focus on checking the soldering conditions of the above three parts.
Most use uno development boards for programming, so pay attention to the jumper connection : the jumper may be only plugged into the breadboard without touching the pad, or the jumper is shorted where you can't see it.
If the contact is bad, please do not directly hold the jumper to ensure the contact by force! This is because people directly hold the jumper with their hands. Shaking hands may cause the entire self-made development board to be powered on and off at times, and it may also cause the entire development board to short-circuit and damage the chip.
2. Chip problem
See above (1.3.1) Fake chips are emerging in an endless stream. I have bought fake CH340 and also bought fake Atmega328P-AU, and a lot of money wasted. Before buying, you must inspect and complete in many aspects before making a purchase!
Secondly, it is also possible that the contact time is too long and the chip is scalded. It is recommended not to exceed 400°C, otherwise the PCB board and chip will not be able to stand it.
When I operate the cutter head at 385°C, each contact should not exceed 5s. After the chip is dragged and soldered, there is a lot of solder rubbing. If it can't be rubbed off, let the chip cool down for a while before operating.
3. Unable to power on
Specific phenomenon: Connect the USB plug, but the computer does not move.
This is generally a soldering problem of several pins behind the USB interface, or a soldering problem of the chip.
If it is not a soldering problem, it is a problem with the chip itself, see above: chip quality problems or man-made damage.