I have a problem and would like to ask for advice. The microcontroller board I have does not have a UART port, but the pins are reserved, so I want to use the TTL to USB for debugging. Everything was normal before the wiring was done, Keil could download the program, and the LED test program was also fine. But after I wired and powered on the microcontroller, the microcontroller broke down. The error message showed" too many errors" and" flash download failed "Cortex _M4"". I don't know what the reason is. The picture is an example I found. enter image description here I tested it and found that the power supply was normal (my microcontroller power supply is 3.3V); I used the internal crystal oscillator; reset and boot0 were also 3.3V; Keil could detect the chip ID, but it could not download. After canceling the verify in the flash download, it showed that the downloading was successful, but the program did not run correctly. I also tried operations such as erasing the entire chip, but it did not succeed. I checked the manual for the microcontroller pins, and the voltage resistance "min (VDD, VDDA, VDDIO2, VDDUSB) + 4.0", at least it is close to 6V.