An Arduino Leonardo, Arduino Micro or SparkFun Pro Micro has no USB-to-serial chip: the ATmega32U4 is the USB device, and it presents two different…
"The port is busy or doesn't exist" hides three Windows error numbers: 5 (another program owns the port), 2 (no such COM number) and 433 (the USB device vanished). How to read them, the two-minute Process Explorer search that names the process, and the programs that grab serial ports in the background.
"No ST-LINK detected" is a USB or driver problem; "No STM32 target found" is wiring, power or the chip itself. The two messages sorted into four layers, with the exact message table, the UM2237 connection modes, and the ST Community threads that settled each case.
Decode the ESP8266 74880-baud boot line bit by bit, see which boards auto-reset into download mode, and the Linux kernel versions that broke CH340 uploads.
When an Arduino Nano shows up as a COM port but every upload ends in ten lines of “not in sync” and “programmer is…
Port Reset Failed, Set Address Failed, Device Descriptor Request Failed, Codes 10/28/43/45/52: which USB enumeration step failed, what it usually is, and the next step. USBDeview 3.10 hosted with SHA-256.
boot:0x13 is the ESP32 strapping register with GPIO0 high: the board never entered download mode. Every boot code decoded, the auto-reset timing, and the causes of "Failed to connect" in order.
A display that shows up as "Generic PnP Monitor" or drops a resolution is either a five-minute registry override or an eight-pin EEPROM job. Here is how to tell which, and how to do both.