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Why can't we use a male to male usb cable?

A simple male to male cable is irrelevant due to the usb protocol. A PC is a usb host, it can't be a usb device. Usb to usb cables not designed to work with GIMX such as file transfer cables won't work.

Can we use a normal serial port? (usb adapter)

No. The serial port (RS232 port) that can be found on old PCs does not work at the right voltage levels. It works at -12V/+12V instead of 0V/+5V.

I can't flash firmware using Teensy Loader to my Teensy board

Your Teensy is probably FAKE. Buy genuine one.

My DIY USB adapter isn't working. What can I do?

  • Do not use atmega16 or atmega90 boards. There are known issues with these. Recommended one is atmega32u4
  • Double check your wiring as explained here: link. A bad wiring results in failure at startup.
  • Check your USB cables, one might be bad. Only use shielded USB cables. A bad cable results in disconnection after a variable time.
  • Connect the USB to UART adapter directly to your computer. Using USB extenders is discouraged, especially on this side of the adapter.
  • Check the wires between the USB to UART adapter (e.g. CP2102) and the AVR USB board (e.g. Teensy 2.0). Make sure there is no bad contact. Loose contact (not soldered and not using jumper wires) is not acceptable and results in inconsistent behavior, such as failure at startup or disconnection after a variable time.
  • Test your USB to UART adapter as explained below.

How can I test my serial adapter?

Only connect the rx pin to the tx pin.
Download the Arduino IDE.
Start the Arduino IDE (don't forget to extract the archive...).
Select the right serial port in the "Tools>Serial Port" menu.
Click on "Tools>Serial Monitor".
Type something in the small text area at the top of the serial monitor, and click on the "Send" button.
If your adapter works correctly, you should see what you typed appearing in the main text area.
It is possible to test the serial adapter for different transmission speed (from 9600 to 115200 bauds).

I can't see the COM port in gimx-launcher? (windows+usb adapter)

Check the COM port in the device manager.