Reply To: Teensy controller – manually write controller config file

You could put this file in:
/opt/retropie/configs/all/retroarch-joypads

I think you can call it anything – try “joypadtest.cfg”
It may well need a different driver name

input_device = "2 Serial/Keyboard/Mouse/Joystick"
input_driver = "udev"
input_up_axis = "-1"
input_right_axis = "+0"
input_down_axis = "+1"
input_left_axis = "-0"
input_a_btn = "13"
input_b_btn = "14"
input_x_btn = "15"
input_y_btn = "999"
input_l_btn = "3"
input_r_btn = "4"
input_start_btn = "7"
input_exit_emulator_btn = "7"
input_select_btn = "6"
input_enable_hotkey_btn = "6"

Although this implies you may be better outputting keyboard keys from the Teensy:
https://forums.adafruit.com/viewtopic.php?f=8&t=58901

What you want is yellow text at the bottom of the screen when you start a retroarch based game, indicating it has found the joypad.

Yes, you could be right about the / marks in the name – do you have a file here?
/home/pi/.emulationstation/es_temporaryinput.cfg ?