I have done my own custom PCB using Kinetis KL27. The board should be powered by USB 5V and appeared as a USB device when plugged to the computer.
So far, the board seems to correctly be powered by 5V and the MCU by 3.3V (converted by a LDO). The KL27 MCU can run and be flashed through the ROM Bootloader using UART.
My issue is the board never appear has a USB device (either from the Bootloader ROM or from my own Flash code) when plugged to a computer.
While I was measuring the various USB signals. I realize when I measure the voltage on USB D+ (with the GND) using a multimeter, the board is magically recognized as a USB device when it runs either from the Bootloader ROM or from my own Flash code).
Here is a reduce schematic and layout of my custom PCB.