The MCF54415 board has an USB 2.0 high speed interface using the USB3343 ULPI external transceiver.
Although USB is not used in bootloader, the pins(ULPI_D[7:0]， ULPI_STP， ULPI_NXT, ULPI_DIR) are still configured as ULPI functions. However, this will lead to an issue that when the board is connected to PC and stays in bootloader, the PC pops up the error message "USB Not Recognize".
I mesaured the voltages of DP, DM lines, both are 1.5V. Because of this, after usb hosts detect the voltage, some assume a device is attached, and do the usb enumeration, but fail.
And I found,
- If the ULPI pins are configured as GPIO, the voltage is zero and everything is OK.
- In the default state of power on reset, DP, DM have 15K pull-down, I got this from the specification of USB3343.
I can't understand why, that does not make sense. So I'm wondering what additional action does the MCF54415 do while configuring the pins as ULPI functions.