What is the difference between USB and ULPI? I know they are closely related, but how they are related is not clear to me.
First time I came to know when I was looking at this board (See at the bottom middle): Xilinx Zynq-7000 All Programmable SoC ZC702 Evaluation Kit
USB defines the external interface (physical, electrical, various layers of signalling).
The PHY (physical interface circuitry) that presents USB interfaces also has to interface to the host computer. This is done using a UTMI interface
ULPI is a lower pin-count version of that internal interface. This is beneficial for smaller and lower-cost devices.
Announced on March 1, 2004, the ULPI specification provides a low-pin, low-cost, small form-factor transceiver interface for any USB application
...
ULPI was developed by a group of USB industry leaders to address the need for low-cost USB and OTG PHYs. Existing specifications including UTMI and UTMI+ were developed primarily for Macrocell (IP) development, and are not optimized for use as an external PHY. Using the existing UTMI+ specification as a starting point, the ULPI working group reduced the number of interface signals to 12 pins, with an optional implementation of 8 pins. The package size of PHY and Link IC’s are drastically reduced. This not only lowers the cost of Link and PHY IC’s, but also makes for a smaller PCB.
I came here looking for an answer to the OP's original question and the only answer wasn't what I was looking for, so I did some digging. I didn't find much. The ULPI and related UTMI interfaces were originally devised for FPGA chips since most FPGA chips have little in the way of peripherals.
ULPI-to-USBPHY and UTMI-to-USBPHY chips are available which allow ULPI or UTMI capable chips to communicate on a USB bus. While this makes sense on for FPGA's, some MCU's also provide this interface. It means that an MCU can be manufactured for less, but also means that you will need to add a ULPI-to-USBPHY or UTMI-to-USBPHY chip and other circuitry to your solution if you want to access a USB bus. It is generally easier and cheaper to buy an MCU with a USBPHY already on-chip.
