4.5.2. USB Interface Design Guidelines
The Cyclone® V/ Arria® V SoC Hard Processor system can connect its embedded USB MACs directly to industry-standard USB 2.0 ULPI PHYs using the HPS Dedicated I/O that support 1.8V, 2.5V, 3.0V and 3.3V I/O standards. No FPGA routing resources are used and timing is fixed, which simplifies design. This guide describes the design guidelines covering all supported speeds of PHY operation: High-Speed (HS) 480 Mbps, Full-Speed (FS) 12 Mbps, and Low-Speed (LS) 1.5 Mbps.
GUIDELINE: Design the board to support both USB PHY modes where the device supplies the clock versus where an external clock is the source.
The interface between the ULPI MAC and PHY on the Cyclone® V/ Arria® V SoC consists of DATA[7:0], DIR and NXT from the MAC to the PHY and STP from the MAC to the PHY. Lastly a static clock of 60 MHz is driven from the PHY and is required for operation, including some register accesses from the HPS to the USB MAC. Ensure the PHY manufacturer recommendations for RESET and power-up are followed.
GUIDELINE: Ensure that the USB signal trace lengths are minimized.
At 60 MHz, the period is 16.67 ns and in that time, for example, the clock must travel from the external PHY to the MAC and then the data and control signals must travel from the MAC to the PHY. Because there is a round-trip delay, the maximum length of the CLK and ULPI signals are important. Based on timing data the maximum length is recommended to be less than 7 inches. This is based on a PHY with a 5 ns Tco specification. If the specification is slower the total length must be shortened accordingly.
GUIDELINE: Ensure that signal integrity is considered.
Signal integrity is also important but mostly on the CLK signal driven from the PHY to the MAC in the HPS subsystem. Because these signals are point-to-point with a maximum length, they can usually run unterminated but it is recommended to simulate the traces to make sure the reflections are minimized. Using the 50-ohm output setting from the FPGA is typically recommended unless the simulations show otherwise. A similar setting should be used from the PHY vendor if possible.
GUIDELINE: Design properly for OTG operation, when applicable.
When On-the-Go (OTG) functionality is used, the SoC can become a host or endpoint. When in host mode consider the power delivery, such as when you are supporting a USB Flash drive, or potentially a USB Hard Drive. These power requirements and reverse currents must be accounted for, typically by using external diodes and current limiters such as those used on the Cyclone® V SoC or Arria® V SoC development kits.