Design Considerations for the Microchip USB3320C-EZK Hi-Speed USB 2.0 ULPI PHY
The Microchip USB3320C-EZK is a highly integrated Hi-Speed USB 2.0 ULPI (UTMI+ Low Pin Interface) PHY (Physical Layer) transceiver. It serves as a critical bridge between a link controller (typically embedded within an ASIC, FPGA, or microcontroller) and the USB 2.0 bus. Successful implementation of this component requires careful attention to several key design areas to ensure robust signal integrity, stable power delivery, and proper device functionality.
1. Power Supply and Decoupling Strategy
A stable and clean power supply is paramount for the PHY's analog and digital sections. The USB3320C-EZK features separate power pins for its core logic (VDDCORE: 1.8V), its analog blocks (VDDA: 3.3V), and its I/O buffers (VDDIO: 1.8V or 3.3V). Proper power domain isolation and meticulous decoupling are non-negotiable for optimal performance. Each power pin should be decoupled to its respective ground with a combination of bulk and small-value ceramic capacitors placed as close to the pins as possible. This minimizes noise and prevents voltage droops that can cause data corruption.
2. Clock Source Requirements
The PHY requires a high-quality, low-jitter 60 MHz reference clock input. The performance of this clock directly impacts the jitter performance of the outgoing USB data signals, which must comply with stringent USB 2.0 specifications. It is strongly recommended to use a crystal oscillator (XO) or a clock generator with jitter well below 100ps. The clock trace should be kept short and impedance-controlled, and must be treated as a sensitive signal, away from noisy sources.
3. Impedance Matching and PCB Layout for Signal Integrity
The D+ and D- differential pair signals are the most sensitive nets on the board. They must be routed as a controlled impedance differential pair (90Ω differential impedance for USB). Key layout rules include:
Length Matching: The two traces within the pair must be length-matched to minimize intra-pair skew.
Minimizing Vias: Avoid using vias on these traces if possible; if necessary, use them symmetrically for both signals.
Isolation: Keep the pair away from aggressive noise sources like clocks, switching power supplies, and RF circuits.
Short Stubs: The connection from the PHY to the USB connector must be as direct and short as possible.
4. ULPI Interface Implementation
The ULPI interface significantly reduces pin count (12-pin data bus) compared to the older UTMI+ standard. Correctly managing the ULPI bus timing and control signals (CLK, DIR, STP, NXT) is crucial for reliable communication between the PHY and the link controller. Designers must ensure the FPGA or ASIC logic is configured to meet the setup and hold times specified in the ULPI specification and the USB3320C datasheet. Trace lengths for the ULPI bus should be kept short and matched to avoid timing issues, especially at high speeds.
5. Configuration and Reset Sequencing
The USB3320C-EZK offers configuration options, such as terminating resistors and slew rate control, often set by pulling specific pins (e.g., XCVRSEL, TERMSEL) high or low. The power-on reset (POR) sequence and the timing of the RESET_N signal must be strictly adhered to as outlined in the datasheet. An improper reset sequence can lead to the PHY failing to initialize correctly.
6. ESD Protection and Connector Selection
While the USB3320C-EZK incorporates some on-chip ESD protection, additional external ESD protection diodes are highly recommended on the D+ and D- lines near the USB connector. This provides a robust defense against electrostatic discharge events from user interaction, ensuring long-term reliability. The USB connector itself should be of high quality, providing a robust mechanical connection and good shielding.
ICGOOODFIND: The Microchip USB3320C-EZK is a powerful enabler for adding USB 2.0 Hi-Speed connectivity. A successful design hinges on a rigorous approach to power integrity, impeccable high-speed PCB layout for the differential pair, a low-jitter clock source, and careful attention to the ULPI interface timing. By prioritizing these considerations, designers can achieve a stable, compliant, and high-performance USB interface.
Keywords: Signal Integrity, ULPI Interface, Power Decoupling, Impedance Matching, ESD Protection
