VSC8201DL

Rev. 1.6.3 - 9/29/04

VITESSE - CONFIDENTIAL & PROPRIETARY - DO NOT COPY WITHOUT PERMISSION

- Page 11 of 21 -

Special RGMII Implementation Using a Single 3.3V Supply

This section addresses designers who wish to use the VSC8201 Single Port Gigabit Ethernet PHY in RGMII interface mode at an inter-

VSC8201’s RGMII Interface Description

The VSC8201 PHY is designed, as per the RGMII version 1.2a specification, to support RGMII operation at a 2.5v I/O supply. In addition

to the RGMII requirements specified by the standard, the PHY also has an internal clock delay feature (enabled by setting MII Register

23.8) that adds an internal delay of ~2ns to the RXC and the TXC signals when operating at 2.5v. This feature eliminates the need for

PCB trace delays or 2ns buffers in the path of the RXC and TXC traces.

Due to customer interest in using the VSC8201 PHY in RGMII interface mode with a single 3.3v supply, Vitesse has characterized the

RGMII I/O timing of the PHY under this condition in the following two configurations:

•

Standard RGMII Implementation (i.e. with RXC and TXC trace delays on the PCB)

•

RGMII Implementation using the Internal Clock Delay feature.

Since the VSC8201 PHY was not specifically designed for RGMII operation with a 3.3v I/O supply, certain accommodations must be

made to the board layout in order to ensure a robust interface. The VSC8201 PHY complies with the I/O timing requirements specified in

the RGMII version 1.2a specification. No special board layout considerations other than those mentioned in the standard are required for

this implementation.

RGMII Implementation using the Internal Clock Delay feature with 3.3v I/O Supply

With a 3.3v I/O supply, the worst case internal delay added to the TXC signal is around 3ns. If the TXC, TD[3:0], and TX_CTL traces are

length matched on the PCB, then the PHY may mis-latch data under the worst case TXC delay conditions, resulting in packet errors at

the MAC/PHY transmit interface. Due to this it is important to follow the guidelines mentioned in the section below during board layout.

1. RGMII Version 1.2a