CN0506

Ethernet

Ethernet is the most common type of packet-based physical connection for data networking applications in a local area network (LAN) that is defined as subsections and specifications by the Institute of Electrical and Electronic Engineers (IEEE) 802.3 standards.

There are different speeds and transmission medium associated with Ethernet. However, the focus of this circuit note centers on 10BASE-T, 100BASE-TX, and 1000BASE-T over twisted pair cables, Category 5e (CAT5e) or Category 6 (CAT6), straight through or crossover.

Line and Ring Network Topologies

Typical industrial Ethernet networks are deployed in line or ring topologies. The line and ring network topologies have reduced wiring lengths compared to start networks, and a redundant path for the ring network (see Figure 2). Each device connected to a line or ring network requires two Ethernet ports to pass Ethernet frames along the network.

PHY

A PHY is the physical interface transceiver that implements the physical layer functions of the open systems interconnection (OSI) model. A PHY encodes and decodes the data transmitted and received between devices, maintaining the integrity of the frames and packets (see Figure 3).

PHY Hardware Configuration—Strapping Resistors

The ADIN1300 can be configured to power up ready to establish a link. This PHY hardware configuration uses external strapping resistors and provides a known configuration for power on in an unmanaged application. Typically, in unmanaged applications, users do not configure the PHY over the management data input and output (MDIO). Instead, unmanaged applications rely on the PHY hardware configuration to bring the ADIN1300 up with the appropriate configuration ready to link with a remote PHY partner. When the ADIN1300 powers up, the hardware strapping pins are sampled when the device comes out of reset, and the PHY device then knows how to configure the various functions.

The hardware configuration modes of interest for this circuit note are speed, PHY address, automatic medium dependent interface crossover (Auto-MDIX), and MAC interface. The EVAL-CN0506-FMCZ includes resistor footprints for the various combinations, with a particular default configuration. The resistor components can be inserted or removed if required to change the default hardware configuration.

Consult the ADIN1300 data sheet for more information on using the other features and functions such as energy efficient Ethernet (EEE), energy detect power down, down speed, and software power down.

Physical Layer—MAC Interface

The MAC interface is the wired medium on the CN0506, and there are three MAC interface options, RGMII, RMII, or MII. RGMII supports all speeds up to 1000 Mbps, while MII and RMII support 10 Mbps and 100 Mbps, respectively. RGMII is the default interface on the CN0506.

The two ways to choose which MAC interface is used is either by hardware strapping external resistors or by using a software register configuration. MACIF_SEL0 and MACIF_SEL1 are multifunctional pins within the ADIN1300 (see the ADIN1300 data sheet for additional information). For the CN0506, the MACIF_SEL0 and MACIF_SEL1 pins can be configured to select the MAC interface according to Table 1. Note that the MACIF_SEL0 and MACIF_SEL1 pins have weak internal pull-down resistors. Therefore, if there are no external strapping resistors, the default MAC interface is RGMII with a 2 ns delay.

In this circuit note, the MAC interface selection is done via software configuration by using the GE_RGMII_CFG and GE_RMII_CFG registers within the ADIN1300. There are also footprints for external pull-up and pull-down resistors if users rather configure the MAC interface within the hardware. However, the resistors are not installed and, therefore, the PHY powers up on the EVAL-CN0506-FMCZ with the default RGMII interface.

PHY Address

There are four PHY address pins (PHYAD_x) that allow users to configure the PHY to any of 16 PHY addresses. The PHY addressing enables a system to have up to 16 individually controllable channels from a single controller.

The EVAL-CN0506-FMCZ is currently hardwired to specific addresses, but it can be changed by altering the configuration resistors for each of the channels. Channel 1 is assigned with Address 0001 currently, and Channel 2 is assigned with Address 0010 currently.

Programmable MAC Interface Clock

The ADIN1300 has three MAC interface options, MII, RMII or RGMII. For RGMII and MII interfaces, a 25 MHz clock is required for the ADIN1300, while the RMII requires an external 50 MHz clock. In a user application, a user can choose to place a 25 MHz crystal close to the XTAL_I and XTAL_O pins, or for the RMII use case, the host controller, MAC interface, or switch can provide the required 50 MHz clock directly to the PHY.

The EVAL-CN0506-FMCZ includes two programmable I²C clocks (Y1 and Y2) from 100 kHz to 125 MHz to support each of the ADIN1300 clock needs for the different MAC interfaces.

By default, the clock for each channel is set to 25 MHz on power up. When using the RMII MAC interface, the clock can be programmed to 50 MHz.

Both clocks have the same I²C address, but by using an I²C address translator, LTC4316, these clocks can be programmed individually to be different from each other. The LTC4316 translates each incoming bit by XORing the incoming address to a user configurable translation byte set by a resistive divider network of the chip.

MDI Interface—Integrated Magnetics

The MDI interface connects the ADIN1300 to the Ethernet network, typically through a transformer and RJ45 connector. The CN0506 uses RJ45 connectors with integrated magnetics. Integrated magnetics in the RJ45 connectors typically improve electromagnetic interference (EMI) shielding and have a smaller footprint, requiring shorter trace routing when compared to using discrete magnetics.

Integrated magnetics consist of the RJ45 connector, common-mode chokes, isolation transformers, LEDs, decoupling capacitors, and termination resistors. Designs may opt to use discrete magnetics due to different overvoltage requirements in designs, or if a need for a different layout for a specific EMI is required.

Power Supplies

To reduce the number of power supplies, the supplies for the analog circuitry for the ADIN1300, MDIO, and MAC interfaces are taken from the 3.3 V rails of the FPGA through a ferrite bead to reduce noise into the system.

The digital core of the ADIN1300 requires a 0.9 V supply. This supply is derived on board from the 3.3 V rail using the LT3502 pulse-width modulation (PWM), step-down, dc-to-dc converter, which converts the 3.3 V supply from the FPGA into 0.9 V, consuming a maximum of 0.45 W from the carrier.

Software Overview

The FPGA reference designs provided for use with the CN0506 configure each ADIN1300 independently. Each PHY (ADIN1300) is connected to a designated MAC interface, and there are three supported interface modes between the ADIN1300 and the FPGA: RGMII, MII, and RMII.

For each mode, there is a separate hardware design language (HDL) because some modes require converters (for example, Gigabit MII (GMII) to RGMII). The operating mode must be selected in the HDL to match the mode users wish to use in Linux.

The EVAL-CN0506-FMCZ connects to a standard low pin count (LPC) FMC connector, which makes the software design portable to many different FPGA development boards.

The Linux device tree supported for the different modes and carrier combinations can be found on the CN0506 HDL page. For more information on the standard Analog Devices, Inc., Linux image, see the FPGA Image user guide.

Board Layout Considerations

Ethernet signal layout is critical, especially at gigabit speeds. Signals are routed to RJ45 jacks as 100 Ω, controlled impedance pairs.

Data and clock signals to the carrier, while operating at lower clock rates, have edge rates that necessitate careful layout. Signals on the EVAL-CN0506-FMCZ are kept as short as possible, while signal trace length and impedance matching on the carrier board must be carefully considered when connecting the CN0506. These factors are important to the overall speed and performance of the CN0506 but must be considered separately.

Figure 4 shows the 1000BASE-T maximum droop point from V2 to V1 at 98.7%.

Performance Result

Several tests were conducted using the CN0506, including a mode verification test, a speed test, and a cable length drive test.

The EVAL-CN0506-FMCZ was tested at different modes with increasing cable lengths, and the results are detailed in Table 2 and Table 3 for a 4 meter cable and for a 154 meter cable, respectively, with no packet loss.

Table 2 and Table 3 show the frame checker count registers (FC_FRM_CNT_H and FC_FRM_CNT_L) and the receive error count register (RX_ERR_CNT) readback values of the ADIN1300 at the local and remote Ethernet PHY locations for a short cable transmission and a long cable transmission, respectively.