Functions of Ethernet PHY and Selections

The Ethernet PHY has two main functions.

First, the PHY has a digital domain that interfaces directly to the media access controller (MAC) of a device such as a field-programmable gate array (FPGA), microcontroller (MCU), or central processing unit (CPU). The PHY will have MII to some extent, a 4-bit wide data bus with control and clock lines in the transmit and receive directions. MII comes in various forms, depending on the speed of the MAC and PHY, and has different pin counts. Below table shows the most common MIIs and provides a high-level summary of the pros and cons to consider during the selection process.

Interface Pins (pin count) Speed support (Mbps) Pros Cons
MII RX_D[3:0], RX_CLK, RX_DV, CRS, COL TX_D[3:0], TX_CLK, TX_EN (14) 10, 100 Common pinout, low speed, simple to route, lowest latency No 1-Gbps support, high pin count
Reduced MII (RMII) RX_D[1:0], CRS_DV, TX_D[1:0], TX_EN (6) 10, 100 Reduced pin count Poor deterministic latency (due to first-in, first-out), no 1-Gbps support
Gigabit MII (GMII) RX_D[7:0], GRX_CLK, RX_CTRL, TX_D[7:0], GTX_CLK, TX_CTRL (20) 10, 100, 1000 1-Gbps support, low latency High pin count, not commonly supported
Reduced Gigabit MII (RGMII) RX_D[3:0], RX_CLK, RX_CTRL, TX_D[3:0], TX_CLK, TX_CTRL (12) 10, 100, 1000 1-Gbps support, common pinout Difficult to route, poor electromagnetic compatibility (EMC)
Serial Gigabit MII (SGMII) SO_P, SO_M, SI_P, SI_M (4) 10, 100, 1000 1-Gbps support, common pinout, excellent EMC performance, easy to route More expensive integrated circuits

Table 1: Common MIIs listed according to pin count and speed support

Second, a PHY has a Medium Dependent Interface (MDI) that connects one device (again, an FPGA, MCU or CPU) to another device over physical media. This is generally referred to as the analog domain of the PHY since it is a continuous time-varying signal.

Selecting the right Ethernet PHY for your application based on MDI

Most PHYs’ datasheets show the following specifications and features clearly:

  • Data rates (10 Mbps, 100 Mbps, 1 Gbps).
  • Interface support (MII, RMII, GMII, RGMII, SGMII).
  • Media support (BASE-T, BASE-Te, BASE-TX, BASE-T1).

With this information in mind, you can work through the list beginning with data rate, and match it to the data rate that your end application requires. Next, determine what standard your application typically uses.

As an example, consumer electronics and most industrial applications use 10BASE-Te, 100BASE-TX and 1000BASE-T because PCs support these standards. If your application is automotive, a PHY that supports BASE-T1 is the most suitable solution. The exception to this rule is automotive onboard diagnostic (OBD) ports, which generally use BASE-T or BASE-TX interfaces to (again) support PC connections.

MDI IEEE specification (data rate) Typical systems Medium Pros Cons
10BASE-T/Te IEEE802.3u (10 Mbps) Industrial lighting CAT5 Commonly supported Long reach, Low standby power, Low speed
10BASE-T1L IEEE802.3cg (10 Mbps) Field transmitters; switches; heating, ventilation and air conditioning controllers; escalators Unshielded twisted pair (UTP), shielded twisted pair (STP) Ultra-long reach, bidirectional over single pair, power coupled over the data Low speed
100BASE-TX IEEE802.3u (100 Mbps) PLCs, IP cameras, OBD ports CAT5 Commonly supported, used by field buses High emissions, external components
100BASE-T1 IEEE802.3bu (100 Mbps) Display clusters, head units, gateways, infotainment, avionics communication, robotics, machine vision UTP, STP Low emissions, high immunity, bidirectional over single-pair cables Not as common (no PC connection support), short cable reach
1000BASE-T IEEE802.3ab (1 Gbps) IP cameras, test and measurement CAT6 1-Gbps speed Expensive cable
1000BASE-T1 IEEE802.3bp (1 Gbps) Telematics control unit, gateways, avionics communication, robotics, machine vision UTP, STP 1-Gbps speed, bidirectional over single pair Not as common (no PC connection support), short cable reach

Table 2 outlines common MDIs and the systems in which they’re commonly found.

Most commercial and industrial PHYs support multiple data rates. These PHYs include a mechanism called auto-negotiation, which is a way for PHYs to exchange information about feature support that enables them to link up at the highest possible speed.