ESP32 Ethernet PHY Schematic Design

The ESP32 WiFi/BT SoC from Espressif Systems supports wired ethernet along with wireless networking, which could be a huge advantage for applications that need wired connectivity. Ethernet is still a very popular standard for building and industrial automation. The ESP32 ethernet feature lets you mix wired and wireless connectivity with one low cost design.
While it is not exactly low power, it does offer advantages like PoE (Power over Ethernet), which can be used to power ESP32 directly off the ethernet cable. This saves installation costs and allows ESP32 to be a standalone ethernet device.

ESP32 ethernet PHY interface schematic

The ESP32 ethernet physical interface is shown in the schematic above. It mainly consists of three sections:

  • The PHY chip or interface
  • The 50 MHz oscillator
  • Jack and magnetics

The main sections that you need to pay attention to for proper operation are:

  • Pull-up resistors on ESP32 side of the PHY chip.
  • Series termination resistors for reducing signal reflection and ringing.
  • The 50 ohm pullups on the ethernet jack side of the PHY chip.
  • Proper magnetic jack. Most ethernet jacks are low cost and do not contain any magnetics… you cannot use those directly. You will need external inductive components for using plain old RJ-45 connectors.

ESP32 resources needed for ethernet

Ethernet is rather demanding in terms of GPIO requirements. The IO MUX of the ESP32 cannot be used to flexibly configure GPIOs for ethernet interface. You must connect the specified ESP32 GPIOs to the ethernet PHY chip (the PHY chip could be a LAN8720 or LAN8710, for example).

The GPIOs you will need to reserve for ethernet are listed below for reference (excluding the clock pin):

 ESP32 GPIO Ethernet PHY IO
 GPIO 21 TXEN
 GPIO 19 TXD0
 GPIO 22 TXD1
 GPIO 27 CRS_DV
 GPIO 25 RXD0
 GPIO 26 RXD1
 GPIO 23 MDC
 GPIO 18 MDIO

Finally, let us not forget the ethernet clock pin. The GPIO0 of ESP32 will be connected either to a common clock source or will be sourcing the internal APLL clock to the PHY chip.

In either case, GPIO0 will be used up and the audio PLL may not be available at the same time as the ethernet interface.

External oscillator for ESP32 ethernet PHY

The ESP32 ethernet interface requires a 50MHz clock source to work. The clock must be synced for both the ESP32 and ethernet PHY chip to work properly.

There are 2 ways to obtain 50MHz clock for ethernet PHY:

  • 50MHz external clock IC
  • ESP32 APLL generating the required 50MHz

As mentioned before, using the APLL restricts the use of APLL for audio applications to generate MCLK. However, there are audio DACs, ADCs and codecs that do feature internal PLLs for MCLK.

PCB design and layout

The ethernet interface speed on the ESP32 will not blow your mind. The clock rate on the ethernet lines is moderately fast and needs no special attention while laying out the PCB.

While sloppy layout may not break your design, it is always good to follow basic high speed layout rules for good performance in noisy environments:

  • Pair up differential traces
  • Make sure all series termination resistors on ethernet GPIO lines are placed close to ESP32.
  • Length matching is not necessary, but the trace lengths should not vary wildly.
  • GPIO0 or whatever other GPIO carries the 50 MHz clock signal must have as little capacitive loading as possible.
  • The clock trace should be around the same length as the other signal lines.
  • The traces connecting the ethernet jack and PHY interface chip should preferably be short. They should be routed as differential pairs.
References

Reference 1: LAN8720 datasheet

Reference 2: ESP32 datasheet

Reference 3: ESP32-EVB documentation

Reference 4: ESP32 AT firmware documentation (ethernet)

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