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ESP32_AT_Ethernet.md

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Overview

Initialises the Ethernet interface and enables it, then sends DHCP requests and tries to obtain a DHCP lease. If successful then you will be able to ping the device.

PHY Configuration

Use make menuconfig to set the PHY model and the PHY address, and configure the SMI I/O pins (see below). These configuration items will vary depending on the hardware configuration you are using.

The default configuration is correct for Espressif's Ethernet board with TLK110 PHY. Other hardware will require different configuration and/or changes to the default.

PHY Address

The PHY address depends on the hardware and the PHY configuration. Consult the documentation/datasheet for the PHY hardware you have.

  • Address 31 (default) for Espressif's Ethernet board with TLK110 PHY
  • Address 1 for the common Waveshare LAN8720 PHY breakout
  • Address 0 for other LAN8720 breakouts

If the PHY address is incorrect then the EMAC will initialise, but all attempts to read/write configuration registers on the PHY will fail.

PHY Clock Wiring

The ESP32 and the Ethernet PHY need a common 50MHz reference clock. This clock can either be be provided externally by a crystal oscillator (e.g. crystal connected to the PHY or a seperate crystal oscillator) or internally by using the EPS32's APLL.

Because of its freqency the signal integrity has to be observed (ringing, capacitive load, resisitive load, skew, length of PCB trace). It is recommended to add a 33Ω resistor in series to reduce ringing.

Possible configurations of the 50MHz clock signal:

Mode GPIO Pin Signal name Notes
external GPIO0 EMAC_TX_CLK Input of 50MHz PHY clock
internal GPIO0 CLK_OUT1 Output of 50MHz APLL clock. Signal quality might be an issue.
internal GPIO16 EMAC_CLK_OUT Output of 50MHz APLL clock.
internal GPIO17 EMAC_CLK_180 Inverted output of 50MHz APLL clock. Found to be best suitable for LAN8720 with long signal lines.

External PHY Clock

The external reference clock of 50MHz must be supplied on GPIO0. See note about GPIO0 below.

Internal PHY Clock

The ESP32 can generate a 50MHz clock using its APLL. When the APLL is already used as clock source for other purposes (most likely I²S) external PHY has to be used.

On different test setups clock output on GPIO0 was found unstable because in most designs the signal path is not ideal for this high frequency (the PCB trace has several devices added to it and therefore the capacitive load is relatively high)

The inverted clock signal EMAC_CLK_180 was found working best with a LAN8720 PHY.

RMII PHY Wiring

The following PHY connections are required for RMII PHY data connections. These GPIO pin assignments cannot be changed.

GPIO RMII Signal ESP32 EMAC Function Notes
GPIO21 TX_EN EMAC_TX_EN
GPIO19 TX0 EMAC_TXD0
GPIO22 TX1 EMAC_TXD1
GPIO25 RX0 EMAC_RXD0
GPIO26 RX1 EMAC_RXD1
GPIO27 CRS_DV EMAC_RX_DRV

RMII PHY SMI Wiring

The following PHY connections are required for RMII PHY SMI (aka MDIO) management interface. These GPIO pin assignments can be changed to any unused GPIO pin.

For the AT, these pins are configured via make menuconfig under the configuration.

Default GPIO RMII Signal Notes
GPIO23 MDC Output to PHY
GPIO18 MDIO Bidirectional

The defaults in the menuconfig are correct for Espressif's Ethernet development board.

Note about GPIO0

Because GPIO0 is a strapping pin for entering UART flashing mode on reset, care must be taken when also using this pin as EMAC_TX_CLK. If the clock output from the PHY is oscillating during reset, the ESP32 may randomly enter UART flashing mode.

One solution is to use an additional GPIO as a "power pin", which either powers the PHY on/off or enables/disables the PHY's own oscillator. This prevents the clock signal from being active during a system reset. For this configuration to work, GPIO0 also needs a pullup resistor and the "power pin" GPIO will need a pullup/pulldown resistor - as appropriate in order to keep the PHY clock disabled when the ESP32 is in reset.

See the source code to see how the "power pin" GPIO can be managed in software.

The defaults is to using GPIO17 for this function, but it can be overriden. On Espressif's Ethernet development board, GPIO17 is the power pin used to enable/disable the PHY oscillator.