Espressif ESP32 Technical Reference Manual page 237

10 Ethernet Media Access Controller (MAC)
10.6.2 RMII (Reduced Media-Independent Interface)
RMII interface signals are shown in figure 10-5.
10.6.2.1 RMII Interface Signal Description
The Reduced Media-Independent Interface (RMII) specification reduces the number of pins between the mi-
crocontroller's external peripherals and the external PHY at a data transmission rate of 10 Mbit/s or 100 Mbit/s.
According to the IEEE 802.3u standard, MII includes 16 pins that contain data and control signals. The RMII
specification reduces 62.5% of the pins to the number of seven.
RMII has the following features:
• Support for an operating rate of 10 Mbit/s or 100 Mbit/s
• The reference clock frequency must be 50 MHz.
• The same reference clock must be provided to the MAC and the external Ethernet PHY. The PHY provides
independent 2-bit-wide TX and RX data paths.
• Note: If Wi-Fi and Ethernet are used simultaneously, the RMII clock cannot be generated by the internal
APLL clock, as it would result in clock instability. In this case, please use an external PHY or external clock
source to provide the reference clock.
10.6.2.2 RMII Clock
The configuration of the RMII clock is as figure
Espressif Systems
Figure 10-5. RMII Interface
10-6 shows.
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