Espressif ESP32 Technical Reference Manual page 228

10 Ethernet Media Access Controller (MAC)
Figure 10-2. Ethernet Block Diagram
Ethernet MAC consists of the MAC-layer configuration register module and three layers: EMAC_CORE (MAC Core
Layer), EMAC_MTL (MAC Transition Layer), and EMAC_DMA (Direct Memory Access). Each of these three layers
has two directions: Tx and Rx. They are connected to the system through the Advanced High-Performance Bus
(AHB) and the Advanced Peripheral Bus (APB) on the chip. Off the chip, they communicate with the external
PHY through the MII and RMII interfaces to establish an Ethernet connection.
10.2 EMAC_CORE
The MAC supports many interfaces with the PHY chip. The PHY interface can be selected only once after reset.
The MAC communicates with the application side (DMA side), using the MAC Transmit Interface (MTI), MAC
Receive Interface (MRI) and the MAC Control Interface (MCI).
10.2.1 Transmit Operation
A transmit operation is initiated when the MTL Application pushes in data at the time a response signal is as-
serted. When the SOF (start of frame) signal is detected, the MAC accepts the data and begins transmitting
to the RMII or MII. The time required to transmit the frame data to the RMII or MII, after the application initiates
transmission, varies, depending on delay factors like IFG delay, time to transmit Preamble or SFD (Start Frame
Delimiter), and any back-off delays in half-duplex mode. Until then, the MAC does not accept the data received
from MTL by de-asserting the ready signal.
After the EOF (end of frame) is transmitted to the MAC, the MAC completes the normal transmission and yields
the Transmit Status to the MTL. If a normal collision (in half-duplex mode) occurs during transmission, the MAC
makes valid the Transmit Status in the MTL. It then accepts and drops all further data until the next SOF is
received. The MTL block should retransmit the same frame from SOF upon observing a retry request (in the
Status) from the MAC.
The MAC issues an underflow status if the MTL is not able to provide the data continuously during transmission.
During the normal transmission of a frame from MTL, if the MAC receives an SOF without getting an EOF for the
previous frame, it ignores the SOF and considers the new frame as a continuation of the previous one.
Espressif Systems 228 ESP32 TRM (Version 5.2)
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