Espressif ESP32-S2 User Manual

Chapter 2. API Reference
Destination Address The destination address field contains a 6-byte length MAC address of the device that the
packet is directed to. If the Least Significant bit in the first byte of the MAC address is set, the address is a multi-cast
destination. For example, 01-00-00-00-F0-00 and 33-45-67-89-AB-CD are multi-cast addresses, while 00-00-00-
00-F0-00 and 32-45-67-89-AB-CD are not. Packets with multi-cast destination addresses are designed to arrive and
be important to a selected group of Ethernet nodes. If the destination address field is the reserved multi-cast address,
i.e. FF-FF-FF-FF-FF-FF, the packet is a broadcast packet and it will be directed to everyone sharing the network.
If the Least Significant bit in the first byte of the MAC address is clear, the address is a uni-cast address and will be
designed for usage by only the addressed node.
Normally the EMAC controller incorporates receive filters which can be used to discard or accept packets with multi-
cast, broadcast and/or uni-cast destination addresses. When transmitting packets, the host controller is responsible
for writing the desired destination address into the transmit buffer.
Source Address The source address field contains a 6-byte length MAC address of the node which created the
Ethernet packet. Users of Ethernet must generate a unique MAC address for each controller used. MAC addresses
consist of two portions. The first three bytes are known as the Organizationally Unique Identifier (OUI). OUIs are
distributed by the IEEE. The last three bytes are address bytes at the discretion of the company that purchased the
OUI. More information about MAC Address used in ESP-IDF, please see MAC Address Allocation.
When transmitting packets, the assigned source MAC address must be written into the transmit buffer by the host
controller.
Type / Length The type/length field is a 2-byte field, if the value in this field is <= 1500 (decimal), it is considered
a length field and it specifies the amount of non-padding data which follows in the data field. If the value is >= 1536,
it represents the protocol the following packet data belongs to. The following are the most common type values:
• IPv4 = 0800H
• IPv6 = 86DDH
• ARP = 0806H
Users implementing proprietary networks may choose to treat this field as a length field, while applications imple-
menting protocols such as the Internet Protocol (IP) or Address Resolution Protocol (ARP), should program this field
with the appropriate type defined by the protocol’s specification when transmitting packets.
Payload The payload field is a variable length field, anywhere from 0 to 1500 bytes. Larger data packets will
violate Ethernet standards and will be dropped by most Ethernet nodes. This field contains the client data, such as an
IP datagram.
Padding and FCS The padding field is a variable length field added to meet IEEE 802.3 specification requirements
when small data payloads are used. The DA, SA, type, payload and padding of an Ethernet packet must be no smaller
than 60 bytes. Adding the required 4-byte FCS field, packets must be no smaller than 64 bytes. If the data field is
less than 46 bytes long, a padding field is required.
The FCS field is a 4-byte field which contains an industry standard 32-bit CRC calculated with the data from the
DA, SA, type, payload and padding fields. Given the complexity of calculating a CRC, the hardware normally will
automatically generate a valid CRC and transmit it. Otherwise, the host controller must generate the CRC and place
it in the transmit buffer.
Normally, the host controller does not need to concern itself with padding and the CRC which the hardware EMAC
will also be able to automatically generate when transmitting and verify when receiving. However, the padding and
CRC fields will be written into the receive buffer when packets arrive, so they may be evaluated by the host controller
if needed.
Note: Besides the basic data frame described above, there’re two other common frame types in 10/100 Mbps
Ethernet: control frames and VLAN tagged frames. They’re not supported in ESP-IDF.
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Espressif ESP32-S2 Specifications

General

Brand	Espressif
Model	ESP32-S2
Category	Single board computers
Language	English