Espressif ESP32 Technical Reference Manual page 232

10 Ethernet Media Access Controller (MAC)
10.4 MAC Address Filtering
Address filtering will check the destination and source addresses of all received frames and report the address
filtering status accordingly. For example, filtered frames can be identified either as multicast or broadcast.The
address check, then, is based on the parameters selected by the application (Frame Filter Registers).
Physical (MAC) addresses are used for address checking during address filtering.
10.4.1 Unicast Destination Address Filtering
The MAC supports up to 8 MAC addresses for perfect filtering of unicast addresses. If a perfect filtering is
selected (by resetting bit[1] in the Frame Filter Register), the MAC compares all 48 bits of the received unicast
address with the programmed MAC address to determine if there is a match. By default, EMACADDR0 is always
enabled, and the other addresses (EMACADDR0 ~ EMACADDR7) are selected by a separate enable bit. When
the individual bytes of the other addresses (EMACADDR0 ~ EMACADDR7) are compared with the DA bytes
received, the latter can be masked by setting the corresponding Mask Byte Control bit in the register to 1. This
facilitates the DA group address filtering.
10.4.2 Multicast Destination Address Filtering
The MAC can be programmed to pass all multicast frames by setting the Pass All Multicast (PAM) bit in the Frame
Filter Register to 1. If the PAM bit is reset, the MAC will filter multicast addresses, according to Bit[2] in the Frame
Filter Register.
In perfect filtering mode, the multicast address is compared with the programmed MAC Destination Address
Registers (EMACADDR0 ~ EMACADDR7). Group address filtering is also supported.
10.4.3 Broadcast Address Filtering
The MAC does not filter any broadcast frames in the default mode. However, if the MAC is programmed to reject
all broadcast frames, which can happen by setting the Disable Broadcast Frames (DBF) bit in the Frame Filter
Register to 1, all broadcast frames will be discarded.
10.4.4 Unicast Source Address Filtering
The MAC may also perform a perfect filtering based on the source address field of the received frame. By default,
the Address Filtering Module (AFM) compares the Source Address (SA) field with the values programmed in the
SA register. By setting Bit[30] in the SA register to 1, the MAC Address Register (EMACADDR0 - EMACADDR7)
can be configured to contain SA, instead of Destination Address (DA), for filtering. Group filtering with SA is also
supported. If the Source Address Filter (SAF) enable bit in the Frame Filter Register is set to 1, the MAC discards
frames that do not pass the SA filtering. Otherwise, the result of SA filtering is given as a status bit in the Receive
Status word (Please refer to Table 10-9).
When the SAF enable bit is set to 1, the result of the SA filtering and DA filtering is AND'ed to determine whether
or not to forward the frame. Any frame that fails to pass will be discarded. Frames need to pass both filterings
in order to be forwarded to the application.
10.4.5 Inverse Filtering Operation
For both destination address (DA) and source address (SA) filtering, you can invert the results matched through
the filtering at the final output. The inverse filtering of DA and SA are controlled by the DAIF and SAIF bits,
respectively, in the Frame Filter Register. The DAIF bit applies to both unicast and multicast DA frames. When
DAIF is set to 1, the result of unicast or multicast destination address filtering will be inverted. Similarly, when the
SAIF bit is set to 1, the result of unicast SA filtering is reversed.
Espressif Systems 232 ESP32 TRM (Version 5.2)
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