Altera Low Latency Ethernet 10G MAC - Manual (Page 37)

Frame Length
The frame length must be at least 64 (0x40) bytes and not exceed the following maximum value for the
different frame types:
• Basic—The value in the
rx_frame_maxlength
register.
• VLAN tagged—The value in the
rx_frame_maxlength
register plus four bytes when the
rx_vlan_detection[0]
register bit is 0; or the value in the
rx_frame_maxlength
register when the
rx_vlan_detection[0]
register bit is set to 1.
• Stacked VLAN tagged—The value in the
rx_frame_maxlength
register plus eight bytes when the
rx_vlan_detection[0]
register bit is 0; or the value in the
rx_frame_maxlength
register when the
rx_vlan_detection[0]
register bit is set to 1.
The following error bits represent frame length violations:
•
avalon_st_rx_error[2]
—undersized frames.
•
avalon_st_rx_error[3]
—oversized frames.
Payload Length
The MAC IP core checks the payload length for frames other than control frames when the VLAN and
stacked VLAN detection is disabled. The MAC RX keeps track of the actual payload length as it receives a
frame and checks the actual payload length against the length/type or client length/type field. The payload
length must be between 46 (0x2E) and 1500 (0x5DC). For VLAN and stacked VLAN frames, the
minimum payload length is 42 (0x2A) or 38 (0x26) respectively and not exceeding the maximum value of
1500 (0x5DC).
For an invalid payload length, the MAC RX sets the
avalon_st_rx_error[4]
bit to 1. This error occurs
when the actual payload length is less than the value of the length/type field. If the actual payload length is
more than the value of the length/type field, the MAC RX assumes that the frame contains excessive
padding and does not set this error bit to 1.
CRC and Padding Bytes Removal
By default, the MAC RX forwards receive frames to the client without removing the CRC field and
padding bytes from the frames. You can configure the MAC RX to remove the CRC field by setting the
rx_padcrc_control
register to 1. To remove both the CRC field and padding bytes, set the
rx_padcrc_control
register to 3.
When enabled, the MAC RX removes padding bytes from receive frames whose payload length is less
than the following values for the different frame types:
• 46 bytes for basic frames
• 42 bytes for VLAN tagged frames
• 38 bytes for stacked VLAN tagged frames
The MAC RX removes padding bytes only when the VLAN and stacked VLAN detection is enabled
(
rx_vlan_detection[0]
= 0). Otherwise, the MAC RX does not remove padding bytes even if padding
bytes removal is enabled.
UG-01144
2014.12.15
Frame Length 3-15
Functional Description of LL Ethernet 10G MAC Altera Corporation
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