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Receiving Inbound Frames

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Maximum Permitted Frame Length

The maximum legal length of a frame specified in IEEE 802.3-2005is 1518 bytes for non- VLAN tagged frames. VLAN tagged frames may be extended to 1522 bytes. When jumbo frame handling is disabled and the core receives a frame which exceeds the maximum legal length, rx_bad_frame is asserted. When jumbo frame handling is enabled, frames which are longer than the legal maximum are received in the same way as shorter frames. For more information about enabling and disabling jumbo frame handling, see “Configuration Registers,” on page 78.

Length/Type Field Error Checks

Enabled

Default operation is with the length/type error checking enabled (see “Receiver Configuration,” on page 79). In this mode, the following checks are made on all frames received. If either of these checks fail, the frame is marked as bad.

•A value in the length/type field that is greater than or equal to decimal 46, but less than decimal 1536 (a length interpretation), is checked against the actual data length received.

•A value in the length/type field that is less than decimal 46 is checked to see that the data field is padded to exactly 46 bytes (so that the resultant frame is a minimum frame size of 64 bytes total in length).

Furthermore, if padding is indicated (the length/type field is less than decimal 46) and client-supplied FCS passing is disabled, the length value in the length/type field will be used to deassert rx_data_valid after the indicated number of data bytes so that the padding bytes are removed from the frame. See “Client-Supplied FCS Passing.”

Disabled

When the length/type error checking is disabled and the length/type field has a length interpretation, the MAC does not check the length value against the actual data length received. See “Receiver Configuration” in Chapter 8. A frame containing only this error is marked as good.

However, if the length/type field is less than decimal 46 then the MAC will mark a frame as bad if it is not the minimum frame size of 64 bytes.

If padding is indicated and client-supplied FCS passing is disabled, then a length value in the length/type field will not be used to deassert rx_data_valid. Instead, rx_data_valid is deasserted before the start of the FCS field, and any padding is not removed from the frame.

1-Gigabit Ethernet MAC v8.5 User Guide

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UG144 April 24, 2009

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Xilinx UG144 manual Maximum Permitted Frame Length, Length/Type Field Error Checks, Enabled, Disabled
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UG144 specifications

The Xilinx UG144, a comprehensive user guide for the versatile Zynq-7000 SoC (System on Chip) architecture, serves as an essential resource for developers and engineers designing embedded systems. Emphasizing the blend of programmable logic and processing power, this guide highlights the array of features and technologies that make the Zynq-7000 series particularly attractive for a wide range of applications.

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