The RC interface supports a straddle option that allows up to four TLPs to be transferred over the interface in the same beat. This option can be enabled during core configuration in the Vivado IDE. When enabled, the core may start a new Completion TLP on byte lanes 0, 16, 32, or 48. Thus, with this option enabled, it is possible for the core to send four Completion TLPs entirely in the same beat on the AXI bus, if each of them has a payload of size one Dword or less. The straddle option can only be used when the RC interface is configured in the Dword-aligned mode.
When the Requester Completion (RC) interface is configured for a width of 256 bits, depending on the type of TLP and Payload size, there can be significant interface utilization inefficiencies, if a maximum of 1 TLP is allowed to start or end per interface beat. This inefficient use of RC interface can lead to overflow of the completion FIFO when Infinite Receiver Credits are advertized. You must either:
• Restrict the number of outstanding Non Posted requests, so as to keep the total number of completions received less than 64 and within the completion of the FIFO size selected, or
• Use the RC interface straddle option. See This Figure for waveforms showing this option.
The straddle option, available only on the 256-bit wide RC interface, is enabled through the Vivado IP catalog. See Design Flow Steps for instructions on enabling the option in the IP catalog. When this option is enabled, the integrated block can start a new Completion TLP on byte lane 16 when the previous TLP has ended at or before byte lane 15 in the same beat. Thus, with this option enabled, it is possible for the integrated block to send multiple Completion TLPs entirely in the same beat on the RC interface, if neither of them has more than one Dword of payload.
The straddle setting is only available when the interface width is set to 256 bits, and the RC interface is set to Dword-aligned mode.
Table: Valid Combinations of Interface Width, Alignment Mode, and Straddle lists the valid combinations of interface width, addressing mode, and the straddle option.