9.4.1. Avalon-ST Interface
An Avalon‑ST interface connects the Application Layer and the Transaction Layer. This is a point‑to‑point, streaming interface designed for high throughput applications. The Avalon‑ST interface includes the RX and TX datapaths.
For more information about the Avalon‑ST interface, including timing diagrams, refer to the Avalon Interface Specifications.
The RX datapath transports data from the Transaction Layer to the Application Layer’s Avalon‑ST interface. Masking of non-posted requests is partially supported. Refer to the description of the rx_st_mask signal for further information about masking.
The TX datapath transports data from the Application Layer's Avalon-ST interface to the Transaction Layer. The Hard IP provides credit information to the Application Layer for posted headers, posted data, non‑posted headers, non‑posted data, completion headers and completion data.
The Application Layer may track credits consumed and use the credit limit information to calculate the number of credits available. However, to enforce the PCI Express Flow Control (FC) protocol, the Hard IP also checks the available credits before sending a request to the link, and if the Application Layer violates the available credits for a TLP it transmits, the Hard IP blocks that TLP and all future TLPs until credits become available. By tracking the credit consumed information and calculating the credits available, the Application Layer can optimize performance by selecting for transmission only the TLPs that have credits available.