The PIO design is a simple target-only application that interfaces with the Endpoint for the PCIe core Transaction (AXI4-Stream) interface and is provided as a starting point for you to build your own designs. These features are included:
- In Address Align Mode, four transaction-specific 2 KB target regions using the internal FPGA block RAMs, providing a total target space of 8,192 bytes.
- In the Address Align Mode, supports single Dword payload Read and Write PCI Express transactions to 32-/64-bit address memory spaces and I/O space with support for completion TLPs.
- In the case of Dword Align Mode, the PIO Design supports multiple Dword payload (Up to 256 DW) read and write PCI Express transactions to 32-bit Address Memory Spaces with support for completion TLPs. Uses the BAR ID[2:0] and Completer Request Descriptor[114:112] of the core to differentiate between TLP destination Base Address Registers.
- Provides separate implementations optimized for 64-bit, 128-bit, 256-bit, and 512-bit AXI4-Stream interfaces.
The following figure illustrates the PCI Express system architecture components, consisting of a Root Complex, a PCI Express switch device, and an Endpoint for PCIe. PIO operations move data downstream from the Root Complex (CPU register) to the Endpoint, and/or upstream from the Endpoint to the Root Complex (CPU register). In either case, the PCI Express protocol request to move the data is initiated by the host CPU.
Data is moved downstream when the CPU issues a store register to a MMIO address command. The Root Complex typically generates a Memory Write TLP with the appropriate MMIO location address, byte enables, and the register contents. The transaction terminates when the Endpoint receives the Memory Write TLP and updates the corresponding local register.
Data is moved upstream when the CPU issues a load register from a MMIO address command. The Root Complex typically generates a Memory Read TLP with the appropriate MMIO location address and byte enables. The Endpoint generates a Completion with Data TLP after it receives the Memory Read TLP. The Completion is steered to the Root Complex and payload is loaded into the target register, completing the transaction.