Understanding an FPGA Architecture - 2022.1 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

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2022.1 English

Before migrating any design to a new target platform, you should have a fundamental understanding of the FPGA architecture. The following diagram shows the floorplan of a Xilinx® FPGA device. The concepts to understand are:

  • SSI Devices
  • SLRs
  • SLR routing resources
  • Memory interfaces
Figure 1. Physical View of Xilinx FPGA with Four SLR Regions
Tip: The FPGA floorplan shown above is for a SSI device with four SLRs where each SLR contains a DDR Memory interface.

Stacked Silicon Interconnect Devices

A SSI device is one in which multiple silicon dies are connected together through silicon interconnect, and packaged into a single device. An SSI device enables high-bandwidth connectivity between multiple die by providing a much greater number of connections. It also imposes much lower latency and consumes dramatically lower power than either a multiple FPGA or a multi-chip module approach, while enabling the integration of massive quantities of interconnect logic, transceivers, and on-chip resources within a single package. The advantages of SSI devices are detailed in Xilinx Stacked Silicon Interconnect Technology Delivers Breakthrough FPGA Capacity, Bandwidth, and Power Efficiency (WP380).

Super Logic Region

An SLR is a single FPGA die slice contained in an SSI device. Multiple SLR components are assembled to make up an SSI device. Each SLR contains the active circuitry common to most Xilinx FPGA devices. This circuitry includes large numbers of:

  • LUTs
  • Registers
  • I/O Components
  • Gigabit Transceivers
  • Block Memory
  • DSP Blocks

One or more kernels can be implemented within an SLR. A single kernel can be placed across multiple SLRs if needed.

SLR Routing Resources

The custom hardware implemented on the FPGA is connected via on-chip routing resources. There are two types of routing resources in an SSI device:

Intra-SLR Resources
Intra-SLR routing resource are the fast resources used to connect the hardware logic. The Vitis technology automatically uses the most optimal resources to connect the hardware elements when implementing kernels.
Super Long Line (SLL) Resources
SLLs are routing resources running between SLRs, used to connect logic from one region to the next. These routing resources are slower than intra-SLR routes. However, when a kernel is placed in one SLR, and the DDR it connects to is in another, the Vitis technology automatically implements dedicated hardware to use SLL routing resources without any impact to performance. More information on managing placement are provided in Modifying Kernel Placement.

Memory Interfaces

Each SLR contains one or more memory interfaces. These memory interfaces are used to connect to the DDR memory where the data in the host buffers is copied before kernel execution. Each kernel reads data from the DDR memory and writes the results back to the same DDR memory. The memory interface connects to the pins on the FPGA and includes the memory controller logic.