Cyclone® V Hard Processor System Technical Reference Manual

ID 683126
Date 8/28/2023
Public
Document Table of Contents

9.3.4. Functional Description of the Lightweight HPS-to-FPGA Bridge

The lightweight HPS-to-FPGA bridge provides a lower-performance interface to the FPGA fabric. This interface is useful for accessing the control and status registers of soft peripherals. The bridge provides a 2 MB address space and access to logic, peripherals, and memory implemented in the FPGA fabric. The MPU subsystem, direct memory access (DMA) controller, and debug access port (DAP) can use the lightweight HPS-to-FPGA bridge to access the FPGA fabric or GPV. Master interfaces in the FPGA fabric can also use the lightweight HPS-to-FPGA bridge to access the GPV registers in all three bridges.

The bridge master exposed to the FPGA fabric has a fixed data width of 32 bits. The slave interface of the bridge in the HPS logic has a fixed data width of 32 bits.

Use the lightweight HPS-to-FPGA bridge as a secondary, lower-performance master interface to the FPGA fabric. With a fixed width and a smaller address space, the lightweight bridge is useful for low-bandwidth traffic, such as memory-mapped register accesses to FPGA peripherals. This approach diverts traffic from the high-performance HPS-to-FPGA bridge, and can improve both register access latency and overall system performance.

Table 52.  Lightweight HPS-to-FPGA Bridge PropertiesThis table lists the properties of the lightweight HPS-to-FPGA bridge, including the master interface exposed to the FPGA fabric.
Bridge Property L3 Slave Interface FPGA Master Interface

Data width

32 bits

32 bits

Clock domain

l4_mp_clk h2f_lw_axi_clk

Byte address width

32 bits

21 bits

ID width

12 bits

12 bits

Read acceptance

16 transactions

16 transactions

Write acceptance

16 transactions

16 transactions

Total acceptance

32 transactions

32 transactions

The lightweight HPS-to-FPGA bridge has three master interfaces. The master interface connected to the FPGA fabric provides a lightweight interface from the HPS to custom logic in the FPGA fabric. The two other master interfaces, connected to the HPS-to-FPGA and FPGA-to-HPS bridges, allow you to access the GPV registers for each bridge.

The lightweight HPS-to-FPGA bridge also has a set of registers GPV to control the behavior of its four interfaces (one slave and three masters).

The GPV allows you to set the bridge’s issuing capabilities to support single or multiple transactions. The GPV also lets you set a write tidemark through the wr_tidemark register, to control how much data is buffered in the bridge before data is written to slaves in the FPGA fabric.

Note: It is critical to provide correct clock settings for the lightweight HPS-to-FPGA bridge, even if your design does not use this bridge. The l4_mp_clk clock is required for GPV access on the HPS-to-FPGA and FPGA-to-HPS bridges.