The RTL blackbox enables the use of existing Verilog RTL IP in an HLS project. This lets you add RTL code to your C/C++ code for synthesis of the project by Vitis HLS. The RTL IP can be used in a sequential, pipeline, or dataflow region. Refer to Vitis-HLS-Introductory-Examples/Misc/rtl_as_blackbox on Github for examples of this technique.
Tip: Adding an RTL blackbox to your design will restrict the tool from outputting VHDL code, Because the RTL blackbox must be Verilog, the output will be Verilog only.
Integrating RTL IP into a Vitis HLS project requires the following files:
- C function signature for the RTL code. This can be placed into a header (.h) file.
- Blackbox JSON description file as discussed in JSON File for RTL Blackbox.
- RTL IP files.
To use the RTL blackbox in an HLS project, use the following steps.
- Call the C function signature from within your top-level function, or a sub-function in the Vitis HLS project.
- Add the blackbox JSON description file to your HLS project
using the Add Files command from the
Vitis HLS IDE as discussed in Creating a New Vitis HLS Project, or using the
add_files –blackbox my_file.jsonTip: As explained in the next section, the new RTL Blackbox wizard can help you generate the JSON file and add the RTL IP to your project.
- Run the Vitis HLS design flow for simulation, synthesis, and co-simulation as usual.
Requirements and Limitations
RTL IP used in the RTL blackbox feature have the following requirements:
- Should be Verilog (.v) code.
- Must have a unique clock signal, and a unique active-High reset signal.
- Must have a CE signal that is used to enable or stall the RTL IP.
- Must use the
ap_ctrl_chainprotocol as described in Block-Level Control Protocols.
Within Vitis HLS, the RTL blackbox feature:
- Supports only C++.
- Cannot connect to top-level interface I/O signals.
- Cannot directly serve as the design-under-test (DUT).
- Does not support
- Supports the following interface protocols as described in JSON File for RTL Blackbox:
- The RTL blackbox IP supports the
hls::streaminterface. When this data type is used in the C function, use a
FIFORTL port protocol for this argument in the RTL blackbox IP.
- The RTL blackbox IP supports RAM interface for arrays. For array
arguments in the C function, use one of the following RTL port
protocols for the corresponding argument in the RTL blackbox IP:
- Single port RAM – RAM_1P
- Dual port RAM – RAM_T2P
- Scalars and Input Pointers
- The RTL Blackbox IP supports C scalars and input pointers only in
sequential and pipeline regions. They are not supported in a
dataflow region. When these constructs are used in the C function,
wireport protocol in the RTL IP.
- Inout and Output Pointers
- The RTL blackbox IP supports inout and output pointers only in sequential
and pipeline regions. They are not supported in a dataflow region.
When these constructs are used in the C function, the RTL IP should
ap_vldfor output pointers, and
ap_ovldfor inout pointers.
Tip: All other Vitis HLS design restrictions also apply when using RTL blackbox in your project.