Skip to content

Tutorial: XLS[cc] channels.

This tutorial is aimed at walking you through the implementation and synthesis into Verilog of a sequential C++ block containing channels.

Introduction to channels.

XLS implements channels via a FIFO-based (ready/valid/data) interface.

In C++, these channels are provided by a built-in template class called __xls_channel supporting the two methods: read() and write(val).

It can be aliased to the desired name like this:

template<typename T>
using OutputChannel = __xls_channel<T, __xls_channel_dir_Out>;

template<typename T>
using InputChannel = __xls_channel<T, __xls_channel_dir_In>;

An example of a usage is below, which reads an integer on the input channel, multiplies it by 3, and writes it to the output channel.

class TestBlock {
    InputChannel<int> in;
    OutputChannel<int> out;

    #pragma hls_top
    void Run() {
        auto x =;

Translate into optimized XLS IR.

With the above setup complete, XLS IR can now be generated using a sequence of xlscc and opt_main.

$ ./bazel-bin/xls/contrib/xlscc/xlscc \
  --block_from_class TestBlock --block_pb meta.pb >
$ ./bazel-bin/xls/tools/opt_main >

Below is a quick summary of the options. 1. --block_from_class TestBlock - tells XLS[cc] which class is the top block. 1. --block_pb - tells XLS[cc] where to write the block's metadata description. This must be specified with --block_from_class.

Note that unlike in the prior tutorial, XLS[cc] is used to generate XLS procs rather than functions. This is to support the additional interface requirements of channels.

Note the metadata output

The file meta.pb now contains a description of the block which can be useful for integration. In this example, the result is:

channels {
  name: "in"
  is_input: true
  type: FIFO
  width_in_bits: 32
channels {
  name: "out"
  is_input: false
  type: FIFO
  width_in_bits: 32
name: "TestBlock"

Perform code-generation into a pipelined Verilog block.

With the same IR, you can either generate a combinational block or a clocked pipelined block with the codegen_main tool. In this section, we'll demonstrate how to generate a pipelined block using the above C++ code.

$ ./bazel-bin/xls/tools/codegen_main \
  --generator=pipeline \
  --delay_model="sky130" \
  --output_verilog_path=test_channels.v \
  --module_name=xls_test \
  --top=TestBlock_proc \
  --reset=rst \
  --reset_active_low=false \
  --reset_asynchronous=false \
  --reset_data_path=true \
  --pipeline_stages=5 \
  --flop_inputs=true \
  --flop_outputs=true \
  --flop_inputs_kind=skid \

Below is a quick summary of the options.

  1. --delay_model="sky130" - use the sky130 delay model.
  2. --top=TestBlock_proc - the proc that is the top-level is named TestBlock_proc. This should be the block class name given to XLS[cc] with a _proc suffix appended.
  3. --flop_inputs_kind=skid and --flop_outputs_kind=skid - control what type of I/O buffering is used. In this case, we configure a skid buffer at both the input and output.

Additional XLS[cc] examples.

For developers, it is possible to check if a specific feature is supported by checking for unit tests.