To use hls::stream<>
objects, include
the header file hls_stream.h. Streaming data objects are defined by
specifying the type and variable name. In this example, a 128-bit unsigned integer type is
defined and used to create a stream variable called my_wide_stream
.
#include "ap_int.h"
#include "hls_stream.h"
typedef ap_uint<128> uint128_t; // 128-bit user defined type
hls::stream<uint128_t> my_wide_stream; // A stream declaration
Streams must use scoped naming. Xilinx
recommends using the scoped hls::
naming shown in the
example above. However, if you want to use the hls
namespace, you can rewrite the preceding example as:
#include <ap_int.h>
#include <hls_stream.h>
using namespace hls;
typedef ap_uint<128> uint128_t; // 128-bit user defined type
stream<uint128_t> my_wide_stream; // hls:: no longer required
Given a stream specified as hls::stream<T>
, the type T can be:
- Any C++ native data type
- A Vitis HLS arbitrary precision type
(for example,
ap_int<>
,ap_ufixed<>
) - A user-defined struct containing either of the above types
A stream can also be specified as hls::stream<Type, Depth>
, where Depth indicates the depth of the FIFO
needed in the verification adapter that the HLS tool creates for RTL co-simulation.
Streams can be optionally named. Providing a name for the stream allows the name to be used in reporting. For example, Vitis HLS automatically checks to ensure all elements from an input stream are read during simulation. Given the following two streams:
stream<uint8_t> bytestr_in1;
stream<uint8_t> bytestr_in2("input_stream2");
WARNING: Hls::stream 'hls::stream<unsigned char>.1' contains leftover data, which
may result in RTL simulation hanging.
WARNING: Hls::stream 'input_stream2' contains leftover data, which may result in RTL
simulation hanging.
Any warning on elements left in the streams are reported as follows, where
it is clear that the bytetr_in2 must be bytestr_in2:
When streams are passed into and out of functions, they must be passed-by-reference as in the following example:
void stream_function (
hls::stream<uint8_t> &strm_out,
hls::stream<uint8_t> &strm_in,
uint16_t strm_len
)
Vitis HLS supports both blocking and non-blocking access methods.
A complete design example using streams is provided in the Vitis HLS examples. Refer to the hls_stream
example in the design examples available from the Vitis IDE welcome screen.