pragma HLS stream

Description

By default, array variables are implemented as RAM:

Top-level function array parameters are implemented as a RAM interface port.
General arrays are implemented as RAMs for read-write access.
In sub-functions involved inDATAFLOWoptimizations, the array arguments are implemented using a RAM pingpong buffer channel.
Arrays involved in loop-based DATAFLOW optimizations are implemented as a RAM pingpong buffer channel

If the data stored in the array is consumed or produced in a sequential manner, a more efficient communication mechanism is to use streaming data as specified by theSTREAMpragma, where FIFOs are used instead of RAMs.

Important:When an argument of the top-level function is specified as INTERFACEtype ap_fifo, the array is automatically implemented as streaming.

Syntax

Place the pragma in the C source within the boundaries of the required location.

#pragma HLS stream variable=depth=dim=off

Where:

variable=: Specifies the name of the array to implement as a streaming interface.
depth=: Relevant only for array streaming in DATAFLOW channels. By default, the depth of the FIFO implemented in the RTL is the same size as the array specified in the C code. This options lets you modify the size of the FIFO and specify a different depth.
When the array is implemented in a DATAFLOW region, it is common to the use thedepth=option to reduce the size of the FIFO. For example, in a DATAFLOW region when all loops and functions are processing data at a rate of II=1, there is no need for a large FIFO because data is produced and consumed in each clock cycle. In this case, thedepth=option may be used to reduce the FIFO size to 1 to substantially reduce the area of the RTL design.

Tip:The config_dataflow -depthcommand provides the ability to stream all arrays in a DATAFLOWregion. The depth=option specified here overrides the config_dataflowcommand for the assigned variable.
dim=: Specifies the dimension of the array to be streamed. The default is dimension 1. Specified as an integer from 0 toN, for an array withNdimensions.
off: Disables streaming data. Relevant only for array streaming in dataflow channels.
Tip:The config_dataflow -default_channel fifocommand globally implies a STREAMpragma on all arrays in the design. The offoption specified here overrides the config_dataflowcommand for the assigned variable, and restores the default of using a RAM pingpong buffer based channel.

Example 1

The following example specifies array A[10]to be streaming, and implemented as a FIFO:

#pragma HLS STREAM variable=A

Example 2

In this example array Bis set to streaming with a FIFO depth of 12:

#pragma HLS STREAM variable=B depth=12

Example 3

Array C has streaming disabled. It is assumed to be enabled by config_dataflowin this example:

#pragma HLS STREAM variable=C off