Block-Level I/O Protocols - 2020.2 English - UG1399

ap_ctrl_hs

The following figure shows the behavior of the block-level handshake signals created by the ap_ctrl_hs I/O protocol for a non-pipelined design.

Figure 1. Behavior of ap_ctrl_hs Interface

After reset, the following occurs:

1. The block waits for ap_start to go High before it begins operation.
2. Output ap_idle goes Low immediately to indicate the design is no longer idle.
3. The ap_start signal must remain High until ap_ready goes High. Once ap_ready goes High:
   • If ap_start remains High the design will start the next transaction.
   • If ap_start is taken Low, the design will complete the current transaction and halt operation.
4. Data can be read on the input ports.
5. Data can be written to the output ports.
   Note: The input and output ports can also specify a port-level I/O protocol that is independent of this block-level I/O protocol. For details, see Port-Level I/O Protocols.
6. Output ap_done goes High when the block completes operation.
   Note: If there is an ap_return port, the data on this port is valid when ap_done is High. Therefore, the ap_done signal also indicates when the data on output ap_return is valid.
7. When the design is ready to accept new inputs, the ap_ready signal goes High. Following is additional information about the ap_ready signal:
   • The ap_ready signal is inactive until the design starts operation.
   • In non-pipelined designs, the ap_ready signal is asserted at the same time as ap_done.
   • In pipelined designs, the ap_ready signal might go High at any cycle after ap_start is sampled High. This depends on how the design is pipelined.
   • If the ap_start signal is Low when ap_ready is High, the design executes until ap_done is High and then stops operation.
   • If the ap_start signal is High when ap_ready is High, the next transaction starts immediately, and the design continues to operate.
8. The ap_idle signal indicates when the design is idle and not operating. Following is additional information about the ap_idle signal:
   • If the ap_start signal is Low when ap_ready is High, the design stops operation, and the ap_idle signal goes High one cycle after ap_done.
   • If the ap_start signal is High when ap_ready is High, the design continues to operate, and the ap_idle signal remains Low.

ap_ctrl_chain

The ap_ctrl_chain block-level I/O protocol is similar to the ap_ctrl_hs protocol but provides an additional input port named ap_continue. An active-High ap_continue signal indicates that the downstream block that consumes the output data is ready for new data inputs. If the downstream block is not able to consume new data inputs, the ap_continue signal is Low, which prevents upstream blocks from generating additional data.

The ap_ready port of the downstream block can directly drive the ap_continue port. Following is additional information about the ap_continue port:

• If the ap_continue signal is High when ap_done is High, the design continues operating. The behavior of the other block-level I/O signals is identical to those described in the ap_ctrl_hs block-level I/O protocol.
• If the ap_continue signal is Low when ap_done is High, the design stops operating, the ap_done signal remains High, and data remains valid on the ap_return port if the ap_return port is present.

In the following figure, the first transaction completes, and the second transaction starts immediately because ap_continue is High when ap_done is High. However, the design halts at the end of the second transaction until ap_continue is asserted High.

Figure 2. Behavior of ap_ctrl_chain Interface

ap_ctrl_none

If you specify the ap_ctrl_none block-level I/O protocol, the handshake signal ports (ap_start, ap_idle, ap_ready, and ap_done) are not created. You can use this protocol to create a block without control signals, as used in free-running kernels.

@E [SIM-345] Cosim only supports the following 'ap_ctrl_none' designs: (1) 
combinational designs; (2) pipelined design with task interval of 1; (3) designs with 
array streaming or hls_stream ports.
@E [SIM-4] *** C/RTL co-simulation finished: FAIL ***