Flip-flop stages exist in the boundary logic interface (BLI) between the programmable logic (PL) and the high-performance XPIO and between the PL and AI Engine interface tiles. The BLI flip-flop resources can optimize the interface timing by registering signals coming into and going out of the PL. BLI flip-flop resources have the following restrictions:
- BLI only supports flip-flops with asynchronous clear (FDCE) or a synchronous reset (FDRE) with the R-pin tied to GND.
- BLI only supports FDCE and FDRE with INIT value of 0.
- All BLI flip-flops in a site must share the same active CLR signal or inactive GND R signal.
- FDCE and FDRE can be mixed in a site only if the CLR pin and R-pins are tied to GND.
- All BLI flip-flops in a site must share the same CE signal.
By default, flip-flops are not placed in BLI flip-flop resources if the flip-flops are simply connected to an XPIO bank or AI Engine interface tile resources. The BLI constraint must be used for the Vivado tools to place flip-flops in the BLI flip-flop resources. If the BLI constraint cannot be met due violation of the restrictions, the flip-flop is placed in the PL. Nets of PL flip-flops interfacing with resources using the BLI perform a route-through of the BLI flip-flop resources. In the following example, the BLI constraint is used for flip-flops in BLI resources driving a flip-flop in the XPIO IOB. The example also demonstrates a PL flip-flop driving a flip-flop in the XPIO IOB and shows the route from the BLI through the BLI flip-flop resources.
set_property BLI TRUE [get_cells myIntf/myBLI_reg]
For more information on BLI in XPIO banks, see this link in the Versal ACAP SelectIO Resources Architecture Manual (AM010).
For more information on the BLI constraint, see BLI in Vivado Design Suite Properties Reference Guide (UG912).