Getting timing closure for HDL Coder IP with multicycle paths

I have created an IP component with HDL Coder and I have integrated this into a Vivado project but I am struggling to get timing closure using multicycle paths.

The FPGA clock rate is set at 50MHz. The IP block carries out some reasonably complex fixed point computations at a slower rate of 250KHz. There are 200 clock cycles to complete these computations. Within the computations there are some 1/Z feedback loops as the calculations for the current cycle depend on the results from the previous cycle. There is also some trivial logic that runs in a single cycle at the 50MHz rate. So, the IP model block has 2 rates; 50MHz and 250KHz. Also, some of the slower rate computations are pipelined on 4 paths so in the HDL Coder generated model there are rates of 50MHz, 250KHz and 1MHz as a result of the pipelining. I have surrounded the slower functionality with 1/Z delays as suggested in the help for multicycle paths.

When I generate the IP block I get a timing controller file _tc.vhd with clock enables at 50 and 200 times slower than the base FPGA clock rate.

-- enb_1_1_1   : identical to clk_enable
-- enb_1_50_0  : 50x slower than clk with last phase
-- enb_1_200_0 : 200x slower than clk with last phase
-- enb_1_200_1 : 200x slower than clk with phase 1
-- 

  ATTRIBUTE keep OF phase_0 : SIGNAL IS "true";
  ATTRIBUTE mcp_info OF phase_0 : SIGNAL IS "SpeedsVibsProc_tc.u1_d50_o0";
  ATTRIBUTE keep OF phase_0_1 : SIGNAL IS "true";
  ATTRIBUTE mcp_info OF phase_0_1 : SIGNAL IS "SpeedsVibsProc_tc.u1_d200_o0";
   

I also generate a multicycle constraints file as follows:

# Multicycle constraints for clock enable: SpeedsVibsProc_tc.u1_d50_o0
set enbregcell [get_cells -hier -filter {mcp_info=="SpeedsVibsProc_tc.u1_d50_o0"}]
set enbregnet [get_nets -of_objects [get_pins -of_objects $enbregcell -filter {DIRECTION == OUT}]]
set reglist [get_cells -of [filter [all_fanout -flat -endpoints_only $enbregnet] IS_ENABLE]]
set_multicycle_path 50 -setup -from $reglist -to $reglist -quiet
set_multicycle_path 49 -hold  -from $reglist -to $reglist -quiet

# Multicycle constraints for clock enable: SpeedsVibsProc_tc.u1_d200_o0
set enbregcell [get_cells -hier -filter {mcp_info=="SpeedsVibsProc_tc.u1_d200_o0"}]
set enbregnet [get_nets -of_objects [get_pins -of_objects $enbregcell -filter {DIRECTION == OUT}]]
set reglist [get_cells -of [filter [all_fanout -flat -endpoints_only $enbregnet] IS_ENABLE]]
set_multicycle_path 200 -setup -from $reglist -to $reglist -quiet
set_multicycle_path 199 -hold  -from $reglist -to $reglist -quiet

I then build the IP block into a Vivado project and build the design; also adding the muticycle constraints file to the project. However, the design fails to meet the timing constraints with some paths with a WNS of around -66ns.

I have manually entered the commands from the constraints file in the TCL command window and all seems fine; no errors or warnings.

If I open up one of the failing paths and look at the schematic I see a very long set of linked blocks and it seems that these are somehow related to the 1/Z feedbacks within the calculations.

Is there anything I can do the model design to make the timing closure easier? Do I need to manually create a different multicycle paths constraints file for example?

Answer 1

[Kiran Kintali]

Please contact [e-mail] for help on this topic. Can you share the Simulink model with us to better answer the question?

[Mike Bardill]

I will work with the UK support team to go over this with the model. Thanks.