The XACTStep and M1 Implementation software allows users to set timing requirements on various paths in the design. By attaching TNM attributes to instances in the design, and then setting point-to-point timespecs, timing-critical paths can be controlled. The process for using timespecs with a Foundation VHDL design is as follows:
1. Attach 'TNM' attributes to PADs, Flip-Flops, RAMs, Latches in the VHDL code. 2. Write a constraint file (.CST for F6.x, .UCF for F1.x) which contains the Timespecs. This constraints file will be read into the Design Manager during implementation.
Refer to the Xilinx Development Systems Reference Guide for more information about using Timespecs and Timing Driven Implementation.
**Note that this solution applies to the Metamor XVHDL compiler only. If using the Express HDL compiler, you have the ability to use the Express Constraints GUI to set up timegroups and timespecs.
Example: Putting TNMs on Instantiated Instances ----------------------------------------------- library IEEE; use IEEE.std_logic_1164.all;
entity DP_RAM is port (WR, DATA, CLK, RST: in std_logic; ADDR0, ADDR1, ADDR2, ADDR3: in std_logic; DPRA0, DPRA1, DPRA2, DPRA3: in std_logic; DPOUT, SPOUT, DOUT: out std_logic); end DP_RAM;
architecture INST of DP_RAM is component RAM16X1D port (D,WE,WCLK,A3,A2,A1,A0, DPRA3,DPRA2,DPRA1,DPRA0: in std_logic; SPO,DPO: out std_logic); end component; attribute TNM: string; attribute TNM of U1: label is "RAMTNM"; begin U1: RAM16X1D port map ( D=>DATA, WE=>WR, WCLK=>CLK, A3=>ADDR3, A2=>ADDR2, A1=>ADDR1, A0=>ADDR0, DPRA3=>DPRA3, DPRA2=>DPRA2, DPRA1=>DPRA1, DPRA0=>DPRA0, SPO=>SPOUT, DPO=>DPOUT); end INST;
Attaching TNM attributes in the VHDL code -----------------------------------------
A) Inferred Components
To attach a TNM attribute to an inferred PAD, Flip-Flop or Latch, use the following syntax:
attribute TNM: string; attribute TNM of <signal_name>: signal is "<tnm_id>";
where <signal_name> is the Q output of the flip-flop or latch, or the port name for the desired pad, and <tnm_id> is the TNM identifier name which you want to assign to the instance.
If a signal is declared as a top-level entity output or inout port, and is also the output of a flip-flop or latch, a TNM attribute attached to that signal will be placed onto the OPAD, and not the flip-flop. In order to attach a TNM attribute to the flip-flop in this case, a dummy signal must be created.
See the resolution titled "Example: Putting TNMs on Inferred Instances" for example VHDL code.
B) Instantiated Components
To attach a TNM attribute to an instantiated flip-flop, RAM or latch, use the following syntax:
attribute TNM: string; attribute TNM of <instance_name>: label is "<tnm_id>";
where <instance_name> is the instance name assigned to the instantiated component in the VHDL code, and <tnm_id> is the TNM identifer name which you want to assign to the instance.
See the resolution titled "Example: Putting TNMs on Instantiated Instances" for example VHDL code.
Writing a Constraint File with Timespecs ----------------------------------------
After attaching TNMs to the desired instances in the VHDL file, you must write a constraint file (<project_name>.CST for F6.x or <project_name>.UCF for F1.x) which will be read by the Xilinx Design Manager during implementation.
Below is an example constraint file. This constraint file assumes that the TNMs INPAD, FLOP_X, FLOP_Y, FLOPOUT, and OUTPAD have been defined.
##CST file for use with Foundation 6.x## timespec="ts01=from:INPAD:to:FLOP_X=20ns"; timespec="ts02=from:FLOP_X:to:FLOP_Y=8ns"; timespec="ts03=from:FLOPOUT:to:OUTPAD=8ns"; timespec="ts04=from:FLOP_Y:to:FLOPOUT=10ns";
##UCF file for use with Foundation F1.x## TIMESPEC TS01 = FROM : INPAD : TO : FLOP_X : 20ns; TIMESPEC TS02 = FROM : FLOP_X : TO : FLOP_Y : 8ns; TIMESPEC TS03 = FROM : FLOPOUT : TO : OUTPAD : 8ns; TIMESPEC TS04 = FROM : FLOP_Y : TO : FLOPOUT : 10ns;
Example: Putting TNMs on Inferred Instances ------------------------------------------- library IEEE; use IEEE.std_logic_1164.all;
entity FLOPS is port (DIN1, CLK, RESET: in std_logic; DOUT1: out std_logic); end FLOPS;
architecture USE_TSPEC of FLOPS is signal X : std_logic; -- Internal flip-flops signal Y : std_logic;
signal DUMMY: std_logic; -- This lets us place a TNM on -- both the flip-flop and on the -- output pad DOUT1. attribute TNM: string; attribute TNM of DOUT1: signal is "OUTPAD"; attribute TNM of DIN1: signal is "INPAD"; attribute TNM of X: signal is "FLOP_X"; attribute TNM of Y: signal is "FLOP_Y"; attribute TNM of DUMMY: signal is "FLOPOUT"; begin process (CLK, RESET) begin if RESET='1' then X <= '0'; Y <= '0'; DUMMY <= '0'; elsif (CLK'event and CLK='1') then X <= DIN1; Y <= X; DUMMY <= Y; end if; end process;
DOUT1 <= DUMMY; -- Pass the output of the last FF to the -- OPAD. This dummy signal is created to -- allow us to put a TNM on the last FF -- whose output is also the top-level -- pad. end USE_TSPEC;