Answers Database
SYNPLIFY: How to change the output slew rate using the xc_fast attribute?
Record #2185
Product Family: Software
Product Line: Synplicity
Product Part: Synplify
Product Version: 5.0
Problem Title:
SYNPLIFY: How to change the output slew rate using the xc_fast attribute?
Problem Description:
Urgency: Standard
General Description:
How to change the output slew rate using the xc_fast attribute?
The slew rate of each output buffer is, by default, reduced, to
minimize power bus transients when switching non-critical signals.
For critical signals, attach the FAST property to output primitives,
output pads, bidirectional pads in the UCF file. Or, through Synplify,
apply the xc_fast in the HDL or the SDC file.
Use this attribute to decrease the transition time for the output driver
and increase the noise in the system. Synplify provides certain
attributes that get passed into the implementation netlist for Xilinx
place and route that affect the input setup times and output transition
times for your I/Os.
Solution 1:
-- VHDL
library IEEE;
use IEEE.std_logic_1164.all;
library synplify;
use synplify.attributes.all;
entity fast_ex is
port (CLK : in STD_LOGIC;
D_IN : in STD_LOGIC_VECTOR (3 downto 0);
Q_OUT : out STD_LOGIC_VECTOR (3 downto 0));
attribute xc_fast of Q_OUT : signal is true;
end fast_ex;
architecture XILINX of fast_ex is
begin
-- D flip-flop
process(CLK)
begin
if rising_edge(CLK) then
Q_OUT <= D_IN;
end if;
end process;
end XILINX;
Solution 2:
// Verilog
module fast_ex (CLK, D_IN, Q_OUT);
input CLK;
input [3:0] D_IN;
output [3:0] Q_OUT /* synthesis xc_fast=1 */;
reg [3:0] Q_OUT;
// D flip-flop
always @(posedge CLK)
Q_OUT <= D_IN;
endmodule
Solution 3:
# SDC
define_attribute <output_port_name> xc_fast 1
End of Record #2185 - Last Modified: 07/15/99 11:22 |
