AR# 11658


5.1i XST - How do I put multiple registered outputs in 3-state conditions into the IOBs?


Keywords: XST, IOB, 3-state, 3state, tristate, tri, state, tri-state, enable, keep, merge, duplicate, register, multiple, bus

Urgency: Standard

General Description:
I am using a bus of flip-flops that must be placed in the IOBs of a device. The outputs of these FFs should placed into 3-state conditions, and the enable signal of the 3-state buffer should be registered.

If one bus is used for the registered enable-signal, all but one of the enable-FFs are optimized out. As the output-load of the last FF is now not 1, it cannot be put into the IOBs. What can I do about this?


The solution is not to create a bus of FFs for the enable signal, but rather to create one flip-flop and place the IOB attribute on it. Then, activate the switch to put the registers/latches into the IOBs in the synthesis properties -- this will duplicate the single FF and put it into the IOBs.

To enable the synthesis properties to automatically push flip-flops into the IOB (this is the default behavior):

1. Highlight the Synthesize process.
2. Right-click on the Synthesize process and select "Properties".
3. Select the Xilinx-Specific Options tab.
4. Change the "Pack I/O Registers into IOBs" field to either "Auto" (the default setting) or "Yes".

The following code illustrates an 8-bit-wide bus for which the outputs are put into 3-state conditions by registers.

NOTE: The enable line of the 3-state buffer should be active Low.


library ieee;
use ieee.std_logic_1164.all;

entity tri_iob is
port(CLK, SEL : in std_logic;
DI : in std_logic_vector (7 downto 0);
DO : out std_logic_vector (7 downto 0));
end tri_iob;

architecture arch_tri_iob of tri_iob is
signal tmp1: std_logic_vector(7 downto 0);
signal tmp2: std_logic;
signal inp_ffs: std_logic_vector(7 downto 0);

attribute IOB : string;
attribute IOB of tmp2 : signal is "true";

process (CLK)
if (CLK'event and CLK='1') then
end if;
end process;

process (CLK)
if (CLK'event and CLK='1') then
end if;
end process;

DO <= tmp1 when tmp2='0' else (others=>'Z');

end arch_tri_iob;


module tri_iob (clk, sel, di, do);

input clk, sel;
input [7:0] di;
output [7:0] do;

reg [7:0] tmp1, inp_ffs;
reg tmp2;

//synthesis attribute IOB tmp2 "true"

always @(posedge clk) begin
inp_ffs <= di;
tmp1 <= inp_ffs;

always @(posedge clk) tmp2 <= sel;

assign do = tmp2 ? 8'bz : tmp1;

AR# 11658
Date 04/20/2007
Status Archive
Type General Article
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