Is it possible to make the code faster via pipelining?

You need a major redesign if you want to achieve high speed.

hi kaz, thank you so much for the comment! appreciate it...you mentioned that i have to balance out pipeline? sorry but i don't quite understand how i can do so? i'll redesigned the long combinational path part as you mentioned.

First the counter need be constrained otherwise it defaults to some 40 or so bits (integer limit) and wouldn't come back to zero until it goes that far and that is not what you want.

Hi kaz! thank you for your prompt reply! and by constraining the counter as you suggested, the throughput is now 14ns! improved by 1 ns!you're a genius! hahaha:) as for the delay balance...does quartus support this? how can i balance the delay when i don't know the delay of the components? does quartus support "after 3ns" command? anyway i can implementing the delay balancing via coding that you know of? thanx for suggesting the use of signals...i'm looking into it but thanx again for all the advice!

I don't expect counter restrain to improve speed much as the main bottleneck is your long comb path.

hi again kaz, thanx for your previous reply...i tried to pipeline the bottleneck using the following code, but it doesn't give me the correct results even after i perform pipeline balancing:

Library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
entity TEA_en is 
port(
    clock: in std_logic;                          --clock input
    input_data: in std_logic_vector (63 downto 0);    --input data
    key : in std_logic_vector (127 downto 0);    --secret key 127 downto 0--
    encrypted_data: out std_logic_vector (63 downto 0)    --output/encrypted data
);
end entity TEA_en;
architecture behave of TEA_en is
--declare signals
signal Key0, Key1, Key2, Key3 : std_logic_vector (31 downto 0);
signal Z, Y : std_logic_vector (31 downto 0);
signal count :integer range 0 to 32:=0;            --constrained counter (counts from 0 to 32) improves throughput by 1 ns!
signal pipeline_0,pipeline_1,pipeline_2,pipeline_3,pipeline_4,pipeline_5,pipeline_6,pipeline_7:std_logic_vector(31 downto 0);
begin
--separate key into four parts
Key0<=key(127 downto 96);
Key1<=key(95 downto 64);
Key2<=key(63 downto 32);
Key3<=key(31 downto 0);
Process(Input_data, clock)
--declare and initialize variable
    Variable delta: std_logic_vector (31 downto 0):=x"9e3779b9";
    Variable sum: std_logic_vector (31 downto 0):=x"00000000";
    Variable Zeq,Yeq,Z,Y: std_logic_vector (31 downto 0);
    Begin
        If(rising_edge(clock)) then
            If (count<1) then            --separate input data into two parts
            Z:=input_data(63 downto 32);    --part 1 (32bits)
            Y:=input_data(31 downto 0);        --part 2 (32bits)
            Else --null;
            End if;
            If (count<32) then
            --Encryption routine algorithms
            sum:=sum+delta;
            
            for i in 1 to 8 loop
                case i is
                when 1=>        pipeline_0    <=( (Z(27 downto 0) & "0000")+Key0);        
                when 2=>        pipeline_1    <=(Z+ sum)    ;
                when 3=>        pipeline_2    <=(("00000" &  Z(31 downto 5))+Key1);
                when 4=>        pipeline_3    <=( (Y(27 downto 0) & "0000")+Key2)    ;
                when 5=>        pipeline_4    <=(Y+ sum)    ;
                when 6=>        pipeline_5    <=(("00000" &  Y(31 downto 5))+Key3)    ;    
                when 7=>        pipeline_6    <=pipeline_0 xor pipeline_1 xor pipeline_2;
                when 8=>        pipeline_7    <=pipeline_3 xor pipeline_4 xor pipeline_5;
                when others=>     null;
                end case;                        
            end loop;
        
            
            
            Y:=Y+pipeline_6;
            Z:=Z+pipeline_7;
            --Output encrypted data
            Encrypted_data<=Y&Z;
            else null;
            end if;
        count<=count+1;    --increase value of count     
        End if;
        
    End process;
end architecture behave;

This is where a good testbench and time to sitdown debugging your code will help.

Indeed as Tricky said you need time and testing to finalise.