Combinational feedback loops

J

john

Guest
Hello, I am getting 47 cominational feedback loops for the section of
my code that I am attaching with this message. I am using QuartusII.
Would anyone advice me that how to get rid of these combinational feed
back loops. I have Else for every IF/Else construct. Please advice!

Thanks
John

Process (DPR_CLK, sel_14bit_mux )
Begin

If (DPR_CLK 'event And DPR_CLK='1') Then

If (sel_14bit_mux = "11") Then
-- Frame Counter --
Address_bus( 5 ) <= Frame_cntr(0);
Address_bus( 6 ) <= Frame_cntr(1);
Address_bus( 7 ) <= Frame_cntr(2);
Address_bus( 8 ) <= Frame_cntr(3);
Address_bus( 9 ) <= Frame_cntr(4);
Address_bus( 10 ) <= Frame_cntr(5);
Address_bus( 11 ) <= Frame_cntr(6);
Address_bus( 12 ) <= Frame_cntr(7);
Address_bus( 13 ) <= Frame_cntr(8);
Address_bus( 14 ) <= Frame_cntr(9); Address_bus( 15
) <= Frame_cntr(10);
Address_bus( 16 ) <= Frame_cntr(11);
Address_bus( 17 ) <= Frame_cntr(12);
Address_bus( 18 ) <= Frame_cntr(13);

-- Channel Counter --
Address_bus( 4 downto 0 ) <= countb_mux_datain;

Else If ( sel_14bit_mux="00")Then
Address_bus( 18 downto 5 ) <= TEMP;

-- Sample Counter --
Address_bus( 4 downto 0 ) <= countc_mux_datain;

Else If ( sel_14bit_mux = "01") Then

Address_bus( 5 ) <= '1';
Address_bus( 6 ) <= '0';
Address_bus( 7 ) <= '0';
Address_bus( 8 ) <= '0';
Address_bus( 9 ) <= '0';
Address_bus( 10 ) <= '0';
Address_bus( 11 ) <= '0';
Address_bus( 12 ) <= '0';
Address_bus( 13 ) <= '0';
Address_bus( 14 ) <= '0';
Address_bus( 15 ) <= '0';
Address_bus( 16 ) <= '0';
Address_bus( 17 ) <= '0';
Address_bus( 18 ) <= '0';
Address_bus( 4 downto 0 ) <= countb_mux_datain;


End If;
End If;
End If;
End If;
If (DPR_CLK 'event And DPR_CLK='1')Then

If( latch_enable ='1') Then

TEMP <= Data_Bus;
Else
End If;
End If;

If (DPR_CLK 'event And DPR_CLK='1')Then

If( latch_CPLD ='1') Then

Final_Temp <= Data_Bus;
Else

End If;

End If;

If (DPR_CLK 'event And DPR_CLK='1')Then

If( P2S_load ='1') Then

Ser_buff(18) <= Final_Temp(0);
Ser_buff(17) <= Final_Temp(1);
Ser_buff(16) <= Final_Temp(2);
Ser_buff(15) <= Final_Temp(3);
Ser_buff(14) <= Final_Temp(4);
Ser_buff(13) <= Final_Temp(5);
Ser_buff(12) <= Final_Temp(6);
Ser_buff(11) <= Final_Temp(7);
Ser_buff(10) <= Final_Temp(8);
Ser_buff(9) <= Final_Temp(9);
Ser_buff(8) <= Final_Temp(10);
Ser_buff(7) <= Final_Temp(11);
Ser_buff(6) <= Final_Temp(12);
Ser_buff(5) <= Final_Temp(13);
Ser_buff(4) <= countb_mux_datain(0) ;
--(LSB....MSB)
Ser_buff(3) <= countb_mux_datain(1) ;
Ser_buff(2) <= countb_mux_datain(2) ;
Ser_buff(1) <= countb_mux_datain(3) ;
Ser_buff(0) <= countb_mux_datain(4) ;
Else
End If;

End If;
-- Reading the counter number (New Addition) --

If (DPR_CLK 'event And DPR_CLK='1') Then
If (Addr_active='1') Then
Address_bus <= "0000000000000000001";
Else
End If;
End If;
 
john wrote:
Hello, I am getting 47 cominational feedback loops for the section of
my code that I am attaching with this message.

Process (DPR_CLK, sel_14bit_mux )
xxxxxxxxxxxxx delete this
Click to expand...

If (DPR_CLK 'event And DPR_CLK='1') Then

If (DPR_CLK 'event And DPR_CLK='1')Then

If (DPR_CLK 'event And DPR_CLK='1')Then

If (DPR_CLK 'event And DPR_CLK='1')Then

If (DPR_CLK 'event And DPR_CLK='1') Then
Click to expand...
You only get one of these per process.

-- Mike Treseler
 
The process you're using is a synchronous one. There should then only
be the clk (and an asynchronous reset if it were existing) in the
sensitivity list. Remove the signal sel_14bit_mux from the sensitivity
list :

Process (DPR_CLK)
Begin
....
end process;

I also advise you to use case statement instead of the if, elsif in the
first if. That would be easier to read :

If (DPR_CLK 'event And DPR_CLK='1') Then

case sel_14bit_mux is
when "11" =>
-- Frame Counter --
...
when "00" =>
...
when "01" =>
..
when others =>

end case;

end if;

Regards,

Arnaud
 
Hi ,

I did change it that there will be one per process. and I am also
keeping the DPR_CLK and Reset in the sensitivity list but the compiler
is giving me the same combinational feedback warnings. If for example,
I change

If( latch_CPLD ='1') Then
Final_Temp <= Data_Bus;
Else
Final_Temp <= others ( => '0' );
End If;
End If;

Then I can get rid of 10 combinational loops out of 42. But I do not
want the Final_Temp assign it to zero because of my design. The code I
mentioned previously is controlled by a state machine.
Please advice!
John
 
john wrote:

I did change it that there will be one per process. and I am also
keeping the DPR_CLK and Reset in the sensitivity list but the compiler
is giving me the same combinational feedback warnings.
Click to expand...
If you are getting combinational feedback, some
assignments have slipped outside of the
"elsif rising_edge(clk)" clause.

Please advice!
Click to expand...
Please run a sim.

-- Mike Treseler
 
The other thing is that my vhdl program is not exactly doing what I
want it to do. For example its not working at 20MHz but working ok for
frequencies less than 15 MHZ. I was wondering that can combinational
loops and ripple clock can effect the functionality of the VHDL code.
Thanks
john
 
The other thing is that my vhdl program is not exactly doing what I
want it to do. For example its not working at 20MHz but working ok for
frequencies less than 15 MHZ. I was wondering that can combinational
loops and ripple clock can effect the functionality of the VHDL code.
Thanks
john
 
The other thing is that my vhdl program is not exactly doing what I
want it to do. For example its not working at 20MHz but working ok for
frequencies less than 15 MHZ. I was wondering that can combinational
loops and ripple clock can effect the functionality of the VHDL code.
Thanks
john
 
Hello,

I am running sim but i need the solution. what should I do to remove
the problem. can combinational feedback loops introduced the problem
like forexample my VHDL code works for clock frequencies less than
15Mhz but does not work for frequencies greater that 15MHz,. Please
advice!

John
 
john wrote:
Hello,

I am running sim but i need the solution. what should I do to remove
the problem. can combinational feedback loops introduced the problem
like forexample my VHDL code works for clock frequencies less than
15Mhz but does not work for frequencies greater that 15MHz,. Please
advice!

John
Click to expand...
I don't know what the "solution" is. You may want to simplify the code
as much as possible, to try to find your "loops". I've also noticed
that you have more "end if" statements that "if" statements. I don't
know if that has anything to do with your problem or not. I find that
good formatting of the source code helps me avoid mistakes like that.
I also like schematics. Sometimes I'll do something really dumb, and a
look at the RTL schematic will quickly show me what I've done wrong,
especially if running a simulation leaves me confused.
HTH
-Dave Pollum
 
Hi Mike,

I did not understand completely. Did you mean that there will be only
one

"If (DPR_CLK 'event And DPR_CLK='1') Then" for the whole process?

Thanks
Regards
John
 
john wrote:

I did not understand completely. Did you mean that there will be only
one

"If (DPR_CLK 'event And DPR_CLK='1') Then" for the whole process?
Click to expand...

For synthesis, yes.

-- Mike Treseler
 
john wrote:

What about the final code for the CPLD?
Click to expand...
That sounds like synthesis to me.

You can use as many processes as you like,
but each should look something like this:
--
begin
if reset = '1' then
init_regs;
elsif rising_edge(clock) then
update_regs;
end if;
update_ports;
end process sync_template;

-- Mike Treseler
 
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