Problem with Clock signals generated by combinational logic

G

GuitarNerd

Guest
Hi guys/gals, im building a network interface in vhdl and trying to
synthesize it to a XILINX Spartan-3 but im getting error messages on
timing which looks like this:

These 38 clock signal(s) are generated by combinatorial logic,
and XST is not able to identify which are the primary clock signals.
Please use the CLOCK_SIGNAL constraint to specify the clock signal(s)
generated by combinatorial logic.

Does anyone know how to set the Clock_signal constraint? A collegue
suggested that maybe the problem is that alot of my design uses Mealy
state machines which might work better in a Moore design.

Any help is appreciated Thanks in advance

Eric
 
Mealy or Moore it dosent matter, but why do you have 38 clock type
signals in your design?
cant say much just from the error message? atleast put the part of code
thats being pointed out.
 
its a large construction with fifos and several components etc ... and
alot of handshacking, all these handshacking signals are confusing the
synthesizer from knowing which is the clk signal. Ive found the page
on XILINX which says how to use the constraints, i will try it later on
today. Thanks for your reply.

GuitarNerd
 
Ok im trying to use these attribute specifications but im not using
them right since it complaining.

attribute clock_signal : string ;
attribute clock_signal of Clk : signal is "?????";

Clk is the name of my clock signal......im not sure what value it
should be passed

GuitarNerd
 
ok it synthesises now but no difference in the warnings

this is what i added into the architecture:

attribute clock_signal : string ;
attribute clock_signal of Clk : signal is "yes";
attribute clock_signal of c_reset : signal is "no";
attribute clock_signal of rodata_read : signal is "no";
attribute clock_signal of mux1_wr_en : signal is "no";
attribute clock_signal of mux1_out : signal is "no";

GuitarNerd
 
GuitarNerd wrote:
Hi guys/gals, im building a network interface in vhdl and trying to
synthesize it to a XILINX Spartan-3 but im getting error messages on
timing which looks like this:

These 38 clock signal(s) are generated by combinatorial logic,
and XST is not able to identify which are the primary clock signals.
Please use the CLOCK_SIGNAL constraint to specify the clock signal(s)
generated by combinatorial logic.

Does anyone know how to set the Clock_signal constraint? A collegue
suggested that maybe the problem is that alot of my design uses Mealy
state machines which might work better in a Moore design.
Click to expand...
Forget the clock constraint.

Sounds like your design doesn't have a synchronous process! You should
post a relevant code snippet, but my guess is your state machine
doesn't have proper synchronous state-register update logic.

-a
 
ok heres is one of the components. It just takes an input from one of
two inputs and places it into a fifo. I get the following warnings when
synthesising it:

rnitop_rni_downstream_Mux1__n0030(rnitop_rni_downstream_Mux1__n0030:O)|
NONE(*)(rnitop_rni_downstream_Mux1_muwr_ack)| 1 |
rnitop_rni_downstream_Mux1__n0031(rnitop_rni_downstream_Mux1__n0031:O)|
NONE(*)(rnitop_rni_downstream_Mux1_vcwr_ack)| 1 |
rnitop_rni_downstream_Mux1_State_FFd5:Q| NONE | 1
|
rnitop_rni_downstream_Mux1__n0032(rnitop_rni_downstream_Mux1__n0032:O)|
NONE(*)(rnitop_rni_downstream_Mux1_fdata_106)| 121 |

Code:

library ieee;
use ieee.std_logic_1164.all;

entity mux1_downstream is

port (
Clk,Reset : in std_logic;
--to vc_downstream
vcdata : in std_logic_vector(127 downto 0);
vcwr_ack : out std_logic;
vcwr_en : in std_logic;
--to FIFO2_downstream
fdata : out std_logic_vector(127 downto 0);
fwr_en : out std_logic;
fwr_ack : in std_logic;
--to mux1_uppstream
mudata : in std_logic_vector(127 downto 0);
muwr_ack : out std_logic;
muwr_en : in std_logic);

end mux1_downstream;

architecture Arch_mux1_downstream of mux1_downstream is

type State_type is (SInit,S0,S1,S1_1,S2,S2_1,S3);
signal State : State_type := SInit;
signal Next_state : State_type := SInit;

begin

Clock: process (Clk,Reset, Next_state)

begin

if Reset = '0' then
State <= SInit;
elsif Clk = '1' and Clk'event then
State <= Next_state;
end if;

end process;


transition: process(State, vcwr_en, vcdata, mudata, muwr_en, fwr_ack)
begin
case State is

when SInit =>

fdata <= (others => '0');
fwr_en <= '0';
vcwr_ack <= '0';
muwr_ack <= '0';
Next_state <= S0;


--------------------------------------------------------------------------------------------------------


when S0 =>

if muwr_en = '1' then
fdata <= mudata;
Next_state <= S1;
elsif vcwr_en = '1' then
fdata <= vcdata;
Next_state <= S2;
else
Next_state <= S0;
end if;


--------------------------------------------------------------------------------------------------------
--for mux
when S1 =>

fwr_en <= '1';
Next_state <= S1_1;

when S1_1 =>

if fwr_ack = '1' then
fwr_en <= '0';
muwr_ack <= '1';
Next_state <= SInit;
else
Next_state <= S1_1;
end if;


-------------------------------------------------------------------------------------------------------
--for vc
when S2 =>

fwr_en <= '1';
Next_state <= S2_1;

when S2_1 =>

if fwr_ack = '1' then
fwr_en <= '0';
vcwr_ack <= '1';
Next_state <= SInit;
else
Next_state <= S2_1;
end if;


--------------------------------------------------------------------------------------------------------



when others =>
fdata <= (others => '0');
fwr_en <= '0';
vcwr_ack <= '0';
muwr_ack <= '0';
end case;
end process;
end Arch_mux1_downstream;
 
One problem I see in this is that you are not assigning to the outputs
in the else clauses inside the transition process and also you need to
have assignments to all outputs in each state.
 
Neo wrote:
One problem I see in this is that you are not assigning to the
outputs
in the else clauses inside the transition process and also you need
to
have assignments to all outputs in each state.
Click to expand...
This is the best argument in favor of single-(clocked)process state
machines.

-a
 
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