Strange Timing Problem

[Kload]

Hi all,

I have an unusual timing problem that I hope someone can help with.
I've constrained the delay between flip flops within one of my VHDL
macros (called COLFILT) to bo 20ns. This constraint is violated which
in itself is not strange. The odd thing is that the paths violating the
constraint all seem pass through another VHDL macro (called ROBAVG -
variable SReg) which is in the same data path as COLFILT but the two
have no direct connections.

(Note: INT_ColumnFilter is a TNM that defines all flipflops in the
COLFILT VHDL macro)

Can anyone clear this up for me? What is a reference to ROBAVG doing in
a timing path that should only include elements from COLFILT? I've
attached an example from the timing analyser and associated code for
your reference.

Thanks in advance.

================================================================================
Timing constraint: TSColFilt = MAXDELAY FROM TIMEGRP "INT_ColumnFilter"
TO TIMEGRP "INT_ColumnFilter" 20 nS ;
40105 items analyzed, 14 timing errors detected.
Maximum delay is 25.241ns.
--------------------------------------------------------------------------------
Slack: -5.241ns path COLFILT/FiltS<8> to COLFILT/FiltS<5> relative to
20.000ns delay constraint

Path COLFILT/Filt<8> to COLFILT/FiltS<5> contains 11 levels of logic:
Path starting from Comp: CLB_R26C47.S1.CLK (from CLK180)
To Delay type Delay(ns) Physical Resource
Logical Resource(s)
------------------------------------------------- --------
CLB_R26C47.S1.XQ Tcko 1.372R COLFILT/FiltS<8>
COLFILT/FiltS_reg<8>
CLB_R19C31.S1.F3 net (fanout=13) 3.545R COLFILT/FiltS<8>
CLB_R19C31.S1.X Tilo 0.738R ROBAVG/Sreg0<5>
COLFILT/C1593
CLB_R22C46.S0.G2 net (fanout=1) 2.903R COLFILT/syn7069
CLB_R22C46.S0.Y Tilo 0.738R COLFILT/N3216
COLFILT/C1585
CLB_R25C47.S0.G3 net (fanout=10) 2.337R COLFILT/C912
CLB_R25C47.S0.Y Tilo 0.738R COLFILT/C839/N27
COLFILT/C1564
CLB_R24C46.S1.F1 net (fanout=1) 1.341R COLFILT/C839/N63
CLB_R24C46.S1.COUT Topcyf 1.445R COLFILT/HPO<4>
COLFILT/C824/C6/C2
COLFILT/C824/C6/C1
COLFILT/C824/C7/C1
CLB_R23C46.S1.CIN net (fanout=1) 0.000R COLFILT/C824/C7/C1/O
CLB_R23C46.S1.COUT Tbyp 0.109R COLFILT/N2915
COLFILT/C824/C8/C1
COLFILT/C824/C9/C1
CLB_R22C46.S1.CIN net (fanout=1) 0.000R COLFILT/C824/C9/C1/O
CLB_R22C46.S1.Y Tciny 0.590R COLFILT/N2917
COLFILT/C824/C10/C1
COLFILT/C824/C11/C0
CLB_R24C47.S0.F4 net (fanout=1) 1.474R COLFILT/N2918
CLB_R24C47.S0.X Tilo 0.738R COLFILT/syn7390
COLFILT/C1414
CLB_R27C47.S1.F2 net (fanout=1) 1.444R COLFILT/syn7390
CLB_R27C47.S1.X Tilo 0.738R COLFILT/syn1816
COLFILT/C1411
CLB_R27C47.S0.F1 net (fanout=1) 0.404R COLFILT/syn1816
CLB_R27C47.S0.X Tilo 0.738R COLFILT/C56/N51
COLFILT/C1407
CLB_R35C56.S0.CE net (fanout=7) 2.901R COLFILT/C56/N51
CLB_R35C56.S0.CLK Tceck 0.948R COLFILT/FiltS<5>
COLFILT/FiltS_reg<4>
-------------------------------------------------
Total (8.892ns logic, 16.349ns route) 25.241ns (to CLK180)
(35.2% logic, 64.8% route)




--
-- IXAcc/ITAcc are outputs to an external accumulator
-- IXSum/ITSum are input from the external accumulators
--
library IEEE;
use IEEE.std_logic_1164.all;

use IEEE.std_logic_arith.all;
use IEEE.std_logic_unsigned.all;

library SYNOPSYS;
use SYNOPSYS.attributes.all;

entity colfilt is
port (clk: in STD_LOGIC;
DataBa: in INTEGER range -32768 to 32767;
DataBb: in INTEGER range -32768 to 32767;
DataInReady: in STD_LOGIC;
Height: in INTEGER range 0 to 31;
It: in INTEGER range -256 to 255;
ITSum: in INTEGER range -32768 to 32767;
Ix: in INTEGER range -256 to 255;
IXSum: in INTEGER range -32768 to 32767;
localDiff: in INTEGER range -256 to 255;
reset: in STD_LOGIC;
Width: in INTEGER range 0 to 511;
AccEn: out STD_LOGIC;
AddrA: out INTEGER range 0 to 255;
AddrB: out INTEGER range 0 to 255;
DataAa: out INTEGER range -32768 to 32767;
DataAb: out INTEGER range -32768 to 32767;
DataOutReady: out STD_LOGIC;
ENA: out STD_LOGIC;
ENB: out STD_LOGIC;
ITAcc: out INTEGER range -32768 to 32767;
ItS: out INTEGER range -2048 to 2047;
IXAcc: out INTEGER range -32768 to 32767;
IxS: out INTEGER range -2048 to 2047;
LoadAcc: out STD_LOGIC;
stopData: out STD_LOGIC);
end;

architecture colfilt_arch of colfilt is

--diagram signal declarations
signal BufRead: INTEGER range 0 to 255;
signal BufWrite: INTEGER range 0 to 255;

-- SYMBOLIC ENCODED state machine: FiltSide
type FiltSide_type is (FS1, FS2, FS3, FS4, FS5, FS6, FS7, FS8, FS9, Init2);
signal FiltSide: FiltSide_type;

-- SYMBOLIC ENCODED state machine: InputSide
type InputSide_type is (Init1, IS1, IS2);
signal InputSide: InputSide_type;

begin
--concurrent signal assignments


FiltSide_machine: process (clk)
--machine variables declarations
variable col: INTEGER range 0 to 511;
variable HPO: INTEGER range 1 to 32;
variable row: INTEGER range 0 to 31;

begin

if clk'event and clk = '1' then
if reset='1' then
FiltSide <= Init2;
HPO:=Height+1;
AddrB<=0;
ENB<='0';
BufRead<=0;
DataOutReady<='0';
row:=0;
col:=0;
AccEn<='0';
LoadAcc<='1';
else
case FiltSide is
when FS1 =>
LoadAcc<='0';
if (localDiff>=2*HPO) or (col=Width) or (col=Width-1) then
FiltSide <= FS2;
end if;
when FS2 =>
AddrB<=BufRead;
ENB<='1';
FiltSide <= FS3;
when FS3 =>
IXAcc<=4*DataBa;
ITAcc<=4*DataBb;
AccEn<='1';
AddrB<=BufRead+1;
FiltSide <= FS4;
when FS4 =>
IXAcc<=DataBa;
ITAcc<=DataBb;
AddrB<=BufRead-1;
FiltSide <= FS5;
when FS5 =>
IXAcc<=DataBa;
ITAcc<=DataBb;
AddrB<=BufRead+HPO;
FiltSide <= FS6;
when FS6 =>
IXAcc<=DataBa;
ITAcc<=DataBb;
AddrB<=BufRead-HPO;
FiltSide <= FS7;
when FS7 =>
IXAcc<=DataBa;
ITAcc<=DataBb;
ENB<='0';
FiltSide <= FS8;
when FS8 =>
IxS<=IXSum;
ItS<=ITSum;
AccEN<='0';
DataOutReady<='1';
BufRead<=BufRead+1;
FiltSide <= FS9;
when FS9 =>
DataOutReady<='0';
LoadAcc<='1';
if row=Height and col=Width then
FiltSide <= FS1;
row:=0;
col:=0;
elsif row/=Height then
FiltSide <= FS1;
row:=row+1;
elsif row=Height then
FiltSide <= FS1;
row:=0;
col:=col+1;
end if;
when Init2 =>
FiltSide <= FS1;
when others =>
null;
end case;
end if;
end if;
end process;



InputSide_machine: process (clk)
--machine variables declarations
variable CBW: INTEGER range 0 to 255;

begin

if clk'event and clk = '1' then
if reset='1' then
InputSide <= Init1;
DataAa<=0;
DataAb<=0;
AddrA<=0;
ENA<='0';
BufWrite<=0;
CBW:=1;
stopData<='0';
else
case InputSide is
when Init1 =>
InputSide <= IS1;
when IS1 =>
if CBW=BufRead then
InputSide <= IS1;
stopData<='1';
elsif DataInReady='1' and CBW/=BufRead then
InputSide <= IS2;
DataAa<=Ix;
DataAb<=It;
AddrA<=BufWrite;
ENA<='1';
stopData<='0';
end if;
when IS2 =>
ENA<='0';
if DataInReady='0' then
InputSide <= IS1;
BufWrite<=BufWrite+1;
CBW:=CBW+1;
end if;
when others =>
null;
end case;
end if;
end if;
end process;

end colfilt_arch;

 

[Allan Herriman]

Hi all,

I have an unusual timing problem that I hope someone can help with.
I've constrained the delay between flip flops within one of my VHDL
macros (called COLFILT) to bo 20ns. This constraint is violated which
in itself is not strange. The odd thing is that the paths violating the
constraint all seem pass through another VHDL macro (called ROBAVG -
variable SReg) which is in the same data path as COLFILT but the two
have no direct connections.

(Note: INT_ColumnFilter is a TNM that defines all flipflops in the
COLFILT VHDL macro)

Can anyone clear this up for me? What is a reference to ROBAVG doing in
a timing path that should only include elements from COLFILT? I've
attached an example from the timing analyser and associated code for
your reference.

Each CLB contains eight flip flops and LUTs. A given CLB may contain
logic from separate parts of your design (e.g ROBAVG and COLFILT).

The CLB "name" is taken from one of the flip flops or LUTs it
contains. Thus your "COLFILT" timing paths may seem to be passing
through "ROBAVG", even though there is no actual relationship between
them (except that they are sharing a CLB).

BTW, use fpga_editor in future.

Regards,
Allan.

 

[Kload]

Thanks,

Yes you are right, I had a look in FPGA editor, and the two macros are
sharing a CLB. This leads me to another question. Is there any way to
stop the timing tools reporting on such path?? Or can I stop the place
and route tools sharing CLBs between macros.

Thanks again, you help has been invaluable.