Prev: what is incorrect about my usage of array with port entity?
Next: Unpredictable design
From: Weng Tianxiang on 24 Feb 2010 21:49
On Feb 24, 5:33 pm, rickman <gnu...(a)gmail.com> wrote:
> On Feb 22, 9:41 pm, Weng Tianxiang <wtx...(a)gmail.com> wrote:
>
>
>
>
>
> > On Feb 18, 1:13 pm, rickman <gnu...(a)gmail.com> wrote:
>
> > > On Feb 18, 11:10 am, Weng Tianxiang <wtx...(a)gmail.com> wrote:
>
> > > > On Feb 17, 7:51 pm, rickman <gnu...(a)gmail.com> wrote:
>
> > > > > On Feb 17, 8:05 pm, Weng Tianxiang <wtx...(a)gmail.com> wrote:
>
> > > > > > On Feb 17, 4:29 pm, rickman <gnu...(a)gmail.com> wrote:
> > > > > > > Fight fire with fire!  The two reports below show that both the
> > > > > > > missing else and the missing assignment (which is also missing in the
> > > > > > > missing else case) produce latches.
>
> > > > > > > @W: CL117 :"C:\arius\boards\tdc_upgrade\tests\latchsynthtest.vhd":
> > > > > > > 57:4:57:7|Latch generated from process for signal Latch, probably
> > > > > > > caused by a missing assignment in an if or case stmt
> > > > > > > @W: CL117 :"C:\arius\boards\tdc_upgrade\tests\latchsynthtest.vhd":
> > > > > > > 40:4:40:7|Latch generated from process for signal Comb, probably
> > > > > > > caused by a missing assignment in an if or case stmt
>
> > > > > > > library ieee;
> > > > > > > use ieee.std_logic_1164.all;
> > > > > > > use ieee.numeric_std.all;
>
> > > > > > > entity LatchSynthTest is
> > > > > > >   port(
> > > > > > >           CLK                   : in    std_logic ;
> > > > > > >           RESET                 : in    std_logic ;
> > > > > > >           C01                   : in    std_logic ;
> > > > > > >           C02                   : in    std_logic ;
> > > > > > >           LatchOutput   : out   std_logic ;
> > > > > > >           CombOutput    : out   std_logic
> > > > > > >           );
> > > > > > > end LatchSynthTest ;
>
> > > > > > > architecture behavior of LatchSynthTest is
> > > > > > >   SIGNAL Latch          : std_logic;
> > > > > > >   SIGNAL Comb           : std_logic;
> > > > > > >   SIGNAL LatchReg       : std_logic;
> > > > > > >   SIGNAL CombReg        : std_logic;
>
> > > > > > > begin
>
> > > > > > >   CombOutput    <= CombReg;
> > > > > > >   LatchOutput   <= LatchReg;
>
> > > > > > >   Process_1 : process(RESET, CLK)
> > > > > > >   begin
> > > > > > >     if (RESET = '1') then
> > > > > > >       LatchReg  <= '0';
> > > > > > >       CombReg   <= '0';
> > > > > > >     elsif (rising_edge(CLK)) then
> > > > > > >       LatchReg  <= Latch;
> > > > > > >       CombReg   <= Comb;
> > > > > > >     end if;
> > > > > > >   end process;
>
> > > > > > >   CombProc : process(CombReg, C01, C02)
> > > > > > >   begin
> > > > > > >     case CombReg is
> > > > > > >       when '0' =>
> > > > > > >         if (C01 = '1') then
> > > > > > >           Comb <= '0';
> > > > > > >         elsif (C02 = '1') then
> > > > > > >           Comb <= '1';
> > > > > > >         else
> > > > > > >           -- Here an assignment statement is missing, but it doesn't
> > > > > > >           -- generate latch.  It is treated as a null statement. -
> > > > > > > Weng
> > > > > > >         end if;
> > > > > > >       when others =>
> > > > > > >         Comb <= '1';
> > > > > > >     end case;
> > > > > > >   end process;
>
> > > > > > >   LatchProc : process(LatchReg, C01, C02)
> > > > > > >   begin
> > > > > > >     case LatchReg is
> > > > > > >       when '0' =>
> > > > > > >         if C01 = '1' then
> > > > > > >           Latch <= '0';
> > > > > > >         elsif C02 = '1' then
> > > > > > >           Latch <= '1';
> > > > > > >           -- Here the else is missing, and it does
> > > > > > >           -- generate latch.  It is treated as a null statement.
> > > > > > >         end if;
> > > > > > >       when others =>
> > > > > > >         Latch <= '1';
> > > > > > >     end case;
> > > > > > >   end process;
>
> > > > > > > end behavior;
>
> > > > > > Hi,
> > > > > > Thank you, Andy, Rick and everyone, I am wrong in the second point:
> > > > > > missing "else" or missing an assignment statement.
>
> > > > > > But my first point is how to generate a latch for a compiler. Rick,
> > > > > > can you see the floor plan to show how the latch is generated: for the
> > > > > > state only or for full states?
>
> > > > > > Weng
>
> > > > > I'm not clear what you mean by "how".  Are you asking about the detail
> > > > > of how it is implemented in the FPGA?  In the Lattice part they used a
> > > > > FF as a latch.  A register is between the latch and the output.  They
> > > > > drive the latch oddly driving both the clock and the async reset
> > > > > inputs with logic, but then if you look at the code what would you
> > > > > think is the clock?  I don't see why they did it the way they did, but
> > > > > it works correctly according to the VHDL.  With only four inputs I
> > > > > would expect they could have just used a single LUT4 and the latch
> > > > > with the clock always enabled.
>
> > > > > Din == '1'
> > > > > Latch Enable == CombReg + C02
> > > > > Async Clear  == ~CombReg * C01
>
> > > > > Is this what you are asking?
>
> > > > > Rick
>
> > > > Rick,
> > > > Yes, that is what I want.
>
> > > > Could you please send the code and a window screen frame using
> > > > Window's Paint so that I can see the full picture.
>
> > > > Thank you.
>
> > > > Weng
>
> > > I'm not clear on what you want.  I posted the full code a couple of
> > > posts back.  What is it that you want a screen shot of?  The text I
> > > quoted was from the Synthesis report.  If you want an image of the
> > > chip editor, the latch only shows in the logic block editor dialog
> > > box.  It is just a check box on a schematic of the functional elements
> > > in the logic block.  Is that of any value to you?
>
> > > Rick
>
> > Rick,
> > Thank you for your help.
>
> > This time I really understand what the Lattice does with your source
> > code in the previous poster.
>
> > Lattice generates a latch for the process of CombProc, paying no
> > attention to what is used.
>
> > And I think Lattice compiler does a very good job by generating the
> > following equations:
>
> > Din == '1'
> > Latch Enable == CombReg + C02
> > Async Clear  == ~CombReg * C01
>
> > "With only four inputs I
> > would expect they could have just used a single LUT4 and the latch
> > with the clock always enabled. "
>
> > No. What you suggest may not work. Or it may work, but is not as
> > simple as the Lattice equations show.
>
> > I would like to see how you would write a LUT4 equation for a latch.
>
> > Weng
>
> Actually, I'm not certain the code you show (that I got from the
> Lattice Logic Block Editor) is exactly the same as my VHDL
> description.  For them to match, the latch enable would have to have
> priority over the reset and that is not a very normal feature in a
> latch.
>
> case CombReg is
>   when '0' =>
>     if (C01 = '1') then
>       Comb <= '0';
>     elsif (C02 = '1') then
>       Comb <= '1';
>     else
>       -- Here an assignment statement is missing, but it doesn't
>       -- generate latch.  It is treated as a null statement. -Weng
>     end if;
>   when others =>
>     Comb <= '1';
> end case;
>
> Notice that once the latch is set to a '1' in the VHDL, there is no
> way to clear it.  When CombReg is a '1', the "others" clause of the
> case is executed which only allows it to be a '1'.  The async clear
> can only be asserted when CombReg is a '0'.  Of course, Comb and
> CombReg are not the same signals, so there is a window between the
> latch being set and the Register output going high where the latch can
> be reset by C01.
>
> There are only four inputs to this logic function "Comb".  A LUT4 can
> implement ***ANY*** logic function of 4 inputs.  So there certainly is
> a way to implement the above VHDL in a single LUT4.  In fact, you
> don't even need the latch.
>
> Comb <= CombReg or (C02 and not C01) or (Comb and not C01);
>
> If you want to use the built in Latch in the FPGA, then I guess you
> have to use a LUT4 to generate the enable and another to generate the
> data (or async clear).
>
> Enable <= CombReg or C01 or C02;
> DataIn <= CombReg or not C01;
>
> There is no savings by only using 2 of the 4 inputs on a LUT4 but
> there is some advantage to using the reset input to a Latch.  I think
> it may avoid potential race conditions when only one input switches.
> My logic will have some problems, for example CombReg = 0, C02 = 0 and
> C01 = 1.  Bring C01 low and it will either stay clear or set the latch
> depending on which of the two paths are faster.  Hmmm, maybe the tools
> aren't so stupid after all.  In essence, they are using the enable as
> a set and the async reset as a... well, a reset!
>
> Rick

Rick

Actually it is not the fault of Lattice, but of your VHDL code.

In your example,

case CombReg is
when '0' =>
if (C01 = '1') then
Comb <= '0';
elsif (C02 = '1') then
Comb <= '1';
else
-- Here an assignment statement is missing, but it doesn't
-- generate latch. It is treated as a null statement. -Weng
end if;
when others =>
Comb <= '1'; <-- lock here by VHDL, not by Lattice compiler
end case;

Process_1 : process(RESET, CLK)
begin
if (RESET = '1') then
LatchReg <= '0';
CombReg <= '0';
elsif (rising_edge(CLK)) then
LatchReg <= Latch;
CombReg <= Comb; <-- lock here by VHDL, not by Lattice
compiler
end if;
end process;

Weng
From: rickman on 25 Feb 2010 06:49
On Feb 24, 9:49 pm, Weng Tianxiang <wtx...(a)gmail.com> wrote:
>       CombReg   <= Comb; <-- lock here by VHDL, not by Lattice
>
> Weng

Who said anything about it be the fault of the tools? I said that it
appeared there was a difference, but then I realized that the async
reset must have priority over the latch enable because it can still be
reset once set up to the point that the value of Comb is captured by
CombReg. Then there is no longer a way to reset the latch.

This is the sort of stuff that makes working with latches difficult.
If the combinatorial logic had been described in the sequential
process, there would have been no possibility of generating a latch.

Rick
From: Andy on 25 Feb 2010 14:34
On Feb 25, 5:49 am, rickman <gnu...(a)gmail.com> wrote:
> If the combinatorial logic had been described in the sequential
> process, there would have been no possibility of generating a latch.

Amen!

Andy
From: Weng Tianxiang on 25 Feb 2010 20:01
On Feb 25, 11:34 am, Andy <jonesa...(a)comcast.net> wrote:
> On Feb 25, 5:49 am, rickman <gnu...(a)gmail.com> wrote:
>
> > If the combinatorial logic had been described in the sequential
> > process, there would have been no possibility of generating a latch.
>
> Amen!
>
> Andy

Rick,
Try this one and there is no lock problem any more in VHDL. See what
happens.

CombProc : process(CombReg, C01, C02)
begin
case CombReg is
when '1' => <-- only change, and not locked again
if (C01 = '1') then
Comb <= '0';
elsif (C02 = '1') then
Comb <= '1';
else
null;
end if;
when others =>
Comb <= '1';
end case;
end process;

Thank you.

Weng
First  |  Prev  | 
Pages: 1 2 3 4 5
Prev: what is incorrect about my usage of array with port entity?
Next: Unpredictable design