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VHDL and Latch

 
 
KJ
Guest
Posts: n/a
 
      03-07-2007

"Weng Tianxiang" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ups.com...
>
> Hi Kevin,
> In my first posting, there are 3 questions:
> 1. Why doesn't compiler generate an error for unintended latch
> generation?

Simply put because it is not an 'error' to write code that infers a latch.
To presume that every latch is 'unintended' would be wrong.

> This question is satisfactorily answered: there is a new attribute to
> give errors if an unintended latch is to generate. It says that some
> other people had already observed the situation long time ago.
>

If you choose to use that attribute and find a tool that supports it then by
all means use it. But keep in mind that if you forget to use the new
attribute (because people are human) that you can still end up in the same
situation. It's up to you if that's the coding style you prefer to use to
avoid unintended latches, personally I prefer other coding styles but the
attribute is at least an improvement and gives another method which is good.

No matter which of these two methods you use, you can still get an
'unintended latch' with the following type of code which infers a latch

Q <= (C and D) or (not(C) and Q);

This is obviously a very stripped down example, a 'real' design might be
much more complicated, but you have to train yourself to look at every
concurrent statement and make sure that you don't see the output on both
sides of the equation.

It gets worse though because the 'latch' could be a function of multiple
concurrent statements that happen to loop back. The synthesizer report that
it finds a combinatorial loop works to catch that but nothing in the
language prevents it (nor should it, since it may not be 'unintended').

> 2. If the above error is given an error, VHDL may include a special
> statement to generate a latch.

If you want a 'special' statement that creates a latch, then write one and
instantiate it until your heart is content....but don't presume that
everyone else agrees with you and that the language itself should only
support your coding style for creating a latch and any other method should
cause an error.

> This is another big problem: I don't know how ASIC people generate a
> real latch using VHDL? I think they may most likely use latch library
> to generate special latch instead of using VHDL statements.

That doesn't preclude them from inferring a latch in other ways.

> I read an
> article about asynchronous FIFO written by two engineers one of whom
> is Pete Alfke of Xilinx (it is the best article I have read in my
> life). In the paper they say that they fail to generate two or 3
> latches in their design using Xilinx chip.

No idea what you mean by this...but reading on

> If so, it seems to me that
> there is no reliable statement in VHDL to generate a latch for a FPGA
> chip.

Unless the underlying hardware has a hard latch available as a resource to
use and most (if not all) FPGAs do not, then I would agree that there is no
reliable way to generate a latch inside an FPGA in any language, not just
VHDL. The lack of latches in FPGAs though has nothing at all to do with
VHDL or any language it is simply recognition by the FPGA suppliers that
there is very little demand for a hard latch inside the FPGA. A flip flop
would have the exact same coding issues, the only reason it's not a
'problem' is because there are hard flip flops inside the FPGA.

The 'problem' with inferring any storage element (latch, flip flop, big
hunk-o-memory, fifo, etc.) is when the underlying hardware doesn't directly
support the inferred element and it needs to be cobbled together from basic
logic blocks. In the FPGA/CPLD world, it is very difficult to meet internal
setup, hold time and 'no glitching' requirements between two arbitrary
signals.

Remember also, that you can build ANY logical element strictly from NAND
gates or NOR gates....so in theory you could write your code using only
these two building blocks. Were you to do so and target an FPGA, it would
most certainly fail if that design had any sort of storage.

>
> 3. If the example would generate a latch for signalA, how it is
> generated?

Same as any other logical description....it gets mapped to whatever internal
resources exist in the device (LUTs, macrocells, flip flops, latches,
whatever the target device supports).

>
> KJ answered the question. If the equation KJ suggested is true, it
> would like the following:
> if (state = stateA_S and a = "000001") then
> signalA <= '1';
> end if;
>
> Finally I realized KJ saying is correct.
>

You're welcome.

Kevin Jennings


 
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Weng Tianxiang
Guest
Posts: n/a
 
      03-07-2007
On Mar 6, 5:37 pm, "KJ" <(E-Mail Removed)> wrote:
> "Weng Tianxiang" <(E-Mail Removed)> wrote in message
>
> news:(E-Mail Removed) ups.com...
>
> > Hi Kevin,
> > In my first posting, there are 3 questions:
> > 1. Why doesn't compiler generate an error for unintended latch
> > generation?

>
> Simply put because it is not an 'error' to write code that infers a latch.
> To presume that every latch is 'unintended' would be wrong.
>
> > This question is satisfactorily answered: there is a new attribute to
> > give errors if an unintended latch is to generate. It says that some
> > other people had already observed the situation long time ago.

>
> If you choose to use that attribute and find a tool that supports it then by
> all means use it. But keep in mind that if you forget to use the new
> attribute (because people are human) that you can still end up in the same
> situation. It's up to you if that's the coding style you prefer to use to
> avoid unintended latches, personally I prefer other coding styles but the
> attribute is at least an improvement and gives another method which is good.
>
> No matter which of these two methods you use, you can still get an
> 'unintended latch' with the following type of code which infers a latch
>
> Q <= (C and D) or (not(C) and Q);
>
> This is obviously a very stripped down example, a 'real' design might be
> much more complicated, but you have to train yourself to look at every
> concurrent statement and make sure that you don't see the output on both
> sides of the equation.
>
> It gets worse though because the 'latch' could be a function of multiple
> concurrent statements that happen to loop back. The synthesizer report that
> it finds a combinatorial loop works to catch that but nothing in the
> language prevents it (nor should it, since it may not be 'unintended').
>
> > 2. If the above error is given an error, VHDL may include a special
> > statement to generate a latch.

>
> If you want a 'special' statement that creates a latch, then write one and
> instantiate it until your heart is content....but don't presume that
> everyone else agrees with you and that the language itself should only
> support your coding style for creating a latch and any other method should
> cause an error.
>
> > This is another big problem: I don't know how ASIC people generate a
> > real latch using VHDL? I think they may most likely use latch library
> > to generate special latch instead of using VHDL statements.

>
> That doesn't preclude them from inferring a latch in other ways.
>
> > I read an
> > article about asynchronous FIFO written by two engineers one of whom
> > is Pete Alfke of Xilinx (it is the best article I have read in my
> > life). In the paper they say that they fail to generate two or 3
> > latches in their design using Xilinx chip.

>
> No idea what you mean by this...but reading on
>
> > If so, it seems to me that
> > there is no reliable statement in VHDL to generate a latch for a FPGA
> > chip.

>
> Unless the underlying hardware has a hard latch available as a resource to
> use and most (if not all) FPGAs do not, then I would agree that there is no
> reliable way to generate a latch inside an FPGA in any language, not just
> VHDL. The lack of latches in FPGAs though has nothing at all to do with
> VHDL or any language it is simply recognition by the FPGA suppliers that
> there is very little demand for a hard latch inside the FPGA. A flip flop
> would have the exact same coding issues, the only reason it's not a
> 'problem' is because there are hard flip flops inside the FPGA.
>
> The 'problem' with inferring any storage element (latch, flip flop, big
> hunk-o-memory, fifo, etc.) is when the underlying hardware doesn't directly
> support the inferred element and it needs to be cobbled together from basic
> logic blocks. In the FPGA/CPLD world, it is very difficult to meet internal
> setup, hold time and 'no glitching' requirements between two arbitrary
> signals.
>
> Remember also, that you can build ANY logical element strictly from NAND
> gates or NOR gates....so in theory you could write your code using only
> these two building blocks. Were you to do so and target an FPGA, it would
> most certainly fail if that design had any sort of storage.
>
>
>
> > 3. If the example would generate a latch for signalA, how it is
> > generated?

>
> Same as any other logical description....it gets mapped to whatever internal
> resources exist in the device (LUTs, macrocells, flip flops, latches,
> whatever the target device supports).
>
>
>
> > KJ answered the question. If the equation KJ suggested is true, it
> > would like the following:
> > if (state = stateA_S and a = "000001") then
> > signalA <= '1';
> > end if;

>
> > Finally I realized KJ saying is correct.

>
> You're welcome.
>
> Kevin Jennings


Hi KJ,
1. Q <= (C and D) or (not(C) and Q);
I have never seen such equation in my coding experiences and have no
idea how this equation would be written. The logic result is beyond a
reason. Could you please write it in equivalent latch equation in
informal VHDL?

2. FPGA of Xilinx chip really has latch primative and one may use it
using latch primative to call it. But it is hard to refer to it in
VHDL. I don't mean VHDL should have included latch statement, what I
mean is VHDL really lacks the statement element to refer to a latch in
a clear and reliable way and the lack can be easily corrected.

Thank you.

Weng

 
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Daniel S.
Guest
Posts: n/a
 
      03-07-2007
Weng Tianxiang wrote:
> On Mar 6, 5:37 pm, "KJ" <(E-Mail Removed)> wrote:
>> "Weng Tianxiang" <(E-Mail Removed)> wrote in message
>>
>> news:(E-Mail Removed) ups.com...
>>
>>> Hi Kevin,
>>> In my first posting, there are 3 questions:
>>> 1. Why doesn't compiler generate an error for unintended latch
>>> generation?

>> Simply put because it is not an 'error' to write code that infers a latch.
>> To presume that every latch is 'unintended' would be wrong.
>>
>>> This question is satisfactorily answered: there is a new attribute to
>>> give errors if an unintended latch is to generate. It says that some
>>> other people had already observed the situation long time ago.

>> If you choose to use that attribute and find a tool that supports it then by
>> all means use it. But keep in mind that if you forget to use the new
>> attribute (because people are human) that you can still end up in the same
>> situation. It's up to you if that's the coding style you prefer to use to
>> avoid unintended latches, personally I prefer other coding styles but the
>> attribute is at least an improvement and gives another method which is good.
>>
>> No matter which of these two methods you use, you can still get an
>> 'unintended latch' with the following type of code which infers a latch
>>
>> Q <= (C and D) or (not(C) and Q);
>>
>> This is obviously a very stripped down example, a 'real' design might be
>> much more complicated, but you have to train yourself to look at every
>> concurrent statement and make sure that you don't see the output on both
>> sides of the equation.
>>
>> It gets worse though because the 'latch' could be a function of multiple
>> concurrent statements that happen to loop back. The synthesizer report that
>> it finds a combinatorial loop works to catch that but nothing in the
>> language prevents it (nor should it, since it may not be 'unintended').
>>
>>> 2. If the above error is given an error, VHDL may include a special
>>> statement to generate a latch.

>> If you want a 'special' statement that creates a latch, then write one and
>> instantiate it until your heart is content....but don't presume that
>> everyone else agrees with you and that the language itself should only
>> support your coding style for creating a latch and any other method should
>> cause an error.
>>
>>> This is another big problem: I don't know how ASIC people generate a
>>> real latch using VHDL? I think they may most likely use latch library
>>> to generate special latch instead of using VHDL statements.

>> That doesn't preclude them from inferring a latch in other ways.
>>
>>> I read an
>>> article about asynchronous FIFO written by two engineers one of whom
>>> is Pete Alfke of Xilinx (it is the best article I have read in my
>>> life). In the paper they say that they fail to generate two or 3
>>> latches in their design using Xilinx chip.

>> No idea what you mean by this...but reading on
>>
>>> If so, it seems to me that
>>> there is no reliable statement in VHDL to generate a latch for a FPGA
>>> chip.

>> Unless the underlying hardware has a hard latch available as a resource to
>> use and most (if not all) FPGAs do not, then I would agree that there is no
>> reliable way to generate a latch inside an FPGA in any language, not just
>> VHDL. The lack of latches in FPGAs though has nothing at all to do with
>> VHDL or any language it is simply recognition by the FPGA suppliers that
>> there is very little demand for a hard latch inside the FPGA. A flip flop
>> would have the exact same coding issues, the only reason it's not a
>> 'problem' is because there are hard flip flops inside the FPGA.
>>
>> The 'problem' with inferring any storage element (latch, flip flop, big
>> hunk-o-memory, fifo, etc.) is when the underlying hardware doesn't directly
>> support the inferred element and it needs to be cobbled together from basic
>> logic blocks. In the FPGA/CPLD world, it is very difficult to meet internal
>> setup, hold time and 'no glitching' requirements between two arbitrary
>> signals.
>>
>> Remember also, that you can build ANY logical element strictly from NAND
>> gates or NOR gates....so in theory you could write your code using only
>> these two building blocks. Were you to do so and target an FPGA, it would
>> most certainly fail if that design had any sort of storage.
>>
>>
>>
>>> 3. If the example would generate a latch for signalA, how it is
>>> generated?

>> Same as any other logical description....it gets mapped to whatever internal
>> resources exist in the device (LUTs, macrocells, flip flops, latches,
>> whatever the target device supports).
>>
>>
>>
>>> KJ answered the question. If the equation KJ suggested is true, it
>>> would like the following:
>>> if (state = stateA_S and a = "000001") then
>>> signalA <= '1';
>>> end if;
>>> Finally I realized KJ saying is correct.

>> You're welcome.
>>
>> Kevin Jennings

>
> Hi KJ,
> 1. Q <= (C and D) or (not(C) and Q);
> I have never seen such equation in my coding experiences and have no
> idea how this equation would be written. The logic result is beyond a
> reason. Could you please write it in equivalent latch equation in
> informal VHDL?


That equation is written exactly as-is and would work in theory.
Glitches on Q while C goes from 1 to 0 may break the feedback loop or
make it metastable in practice.

Another VHDL way of putting it would be:

Q <= D when C = '1' else Q;

An equivalent VHDL process would be:

process(c,d)
begin
if(c='1') then
q <= d;
end if;
end process;

Exactly what each of these would actually infer is tool- and
hardware-specific. The only thing you can do is check your synthesis
reports to determine exactly what was synthesized. This applies to all
matters of getting HDL to map onto specific FPGA resources.

> 2. FPGA of Xilinx chip really has latch primative and one may use it
> using latch primative to call it. But it is hard to refer to it in
> VHDL. I don't mean VHDL should have included latch statement, what I
> mean is VHDL really lacks the statement element to refer to a latch in
> a clear and reliable way and the lack can be easily corrected.


Latches: "if somesig='1' then ... end if;"
Latches(2): see equations and process above

FFs: "if clk='1' and clk'event then ... end if;"
FFs(2): "if rising_edge(clk) then ... end if;"

That seems like plenty straight-forward VHDL to me. Latches are active
on levels, FFs are active on edges. Simple.

Xilinx devices do not have dedicated latches BUT they do allow you to
program a slice FF to bypass HALF of it so it can be used as a latch. As
far as power is concerned though, the whole FF is still connected to the
clock so you most likely won't be saving any power here. BTW, the
configuration logic that goes behind FPGA FFs uses more surface area
than the FF itself and the "latch bypass logic" probably uses nearly as
much power as a pure FF.

To reach high clock speeds, FPGAs and ASICs need their pipeline storage
elements to be able to forward values from D to Q with as little setup,
hold and settling time as possible to maximize timing margins between
clock edges. This is most effortlessly and reliably achieved with FFs,
hence the dominance of FFs in the world of high-speed digital systems
and the FF-oriented nature of FPGAs even though they cost a little more
surface area and power.

Simply put, FFs are the path of least unnecessary pain.

If you want lower power FPGAs, some flexibility has to be sacrificed. If
Xilinx stripped the latch option from slice FFs, it would reduce the
slice FF cells' power and size by 15-20% while making them a little bit
faster. I wonder how many would have a problem with this.
 
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KJ
Guest
Posts: n/a
 
      03-07-2007

"Weng Tianxiang" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> 1. Q <= (C and D) or (not(C) and Q);
> I have never seen such equation in my coding experiences and have no
> idea how this equation would be written. The logic result is beyond a
> reason. Could you please write it in equivalent latch equation in
> informal VHDL?

It's the same old transparent latch that we've been talking about all along.
The following forms are exactly equivalent logic and will result in the
exact same synthesized result.
#1 -- Traditional form of writing a latch
process(C, D)
begin
if (C='1') then
Q <= D;
end if;
end process;

#2 -- Another traditional form of writing a latch.
Q <= D when (C = '1');

#3 -- Latch equations written as a sum of products
Q <= (C and D) or (not(C) and Q);

#4 -- Latch equation written in a sum of products form that includes a
'cover' term
Q <= (C and D) or (not(C) and Q) or (D and Q);

Try all four of the above out and run them through synthesis and you should
get the exact same fitted equations for all 4.

Of the four different forms, only #4 is written in a way that will not have
timing issues that are caused simply by the implementation (over which you
have no real control in an FPGA). However, the last term ".. or (D and Q)"
is redundant and any synthesizer worth a dime will 'optomize the logic' and
implement it as form #3. This 'optomization' though creates a logic hazard
when 'C' switches from '1' to '0'. This logic hazard will in turn show up
as a failure in the latch operation. Specifically, what you'll run into is
that even if you have 'D' set up oodles of time before 'C' switches from '1'
to '0', the output 'Q' can latch itself at either '1' or '0' (depending on
the targetted part) because of differences in prop delay in computing the
first two or terms or glitches in 'C'. That last "D and Q" term is
absolutely required in order to even have a shot at inferring a latch that
will actually work but since it is logically 'redundant' it will always get
optomized away. If you hark back to Boolean Logic 101, you may recall that
adding redundant terms is the method you employ to get rid of logic hazards.
The FPGA LUT implementation leaves even more to the imagination as you
wonder just what those RAM outputs are doing with this combinatorial loop
and whether 'Q' will glitch and cause even more havoc since it would feed
back to the address input of the LUT.

The bottom line is
- There are various forms of writing source code that infers a latch.
- Whether a latch is a 'good' thing or a 'bad' thing depends on the
targetted device itself and whether that device actually has a hard latch
and a way to guarantee the timing and glitch requirements of 'D' and 'C'.
- Inferring any storage element (flops, latches, memory) needs to be done in
such a way that the storage is not implemented as a collection of
LUTs/macrocells but instead uses hard storage resources of the device.

>
> 2. FPGA of Xilinx chip really has latch primative and one may use it
> using latch primative to call it. But it is hard to refer to it in
> VHDL.

You'll still have trouble meeting and guaranteeing the timing and glitch
requirements on 'D' relative to 'C' in order to get it to work correctly for
arbitrary signals 'D' and 'C'. If it does work, it likely won't scale well
(i.e. if you try to infer thousands of these as you would like to)

> I don't mean VHDL should have included latch statement, what I
> mean is VHDL really lacks the statement element to refer to a latch in
> a clear and reliable way and the lack can be easily corrected.
>

I've shown above four different forms for inferring the exact same latch
(i.e. synthesizes to the exact same thing). Which form you prefer (or if
you prefer to make up your own) to use is up to you. Making up a new method
though will not prohibit the earlier forms from being used and outlawing the
other forms gets in the way of people who do use latches and are doing so in
a way that they can control the various issues so you're not helping them by
outlawing their method.

Kevin Jennings


 
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Weng Tianxiang
Guest
Posts: n/a
 
      03-07-2007
On Mar 7, 3:02 am, "KJ" <(E-Mail Removed)> wrote:
> "Weng Tianxiang" <(E-Mail Removed)> wrote in message
>
> news:(E-Mail Removed) oups.com...> 1. Q <= (C and D) or (not(C) and Q);
> > I have never seen such equation in my coding experiences and have no
> > idea how this equation would be written. The logic result is beyond a
> > reason. Could you please write it in equivalent latch equation in
> > informal VHDL?

>
> It's the same old transparent latch that we've been talking about all along.
> The following forms are exactly equivalent logic and will result in the
> exact same synthesized result.
> #1 -- Traditional form of writing a latch
> process(C, D)
> begin
> if (C='1') then
> Q <= D;
> end if;
> end process;
>
> #2 -- Another traditional form of writing a latch.
> Q <= D when (C = '1');
>
> #3 -- Latch equations written as a sum of products
> Q <= (C and D) or (not(C) and Q);
>
> #4 -- Latch equation written in a sum of products form that includes a
> 'cover' term
> Q <= (C and D) or (not(C) and Q) or (D and Q);
>
> Try all four of the above out and run them through synthesis and you should
> get the exact same fitted equations for all 4.
>
> Of the four different forms, only #4 is written in a way that will not have
> timing issues that are caused simply by the implementation (over which you
> have no real control in an FPGA). However, the last term ".. or (D and Q)"
> is redundant and any synthesizer worth a dime will 'optomize the logic' and
> implement it as form #3. This 'optomization' though creates a logic hazard
> when 'C' switches from '1' to '0'. This logic hazard will in turn show up
> as a failure in the latch operation. Specifically, what you'll run into is
> that even if you have 'D' set up oodles of time before 'C' switches from '1'
> to '0', the output 'Q' can latch itself at either '1' or '0' (depending on
> the targetted part) because of differences in prop delay in computing the
> first two or terms or glitches in 'C'. That last "D and Q" term is
> absolutely required in order to even have a shot at inferring a latch that
> will actually work but since it is logically 'redundant' it will always get
> optomized away. If you hark back to Boolean Logic 101, you may recall that
> adding redundant terms is the method you employ to get rid of logic hazards.
> The FPGA LUT implementation leaves even more to the imagination as you
> wonder just what those RAM outputs are doing with this combinatorial loop
> and whether 'Q' will glitch and cause even more havoc since it would feed
> back to the address input of the LUT.
>
> The bottom line is
> - There are various forms of writing source code that infers a latch.
> - Whether a latch is a 'good' thing or a 'bad' thing depends on the
> targetted device itself and whether that device actually has a hard latch
> and a way to guarantee the timing and glitch requirements of 'D' and 'C'.
> - Inferring any storage element (flops, latches, memory) needs to be done in
> such a way that the storage is not implemented as a collection of
> LUTs/macrocells but instead uses hard storage resources of the device.
>
>
>
> > 2. FPGA of Xilinx chip really has latch primative and one may use it
> > using latch primative to call it. But it is hard to refer to it in
> > VHDL.

>
> You'll still have trouble meeting and guaranteeing the timing and glitch
> requirements on 'D' relative to 'C' in order to get it to work correctly for
> arbitrary signals 'D' and 'C'. If it does work, it likely won't scale well
> (i.e. if you try to infer thousands of these as you would like to)
>
> > I don't mean VHDL should have included latch statement, what I
> > mean is VHDL really lacks the statement element to refer to a latch in
> > a clear and reliable way and the lack can be easily corrected.

>
> I've shown above four different forms for inferring the exact same latch
> (i.e. synthesizes to the exact same thing). Which form you prefer (or if
> you prefer to make up your own) to use is up to you. Making up a new method
> though will not prohibit the earlier forms from being used and outlawing the
> other forms gets in the way of people who do use latches and are doing so in
> a way that they can control the various issues so you're not helping them by
> outlawing their method.
>
> Kevin Jennings


Hi KJ,
Thank you very much for your points.

Your answer really opens my eye and expand my knowledge range in FPGA
and VHDL !!!

Weng

 
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Weng Tianxiang
Guest
Posts: n/a
 
      03-08-2007
On Mar 7, 3:02 am, "KJ" <(E-Mail Removed)> wrote:
> "Weng Tianxiang" <(E-Mail Removed)> wrote in message
>
> news:(E-Mail Removed) oups.com...> 1. Q <= (C and D) or (not(C) and Q);
> > I have never seen such equation in my coding experiences and have no
> > idea how this equation would be written. The logic result is beyond a
> > reason. Could you please write it in equivalent latch equation in
> > informal VHDL?

>
> It's the same old transparent latch that we've been talking about all along.
> The following forms are exactly equivalent logic and will result in the
> exact same synthesized result.
> #1 -- Traditional form of writing a latch
> process(C, D)
> begin
> if (C='1') then
> Q <= D;
> end if;
> end process;
>
> #2 -- Another traditional form of writing a latch.
> Q <= D when (C = '1');
>
> #3 -- Latch equations written as a sum of products
> Q <= (C and D) or (not(C) and Q);
>
> #4 -- Latch equation written in a sum of products form that includes a
> 'cover' term
> Q <= (C and D) or (not(C) and Q) or (D and Q);
>
> Try all four of the above out and run them through synthesis and you should
> get the exact same fitted equations for all 4.
>
> Of the four different forms, only #4 is written in a way that will not have
> timing issues that are caused simply by the implementation (over which you
> have no real control in an FPGA). However, the last term ".. or (D and Q)"
> is redundant and any synthesizer worth a dime will 'optomize the logic' and
> implement it as form #3. This 'optomization' though creates a logic hazard
> when 'C' switches from '1' to '0'. This logic hazard will in turn show up
> as a failure in the latch operation. Specifically, what you'll run into is
> that even if you have 'D' set up oodles of time before 'C' switches from '1'
> to '0', the output 'Q' can latch itself at either '1' or '0' (depending on
> the targetted part) because of differences in prop delay in computing the
> first two or terms or glitches in 'C'. That last "D and Q" term is
> absolutely required in order to even have a shot at inferring a latch that
> will actually work but since it is logically 'redundant' it will always get
> optomized away. If you hark back to Boolean Logic 101, you may recall that
> adding redundant terms is the method you employ to get rid of logic hazards.
> The FPGA LUT implementation leaves even more to the imagination as you
> wonder just what those RAM outputs are doing with this combinatorial loop
> and whether 'Q' will glitch and cause even more havoc since it would feed
> back to the address input of the LUT.
>
> The bottom line is
> - There are various forms of writing source code that infers a latch.
> - Whether a latch is a 'good' thing or a 'bad' thing depends on the
> targetted device itself and whether that device actually has a hard latch
> and a way to guarantee the timing and glitch requirements of 'D' and 'C'.
> - Inferring any storage element (flops, latches, memory) needs to be done in
> such a way that the storage is not implemented as a collection of
> LUTs/macrocells but instead uses hard storage resources of the device.
>
>
>
> > 2. FPGA of Xilinx chip really has latch primative and one may use it
> > using latch primative to call it. But it is hard to refer to it in
> > VHDL.

>
> You'll still have trouble meeting and guaranteeing the timing and glitch
> requirements on 'D' relative to 'C' in order to get it to work correctly for
> arbitrary signals 'D' and 'C'. If it does work, it likely won't scale well
> (i.e. if you try to infer thousands of these as you would like to)
>
> > I don't mean VHDL should have included latch statement, what I
> > mean is VHDL really lacks the statement element to refer to a latch in
> > a clear and reliable way and the lack can be easily corrected.

>
> I've shown above four different forms for inferring the exact same latch
> (i.e. synthesizes to the exact same thing). Which form you prefer (or if
> you prefer to make up your own) to use is up to you. Making up a new method
> though will not prohibit the earlier forms from being used and outlawing the
> other forms gets in the way of people who do use latches and are doing so in
> a way that they can control the various issues so you're not helping them by
> outlawing their method.
>
> Kevin Jennings


Hi KJ,
I have another question for you.

1. Is any combinational equation with target signal in both right part
and left part of the equation a latch like your equations 3 or 4 show?

2. Is there any general algorithm to change such an equation to a
latch equation?

Thank you.

Weng

 
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Jim Lewis
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      03-08-2007
Weng
> Hi KJ,
> I have another question for you.
>
>> Q <= (C and D) or (not(C) and Q);

> 1. Is any combinational equation with target signal in both right part
> and left part of the equation a latch like your equations 3 or 4 show?


A latch is the most benign form.
There are oscillators (however when they occur by
accident, they are not necessarily this obvious):
Q <= not Q ;

There is also other strange latch-like behavior.
I don't have an equation for you, but I do remember
analyzing one - it was very interesting.

Cheers,
Jim
 
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Peter Alfke
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      03-08-2007
I think we will agree that a latch needs positive feedback from the
output to at least one of the inputs.
If the feedback is negative (Qbar to D, for example) you end up with
an oscillator.
Peter Alfke

On Mar 8, 9:49 am, Jim Lewis <(E-Mail Removed)> wrote:
> Weng> Hi KJ,
> > I have another question for you.

>
> >> Q <= (C and D) or (not(C) and Q);

>
> > 1. Is any combinational equation with target signal in both right part
> > and left part of the equation a latch like your equations 3 or 4 show?

>
> A latch is the most benign form.
> There are oscillators (however when they occur by
> accident, they are not necessarily this obvious):
> Q <= not Q ;
>
> There is also other strange latch-like behavior.
> I don't have an equation for you, but I do remember
> analyzing one - it was very interesting.
>
> Cheers,
> Jim



 
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KJ
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      03-08-2007
On Mar 8, 11:36 am, "Weng Tianxiang" <(E-Mail Removed)> wrote:
> On Mar 7, 3:02 am, "KJ" <(E-Mail Removed)> wrote:
>
>
>
>
>
> > "Weng Tianxiang" <(E-Mail Removed)> wrote in message

>
> >news:(E-Mail Removed) roups.com...> 1. Q <= (C and D) or (not(C) and Q);
> > > I have never seen such equation in my coding experiences and have no
> > > idea how this equation would be written. The logic result is beyond a
> > > reason. Could you please write it in equivalent latch equation in
> > > informal VHDL?

>
> > It's the same old transparent latch that we've been talking about all along.
> > The following forms are exactly equivalent logic and will result in the
> > exact same synthesized result.
> > #1 -- Traditional form of writing a latch
> > process(C, D)
> > begin
> > if (C='1') then
> > Q <= D;
> > end if;
> > end process;

>
> > #2 -- Another traditional form of writing a latch.
> > Q <= D when (C = '1');

>
> > #3 -- Latch equations written as a sum of products
> > Q <= (C and D) or (not(C) and Q);

>
> > #4 -- Latch equation written in a sum of products form that includes a
> > 'cover' term
> > Q <= (C and D) or (not(C) and Q) or (D and Q);

>
> > Try all four of the above out and run them through synthesis and you should
> > get the exact same fitted equations for all 4.

>
> > Of the four different forms, only #4 is written in a way that will not have
> > timing issues that are caused simply by the implementation (over which you
> > have no real control in an FPGA). However, the last term ".. or (D and Q)"
> > is redundant and any synthesizer worth a dime will 'optomize the logic' and
> > implement it as form #3. This 'optomization' though creates a logic hazard
> > when 'C' switches from '1' to '0'. This logic hazard will in turn show up
> > as a failure in the latch operation. Specifically, what you'll run into is
> > that even if you have 'D' set up oodles of time before 'C' switches from '1'
> > to '0', the output 'Q' can latch itself at either '1' or '0' (depending on
> > the targetted part) because of differences in prop delay in computing the
> > first two or terms or glitches in 'C'. That last "D and Q" term is
> > absolutely required in order to even have a shot at inferring a latch that
> > will actually work but since it is logically 'redundant' it will always get
> > optomized away. If you hark back to Boolean Logic 101, you may recall that
> > adding redundant terms is the method you employ to get rid of logic hazards.
> > The FPGA LUT implementation leaves even more to the imagination as you
> > wonder just what those RAM outputs are doing with this combinatorial loop
> > and whether 'Q' will glitch and cause even more havoc since it would feed
> > back to the address input of the LUT.

>
> > The bottom line is
> > - There are various forms of writing source code that infers a latch.
> > - Whether a latch is a 'good' thing or a 'bad' thing depends on the
> > targetted device itself and whether that device actually has a hard latch
> > and a way to guarantee the timing and glitch requirements of 'D' and 'C'.
> > - Inferring any storage element (flops, latches, memory) needs to be done in
> > such a way that the storage is not implemented as a collection of
> > LUTs/macrocells but instead uses hard storage resources of the device.

>
> > > 2. FPGA of Xilinx chip really has latch primative and one may use it
> > > using latch primative to call it. But it is hard to refer to it in
> > > VHDL.

>
> > You'll still have trouble meeting and guaranteeing the timing and glitch
> > requirements on 'D' relative to 'C' in order to get it to work correctly for
> > arbitrary signals 'D' and 'C'. If it does work, it likely won't scale well
> > (i.e. if you try to infer thousands of these as you would like to)

>
> > > I don't mean VHDL should have included latch statement, what I
> > > mean is VHDL really lacks the statement element to refer to a latch in
> > > a clear and reliable way and the lack can be easily corrected.

>
> > I've shown above four different forms for inferring the exact same latch
> > (i.e. synthesizes to the exact same thing). Which form you prefer (or if
> > you prefer to make up your own) to use is up to you. Making up a new method
> > though will not prohibit the earlier forms from being used and outlawing the
> > other forms gets in the way of people who do use latches and are doing so in
> > a way that they can control the various issues so you're not helping them by
> > outlawing their method.

>
> > Kevin Jennings

>
> Hi KJ,
> I have another question for you.
>
> 1. Is any combinational equation with target signal in both right part
> and left part of the equation a latch like your equations 3 or 4 show?

While Jim and Peter can debate whether or not it's a latch or an
oscillator, suffice it to say that any time you have something of the
form of equations #3 and #4 where you have the same signal on both
sides of the '<=' in a concurrent statement you have some form of
storage or memory being inferred that will cause you the exact same
timing/glitch/hazard issues to deal with as you would if you have
something that matches exactly #3 and #4.

The important thing to take away is not whether it's a latch or an
oscillator or anything else, just that red flags should go off in your
head when you see the output being used on the right side of the
equation. Whether this is a problem or not depends on what the target
device is. In ASICs this will not be a problem, in FPGAs you have to
be very careful or it will be a problem.

>
> 2. Is there any general algorithm to change such an equation to a
> latch equation?

Not sure what you mean....all four forms that I presented are exactly
equivalent 'latch equations', they will produce the same synthesized
result. Equations #1 and #2 involve less typing then #3 and #4.

Also keep in mind, that latches do not need to fit into a single
equation, any time that you form a combinatorial loop you have to be
concerned about how this will get implemented. A combinatorial loop
happens when you have multiple equations, none of which inherently
shows any feedback but taken together the whole set does. A simple
example of this would be

#5
Q <= (C and X) or (not(C) and Y); -- Eqn. 5a
Y <= Q and Z; -- Eqn. 5b

Equation 5a, on the surface appears to be a 2->1 multiplexer where 'C'
is simply used to select between two signals 'X' and 'Y'.

Equation 5b, on the surface appears to be a simple anding of two
signals 'Q' and 'Z'.

But when you put them together the two form a combinatorial loop
because 'Q' depends on 'Y' and 'Y' depends on 'Q'. When you run
through synthesis, these things generally get flagged as warnings
somewhere. The nice thing is that it will find all these loops no
matter how complex the equations or how spread out they are as long as
they are totally self contained within the entire design. By that I
mean, lets say that Equation 5a was implemented in some entity
somewhere in some file and Equation 5b in a totally different entity
in some other file. By themselves, each entity/file will synthesize
just fine. But when you add the code for the entity that ties these
two blocks together that new design and synthesize that new block,
you'll have now formed the loop and the synthesizer should flag the
warning for you. Take those warning seriously if you're designing in
the FPGA/CPLD world.

Kevin Jennings

 
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Jim Lewis
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      03-09-2007
Peter,
No argument from me as you seem to be saying the same
thing I said.

I interpreted Weng's question as being, when a combinational
signal is on both the right and left side of an equation,
is the only hardware solution a latch.

The answer which we both observed is no and the simple
case is an oscillator.

Cheers,
Jim


> I think we will agree that a latch needs positive feedback from the
> output to at least one of the inputs.
> If the feedback is negative (Qbar to D, for example) you end up with
> an oscillator.
> Peter Alfke
>
> On Mar 8, 9:49 am, Jim Lewis <(E-Mail Removed)> wrote:
>> Weng> Hi KJ,
>>> I have another question for you.
>> >> Q <= (C and D) or (not(C) and Q);

>>
>>> 1. Is any combinational equation with target signal in both right part
>>> and left part of the equation a latch like your equations 3 or 4 show?

>> A latch is the most benign form.
>> There are oscillators (however when they occur by
>> accident, they are not necessarily this obvious):
>> Q <= not Q ;
>>
>> There is also other strange latch-like behavior.
>> I don't have an equation for you, but I do remember
>> analyzing one - it was very interesting.
>>
>> Cheers,
>> Jim

>
>

 
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