Synthesis errors?

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I have recently had lots of incorrectly synthesised logic with the
synthesiser I am using.
My latest design occupied approx 20% of a "6 million gate" FPGA, and had a
total of 5
incorrectly synthesised parts (which took some finding).

Can anyone recommend any synthesisers which do not suffer from this sort of

The synth takes approx 1 hour to synth this (much quicker than most of my
"large" designs), and timing far
exceeds the requirements as the clock frequency is low.

If anyone is interested, the sort of errors I was getting were:-

A connection between 2 components was simply not made. An input to the
second component was hardwired to '0'.
Tried the very latest version of the same synth and the problem went away.

The following problems were seen in the latest version:-

OUT_DATA <= IN_DATA & "0";
synthesised to
OUT_DATA <= "0" & IN_DATA;
put this in a clocked process and it synthesised correctly.

had OUT_DATA(0) hardwired to '1'.
replaced with a case statement and it synthesised correctly

if X = -1 then
elsif X= 1 then
  OUT_DATA <= 0 - IN_DATA;
elsif X=0 then
  OUT_DATA <= 0;
  OUT_DATA <= 0 - IN_DATA;
end if;
had OUT_DATA=0 when X=-1
put calculation of 0 - IN_DATA in a separate clocked process, and it
synthesised correctly.

An inferred ROM which synthesised correctly in an earlier version of the
synthesiser, now infers a ROM filled with zeros.
I worked around this by adding a reset to cause the ROM function to be built
from logic. This greatly increased
the size, but this is not a problem for the particular design.

I tried synthing the rogue pieces of code standalone, and they were synthed
correctly (apart from the ROM).


Morrow Electronics Limited,
Milton Keynes,
UK. without the m after my name otherwise it will not be

Re: Synthesis errors?
The Quartus II 3.0 Tool from Altera has a good VHDL/Verilog parser and
synthesis capability. The output of the synthesiser will work only with the
place and route tools from Altera, and you target all of the Altera devices.
A free version of the tools is available from:

- Subroto Datta
Altera Corp.

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Re: Synthesis errors?
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Every synthesizer will suffer 'cause all of them are SW and SW (as well
as HW ;-) has bugs... The only solution to that problem would be to use
an equivalence checker to check the generated Netlist against the RTL.
They are also not bug free, but if the do not share code, the
probability of having the same bug is near to 0. Tools in that area are
for example Verplex LEC but there are a lot more.


Re: Synthesis errors?
I have used a lot of the synthesisers out there and I have had errors with
all of them at one time or another. I have usually tackled them in the way
you describe. Usually I have found particular constructs to be the issue and
generally avoid the ones that cause problems. Generally the best way to find
the bugs is in a post synthesis simulation. Do this ideally with a self
checking testbench that you can run on a spare machine in the background. It
can be a long process on a big design.

To reduce the long synthesis/simulation times consider using modular
synthesis and simulation. Some tools will do this automatically for you.
Using this approach you can virtually ignore "good" modules and look for
problems in the modules you have actually changed either by design or
synthesis. I hope to have a article on the cheap and cheerful approach to
this in our TechiTips page either this month or next.

John Adair
Enterpoint Ltd.

This message is the personal opinion of the sender and not that necessarily
that of Enterpoint Ltd.. Readers should make their own evaluation of the
facts. No responsibility for error or inaccuracy is accepted.

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