Bad synchronous description, but why ?

[Roman Himmes]

Hello,

I am very new to VHDL and I still haven't figured everything out.
In this code I get: "Signal tmprr cannot be synthesized, bad synchronous
description" I understand that this is due to possible that more than one
process is accessing Signals ...

My Process in which I have the Problem is this code:

RR_PROCESS: process(RR,Send,maxDelay)
variable tmprr : std_logic_vector(11 downto 0) := "000000000000";
begin
if RR='1' and RR'event then tmprr := tmprr + 1;
else
if Send='1' then
case maxDelay is
when "00" => tmprr := To_StdLogicVector( To_BitVector(tmprr) sla 1);
when "01" => tmprr := To_StdLogicVector( To_BitVector(tmprr) sla
2);
when others => tmprr := To_StdLogicVector( To_BitVector(tmprr) sla
3);
end case;
sRRight <= EXT(tmprr,8);
tmprr := "000000000000";
end if;
end if;
end process;

Other relevant signals:
signal maxDelay : std_logic_vector (1 downto 0) := "00";
signal Send : std_logic := '0';
and Ports:
RR : in std_logic;
sRRight : out std_logic_vector(7 downto 0));

Where is the problem ? I cannot see it !


Thanx for any comment

Roman Himmes

 

[Mike Treseler]

I am very new to VHDL and I still haven't figured everything out.
In this code I get: "Signal tmprr cannot be synthesized, bad synchronous
description"

Synthesis is expecting:

RR_PROCESS: process(clock, reset) or

RR_PROCESS: process(clock)

Anything else is a "bad synchronous description"
Have a look at some code examples.

-- Mike Treseler

 

[Roman Himmes]

Synthesis is expecting:

RR_PROCESS: process(clock, reset) or

RR_PROCESS: process(clock)

Anything else is a "bad synchronous description"
Have a look at some code examples.

-- Mike Treseler

So every process has to be be synchronized with the clock signal ?
So if you want to make a process sensible to another signal you have to do
something like

process(clock, signal_var)
begin
if clock = '1' and clock'event then
if signal_var='1' then
-- do stuff
end if;
end if;
end process;

-- Roman Himmes

 

[Klaus Falser]

So every process has to be be synchronized with the clock signal ?
So if you want to make a process sensible to another signal you have to do
something like

process(clock, signal_var)
begin
if clock = '1' and clock'event then
if signal_var='1' then
-- do stuff
end if;
end if;
end process;

-- Roman Himmes

I'm really wondering why every VHDL beginner is using
the "if clock = '1' and clock'event" phrase
insteadt of the "rising_edge()" function.

Are all the books around that old or the teachers that oldfashioned?
Please, please, VHDL beginners all over the world, start writing

if rising_edge(clock) then
....
end if;

Roman, to answer you question:
If you have to write VHDL code which has to be synthesized and put
into a FPGA or CPLD, then nearly all the processes are synchronous to
one or more clock signals.
This comes from the fact that the core elements of digital logic
are flip-flops, and flip-flops are active only at the rising or
falling edge of a clock signal.

In your example the "do stuff" is done synchronous to the
clock signal, but only at the rising edges where signal_var = '1'.
Signal_var acts as a "clock enable" signal.

Your process needs NOT to be sensitive to signal_var.
Just to explain :
A process is waiting that one or more signal mentioned in the
sensitivity list changes. After that the body of the process
is executed and the waiting starts again.
Making your process sensitive to signal_var will not change
anything, since the do_stuff statements are only reached when
clock'event is true and clock'event is only true when the clock
signal changes.

Hope this helps, but probabely it's best if you read
some book about digital logic too.

Klaus Falser

 

[Ralf Hildebrandt]

Roman Himmes wrote:

In this code I get: "Signal tmprr cannot be synthesized, bad synchronous
description"

RR_PROCESS: process(RR,Send,maxDelay)
variable tmprr : std_logic_vector(11 downto 0) := "000000000000";
begin
if RR='1' and RR'event then tmprr := tmprr + 1;
else

Do not use else / elsif after an 'event in a preceding if-clause. What
do you expect that this could be in real world?

Note: There is only one exception from this rule: Some (but very few)
synthesis tools support dual-edge flipflops and they are coded using two
succeeding if-clauses containing an 'event. But in general: Don't do this.



Use the template for flipflops:

process(reset, clock)
begin
if (reset='1') then
-- do some asynchronous reset
elsif rising_edge(clock) then
-- do some synchronous stuff
end if;
end process;


Ralf

 

[Andy Peters]

Hello,

I am very new to VHDL and I still haven't figured everything out.
In this code I get: "Signal tmprr cannot be synthesized, bad synchronous
description" I understand that this is due to possible that more than one
process is accessing Signals ...

You should start by reading the documentation that comes with your
synthesis tool. I've used four or five different synthesis tools, and
the docs for each have a section usually titled something like "The Art
Of VHDL Synthesis."

Read it. Your questions will be answered.

-a

 

[Andy Peters]

I'm really wondering why every VHDL beginner is using
the "if clock = '1' and clock'event" phrase
insteadt of the "rising_edge()" function.

Are all the books around that old or the teachers that oldfashioned?

Worse -- the docs that come with the synthesis tools haven't been
updated since the days of VHDL '87. That's why you still see examples
that use std_logic_arith instead of numeric_std.

-a

 

[Bert Cuzeau]

So every process has to be be synchronized with the clock signal ?
So if you want to make a process sensible to another signal you have to do
something like

process(clock, signal_var)
begin
if clock = '1' and clock'event then
if signal_var='1' then
-- do stuff
end if;
end if;
end process;

-- Roman Himmes

Sorry, this is wrong also !
Sensitivity list in this case must be process(clock) _only_.
Neither Clock Enable nor anything else than an async action
should be in such (sequential) sensitivity list.
And as other pointed out, rising_edge is indeed looking better.

Coding in VHDL wihtout basic know-how is, imo, hopeless...

Bert Cuzeau

 

[Duane Clark]

Roman Himmes wrote:




Sorry, this is wrong also !
Sensitivity list in this case must be process(clock) _only_.
Neither Clock Enable nor anything else than an async action
should be in such (sequential) sensitivity list.

While strictly true, it won't make a difference either in simulation or
synthesis. The "clock'event" (which is also implied when using
rising_edge) means that the contained logic will only be executed at the
proper time; a change in signal_var will not cause a clock event. But of
course the code should be written properly.