problem with loop statement

[bxbxb3]

Hi,
Could anyone help me out with this probelm? When ever I try to simulate
the code shown below, my ModelSIM hangs up!

My code is a simple one.

process(clk)
begin
if(clk'event and clk='1') then
if reset='1' then
reg<="000000000000000000000000000000000"; --reg is 33 bits wide(32
downto 0)
generator<="100000100110000010001110110110111"; --total 33 bits
counter<=0;
else
while(not(counter=63)) loop
reg<=reg(31 downto 0) & '0';
if(reg(31)='1') then
reg<=reg xor generator;
end if;
end loop;
end if;
end if;
end process;

 

[bxbxb3]

Sorry, I forgot to mention
counter<=counter+1; after "end if" statement in the while loop.
Thanks.

 

[Jezwold]

The problem is in the fact that you are trying to shift 'reg' left 63
times on the rising edge of the clk signal I would have thought.

 

[Jezwold]

Well o.k 64 times.

 

[ALuPin]

You are trying to increment the SIGNAL counter in a loop within
one clock cycle! You need one VARIABLE.



process(clk)
variable counter : integer range 0 to 63
begin
if rising_edge(clk) then
if reset='1' then
reg <="000000000000000000000000000000000";
generator <="100000100110000010001110110110111";
else
counter := 0;
while(not(counter=63)) loop
reg <= reg(31 downto 0) & '0';
if reg(31)='1' then
reg <= (reg xor generator);
end if;
counter := counter+1;
end loop;
end if;
end if;
end process;

You should always remeber that the change of a signal in one clock cycle
only takes effect one clock cycle later!

What are you trying to implement ?

Rgds
André

 

[Jezwold]

The fact is he is trying to do lots of stuff within one clock cycle.

 

[bxbxb3]

Got it! I must not do so many shifts in a signal clock tick. Actually I
thought the counter in the while loop would increment every clock cycle.
Thanks for the advice.

 

[bxbxb3]

Actually I was trying to develop a code for 32-bit serial CRC generation. I
tried the recommended one and was just checking other methods to do the
same. One of the codes looks like this:

process(clk)
begin
if(clk'event and clk='1') then
if(init='1') then
i<=63;
j<=31;
reg<=data_in & "00000000000000000000000000000000";
generator_poly<="100000100110000010001110110110111";
elsif(init='0') then
i<=i-1;
j<=j-1;
if(reg(i)='1' and i>31) then
reg(i downto j)<=reg(i downto j) xor generator_poly;
elsif(i=31) then
i<=63;
j<=31;
crc_out<=reg(31 downto 0);
end if;
end if;
end if;
end process;

The above code does not synthesis because of (i downto j) clause.

 

[Peter Hermansson]

Hi,
Could anyone help me out with this probelm? When ever I try to simulate
the code shown below, my ModelSIM hangs up!

My code is a simple one.

process(clk)
begin
if(clk'event and clk='1') then
if reset='1' then
reg<="000000000000000000000000000000000"; --reg is 33 bits wide(32
downto 0)
generator<="100000100110000010001110110110111"; --total 33 bits
counter<=0;
else
while(not(counter=63)) loop
reg<=reg(31 downto 0) & '0';
if(reg(31)='1') then
reg<=reg xor generator;
end if;
end loop;
end if;
end if;
end process;

Hi,

Lets say your counter does not equal 63. Then you have (I believe) an
endless loop executing reg <= reg(31 downto 0) & '0';
Why not try a FOR LOOP instead?

Another suggestion: reg could be cleared with reg <= (others => '0');

/Peter