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Forum: FPGA, VHDL & Verilog Sequential Operations and resource sharing


von Carlos (Guest)


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Hi
I'm implementing an equation on VHDL. In this case a Clarke 
transformation.
I did it in 3 different methods.
- First one by a concurrent process using auxiliary signals to register 
the intermediate caclulations.
- Second one using a sublock for the adder and multiplier and and 
instantiating as many as needed pipelining with start/end signals
- Third usign sublocks for adder and multiplier but implementing some 
resource sharing logic to instantiate only one adder and one multiplier.

The result is almost he shame in therms of resources! How is it 
possible? Even the resource sharing method estimated freqency is lower 
than the other methods..

I attach the resource logs and the RTL view on synplify

Seems that Symnplify applies some kind of resource sharing on the 
individual block model?? (in this one there is an inverter for the 
substracttion instead of a full adder, but it does not have so much 
differnece)
On the concurrent methot the resources shouldn't be much higher as it 
implements adders and multipliers individually?

Thanks in advance

von Duke Scarring (Guest)


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Carlos wrote:
> The result is almost he shame in therms of resources! How is it
> possible?
Why not? Your design leeds on all three strategies to a comparable 
solution.
Normally there exists a critical path in every design, so no tool can 
perform magic.

The maximum frequency after synthesis is only a hint. Real numbers 
exists right after Place&Route. They can differ in range 50% to 200% 
from the frequency estimation after the synthesis step.

Duke

von Lothar M. (Company: Titel) (lkmiller) (Moderator)


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Carlos wrote:
> The result is almost he shame in therms of resources!
Kind of correct, although its only a typo... ;-)

> How is it possible?
Obviously doesn't your descriptions of the formula differ very much one 
from the other. Because even when you describe the algo in concurrent or 
a 1, 2 or even in a 3 process code, it has to perform almost the same 
calculations.

> Even the resource sharing method estimated freqency is lower than the
> other methods..
Looks linke there is almost no resseorce sharing at all.


For timing analysis: do a STA and look for the (already mentioned) 
critical path.

von Carlos (Guest)


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>>Obviously doesn't your descriptions of the formula differ very much one
from the other. Because even when you describe the algo in concurrent or
a 1, 2 or even in a 3 process code, it has to perform almost the same
calculations.

Yes, I know. But in concurrent and pipelined blocks aproach it uses 
independent adders and multipliers for each operation.

In resource sharing approach it uses only one adder an one multiplier.

The resource sharing is done by me. Implementing quind of a state 
machine to decide when use the adder and the multiplier. I obtained 
better results using CASE instead of IF/Else statements

But I didnt't take into account that with resource sharing approach I 
was using parallel multipliers instead os serialiced form. Parallel 
multipliers use more logic but are faster than serial ones.

With this two improvements the area reduced to almost 50% and the 
frequency returned to around max 60MHz

But it becomes to slow on simulation to implement serial multipliers and 
respurce sharing...

Thanks

Regards

von Lothar M. (Company: Titel) (lkmiller) (Moderator)


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Carlos wrote:
> But I didnt't take into account that with resource sharing approach I
> was using parallel multipliers instead os serialiced form. Parallel
> multipliers use more logic but are faster than serial ones.
Define "serial multiplier" and "parallel multiplier".

Because the toolchain will simply use the built in silicon multipliers 
in the FPGA.

What you could do is to use such 1 multiplier over several clock cycles 
for  several independent multiplications. But then you have to multiplex 
the factors and the result which generates additional logic around that 
1 multiplier as tradeoff.

von Carlos (Guest)


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I'm not using built in silicon multipliers like Mult18x18 or similar. I' 
just using combinational logic. Indeed Proasic3 does not have 
multipliers.

Parallel multiplication is done by the numeric.std library itself. 
Operation done in one cycle. And then it is registreded and 
scaled(discarding lower bits for floating point operations)
   -- Multiply operation
   mult_data <= signed(a) * signed(b);
 ---
     elsif rising_edge(clk) then           -- rising clock edge
       p <= mult_data(mult_data'high downto g_bit_div_p);

Serial is done by quite more complex code. Accumulate, siftt, xor...

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