Hello FPGA group,
I have to create this sum A+2B+C where A, B, and C are 8 bit std_logic_vector as part of a Sobel function.
What is the best way of doing this?
I have been able to get correct results with the following:
signal sum : std_logic_vector(9 downto 0); begin sum
Umm, you're *not* 'doing 8 bit work'?!?
-- | Mark McDougall | "Electrical Engineers do it | | with less resistance!"
Hello Mark,
OK, it seems to me that the best solution is one eight bit adder with a carry, going into a nine bit adder (maybe not if I don't care about the lower bits) with a carry, giving me a 10 bit sum.
My question is (was), does this VHDL give me
10 bit adders throughout, instead of an optimised solution, on a Xilinx XST platform?And is this, overall, the best way of doing a sum like this?
Mark, is that clearer?
Brad
From the software I know, it likely will generate that, but during place and route any redundant gates will be removed.
The result, then, should be just what you say, unless the addition is done in a different order.
Systems I know about will remove FF's with constant inputs, combine registers with the same inputs, remove gates where all the inputs, or all but one are constant, eliminate inverters by inverting the output of the preceding device, and probably more. Usually there are messages telling what it is doing.
-- glen
[ snip ]
Since your synthesis shows 15 LUTs used, I expect that the sum is the cascade of 2 9-bit adders. Consider that A+C is a 9 bit value but the LSbit is added to zero so it can pass straight through without going to another adder. The second adder takes the top 8 bits of the 9-bit A+C result and adds it to the 8-bit shifted B value. I would have expected 16 LUTs in the two 8-bit adders (with "free" carry-outs) but the synthesizer might have combined something for the LSbits saving you a LUT.
I use Synplify for my synthesis and go to the HDL Analyzer to check the synthesizer's implementation when I have efficiency concerns. I get full visibility into the technology level implementation, no questions.
You're probably doing great on efficiency.
... sorry: Two 8-bit adders with 9-bit results.
Thanks.
And how much does Synplify cost?
An answer not posted to the group:
Macros are not components that can be instantiated in HDL. Macros are only available in ECS, the Xilinx schematic entry tool.
One way to see how Xilinx implements the ADD8 macros is to create the macro using ECS and then looking at the HDL output of ECS (*.VHF file generated after synthesis).
Their sales folks will be happy to talk to you.
Be sure to consider the Synplify or Synplify Pro product with HDL Analyst - an add-on.
While I use Synplify (non-pro) there are benefits with retiming and auto-constraints for aggressive performance in the Pro product worth considering since so much time is spent trying to eke out that last nanosecond in some paths.
In the XACT days there were ways of doing this. You make an empty HDL module with the appropriate I/Os, and if you want to simulate it you can include non-synthesizable code.
At some point after synthesis and before place and route you tell it to remove the dummy and substitute the macro version instead.
It might be that a similar method is available with current tools.
-- glen
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