On 1/10/2013 8:50 AM, GaborSzakacs wrote:
> rickman wrote:
>> How many have used the iCECube2 tool? I am just getting started with
>> it and it seems to be less than optimal. It uses Synplify Pro for
>> synthesis. I wrote code for a simple enabled up/down counter which I
>> expect would use one LUT per bit. But it seems to be using two LUTs
>> per. Instead of running the enable to the register, it is adding the
>> enable into the logic to control whether the bit is inverted or not.
>> This requires an extra input and so an extra LUT per bit. Here is the
>> code involved.
>>
>> ...
>> PORT (
>> SysRst : in std_logic;
>> RateHigh : in std_logic;
>> RateEn : in std_logic;
>> ...
>> signal VFOclkout : std_logic;
>> signal PhaseStepReg : signed (RESOLUTION - 1 downto 0);
>> ...
>> PROCESS (VFOclkout, SysRst) begin
>> if (SysRst ='1') then
>> PhaseStepReg <= to_signed(-VFOSTEPSIZE, RESOLUTION);
>> elsif (rising_edge(VFOclkout)) then
>> if (RateEn = '1') then
>> if (RateHigh = '0') then
>> PhaseStepReg <= PhaseStepReg + 1;
>> else
>> PhaseStepReg <= PhaseStepReg - 1;
>> end if;
>> end if;
>> end if;
>> end process;
>> ...
>>
>> The output, PhaseStepReg, is used in another process. I can't see
>> anything that would affect the synthesis of this counter.
>>
>> I'm wondering if there is something about the architecture of these
>> parts that precludes an up/down counter in one LUT/bit. They use a
>> single input on a register bank for all enables in common, but could
>> that really make them use an extra bank of logic? I may simplify this
>> code to just the up/down counter and send it off to support. So far I
>> haven't gotten much from Lattice support other than the local FAE.
>> Seems they are in a time zone about 12 hours off from mine.
>>
>> Rick
>
> It's been a while since I used any Lattice tools, but I seem to remember
> that Synplify for Lattice has a compiler directive that encourages it
> to use a particular signal as a clock enable. In your case you could
> apply this to RateEn and see if it causes the synthesizer to recognize
> it as an enable to reduce the LUT count. That being said, I have not
> used the ICE series parts and there may still be another reason why
> the tools have not inferred clock enables.
>
> -- Gabor
Thanks for the reply. You might be right. A simple divide by two FF
also uses the logic to implement an enable. I'll look into the
directive. Seems rather retarded really. Most of the time the
synthesizer will even add logic to the enable and with these tools it
doesn't want to use the enable at all.
I'll give the directive a try.
I did put in a ticket for this and we'll see what they say. The iCE40
parts started out with their own tools so it's hard to say what might
have happened when they were brought over to Lattice.
Rick
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