[ISE8.2] DIFF_TERM and unused pin

Tim Verstraete schrieb:

the best thing possible is to use it without using it :)

1) like route the unused input to non-bonded IO, 2) or use in some net in way that the signal isnt really used but XST fails to optimize it out 3) or if you dont use BSCAN you can also route it to TDO pin

all those tricks would keep the net alive. sure it would use some interconnect resources.

Antti

Indeed, that was what i was doing until now ... i just hoped that somewhere there was a constraint that you could use to keep it unoptimized ....

thanks anyway ...

kind regards,

Tim

Antti wrote:

You could try turning off the "Trim unconnected signals" option.

---Matthew Hicks

Look into "keep" attributes. See the Constraints Guide for details.

(Sometimes "keep" attributes don't work though. On registers, they are overridden by "equivalent_register_removal" and result in an obscure error message instead. I haven't tried them on clock buffers)

- Brian

i also thought that but then i read the description:

KEEP Description

KEEP is an advanced mapping constraint and synthesis constraint. When a design is mapped, some nets may be absorbed into logic blocks. When a net is absorbed into a block, it can no longer be seen in the physical design database. This may happen, for example, if the components connected to each side of a net are mapped into the same logic block. The net may then be absorbed into the block containing the components. KEEP prevents this from happening.

KEEP is translated into an internal constraint known as NOMERGE when targeting an FPGA. Messaging from the implementation tools will therefore refer to the system property NOMERGE?not KEEP.

and tried it out but no change ...

i think i will go for antti's idea and just make some small logic to an unbound pin so it does not get removed ...

too bad there is no constraint for this ...

kind regards,

Tim

Brian Drumm> >

indeed but then it does not optimize the part i do want to be optimized ... because it is a global option ... i was just hoping there was some constraint somewhere ...

kind regards,

Tim

yttrium schrieb:

there is ASFAIK no way to use constraints to set options on used by the logic pins.

this is sometimes real problem - you may want to have different unused pins to have optional pulldowns or pullups but this cant be specified unless those pins are used by the design

it would be possible by patching the bitstream, but I am little behind with my bitpatch application at the moment

Antti