Synthesis results when testing for 'X' and 'U'

On Jun 19, 9:01 am, Brian Drummond <brian_drumm...@btconnect.com>
wrote:
> On Wed, 18 Jun 2008 10:10:24 -0700 (PDT), rickman <gnu...@gmail.com>
> wrote:
>
> >Is it important to not use an initial condition controlled by a global
> >reset?  In FPGAs this is typically free, or almost free to use it
> >properly.  In ASICs I guess it is a different matter.
>
> Not sure I understand the question. If you mean use the global reset
> signal to suppress warnings; it won't solve the whole problem.
>
> It'll suppress warnings while Reset is active, but I need to suppress
> warnings for a couple of dozen cycles after Reset. Resetting pipeline
> registers is unnecessary and inefficient (it prevents packing into
> SRL16s for example) so "XXXX" is to be expected for a small but
> deterministic time after reset while the pipelines flush.

I am asking if there is a reason to *not* use the GSR to control the
initial state of the FFs.  I am not sure I understand about the SRLs.
They are implemented using the configuration memory in the LUT.  That
is definitely loaded on power up.  I guess it is not set by the GSR,
but it is well defined even if not controllable from your code.  Do
you need a way to control the values of SRL elements other than on
configuration?

If the SRL is in a determined state on configuration, but this state
can not be restored by user reset of any sort, how does Xilinx
recommend that you model this?

I assume there is some logic in front of the SRL, but that is not
certain I expect.  Either way, each SRL only has a single input and
you can use another LUT to provide a "clamp" to the input or output to
prevent the spread of the 'X' state.  I know you don't want to use
extra resources, but personally, I find that preferable to fighting
the tools (like I am doing now).

Rick

rickman wrote:

> I am asking if there is a reason to *not* use the GSR to control the
> initial state of the FFs.  I am not sure I understand about the SRLs.
> They are implemented using the configuration memory in the LUT.  That
> is definitely loaded on power up.  I guess it is not set by the GSR,
> but it is well defined even if not controllable from your code.  Do
> you need a way to control the values of SRL elements other than on
> configuration?

I don't use the GSR or SRL because they are non-portable and fussy.
I save time by using an explicit reset of every register.

> I know you don't want to use
> extra resources, but personally, I find that preferable to fighting
> the tools (like I am doing now).

I agree with you there.
I prefer saving time to
saving some extra unused flops.

       -- Mike Treseler