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Re: [Discuss-gnuradio] Help: Feedback causing "Flow graph loops"

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Help: Feedback causing "Flow graph loops"
Date: Wed, 14 Aug 2013 17:10:24 -0400

On Fri, Aug 9, 2013 at 1:02 PM, Martin Lülf <address@hidden> wrote:
> Hi,
> since I also had trouble with this issue at first, please allow me to
> clarify this further: GNURadio does not support looping of blocks. Inside a
> single block you can loop as often as you wish and that is exactly how the
> available PLLs in GNURadio are made. Just search the mailing list for more
> details, since this topic has been discussed a couple of times already.
> Yours
> Martin
> Am 09.08.2013 17:17, schrieb Martin Braun (CEL):
>> On Fri, Aug 09, 2013 at 07:31:58AM -0700, tom sutherland wrote:
>>> I am trying to make a simple PLL using a VCO but I get an error
>>> "RuntimeError:flow graph has loops!". Anyway around that?
>> Nope, stock GNU Radio does not support loops. There's plenty of PLLs
>> available already, though.
>> MB

Yes. From a scheduler standpoint, loops, as in traditional control
loops with some required and guaranteed delay, are incredibly
inefficient. And instead of figuring out how to support something like
that, Eric made the decision very early on to just not allow them at

You can use the asynchronous message passing interface to do loops.
However, it's asynchronous, so you're not going to be building a PLL
or something like that which mathematically falls apart if you have
more than 1 sample delay in it. But you can at least use it to feed
back data/metadata/control info if you don't need that kind of
requirement. But if you do, follow Martin and Martin's advice and do
it inside of a block.

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