Re: FLOW disciplines and KCL

Thanks Marq...

One comment, I remember a statement that in verilog-A,
ALL currents are defined as INTO the block..
Thus 
I(a) <+ something;
is the same as 
I(a,gnd!) <+ something;
 If something is positive, the current is INTO the
block.. 
Otherwise its OUTOF. 
this is a completely different thing than the port
direction.. input, output or inout.. which are defined
for the convenience of checking that inputs are driven
by an output, and outputs don't drive other outputs.

If I didn't have to get to an appointment, I'd look
that up now. 

The way that Verilog-A is defined, with node 0 always
being available, and considering 1 block as a
"collection of nodes" for the purposes of KCL.. any
mismatch in the sum of the currents is conveniently
the current to node 0..  If you model carefully you
can avoid useing node 0, and then KCL will have to
apply.. but you will probably need at least 1
reference node besides  "in" and "out" especially if
you are doing a flow discipline for all pins.

Cheers all, sorry I've been missing the calls, we're
getting close to another tapeout.

Jonathan

--- Marq Kole <marq.kole@philips.com> wrote:

> Jonathan,
> 
> A resulting limitation for the signal flow
> disciplines connecting to a 
> conservative discipline would be that a signal flow
> node can have only one 
> conservative instance connected to it, and that all
> signal-flow instances 
> need to have the same port direction, i.e. all in or
> all out.
> 
> Should an inout port direction not be allowed for
> signal flow models: it 
> has to be either in or out. I can image a model
> where a signal-flow port 
> is either read or driven, dependent on a parameter
> setting, but it cannot 
> read and drive at the same time...
> 
> Regards,
> Marq
> 
> 
> Marq Kole
> Competence Leader Analog Simulation, Philips ED&T
> 
> 
> Marq Kole/EHV/RESEARCH/PHILIPS wrote on 02-06-2006
> 16:08:32:
> 
> > Jonathan,
> > 
> > Your reply required some thinking before I could
> answer; I'll also 
> > copy the reflector as I think this is relevant to
> our discussions.
> > 
> > Regards,
> > Marq
> > 
> > 
> > Marq Kole
> > Competence Leader Analog Simulation, Philips ED&T
> > 
> 
> > Jonathan David <jb_david@yahoo.com> wrote on
> 31-05-2006 18:41:25:
> > 
> > > Hi Marq, 
> > > 
> > > thanks for the reply. It looks like you missed
> part of
> > > my point.
> > > 
> > > Let me ask a question;  For a potential nature,
> do you
> > > expect KVL to be obeyed? I do, and I think you
> do
> > > also.. 
> > > V(B) = V(A) + V(B,A)
> > > V(A,gnd) + V(B,A) + V(gnd,B) = 0;
> 
> > This is not necessarily the KVL: in mathematics
> this is also known 
> > as associativity. If you consider 0 to be the
> mathematical ground 
> > i..e reference, then with V(B) = 2 and V(A) = 3
> you say:
> > 
> > 2 = 3 + (2 - 3)
> > (3 - 0) + (2 - 3) + (0 - 2) = 0
> > 
> > > therefor when I flip to the FLOW side, I expect
> KCL to
> > > be obeyed. 
> > > KCL: Sum(I)@node = 0;
> > > 
> > > In fact if it isn't, it wouldn't be possible to
> > > connect the flow type to the flow connection of
> the
> > > compatible conservative discipline. 
> > > 
> > > but your example doesn't show a violation. 
> > > Without the context (how the block is connected)
> we
> > > can't talk about KCL. 
> > > Your example has no context.. its not connected
> up
> > > with any thing else, and without the connection
> nodes,
> > > we have no nodes at which to sum the currents to
> 0.
> > 
> > Yet the block itself is also a node, so the sum of
> currents flowing 
> > into a block should equal the sum of currents
> flowing out of a 
> > block. In a conservative system this would be
> solved by having the 
> > missing current supplied by the ground. That is OK
> as the potential 
> > of the ground stays 0: the energy conservation
> requirement of the 
> > consevative system makes sure that each node
> adheres to the KCL and 
> > each loop of branches to the KVL.
> > 
> > However, in a signal flow environment the ground
> of a flow 
> > discipline needs to be 0; you essentially give
> every flow a value 
> > relative to the reference.
> > 
> > The result is that Tellegen's Theorem (which says
> that the total 
> > energy in a conservative system is equal to 0)
> does not hold for 
> > signal flow disciplines.
> > 
> > > As I read it, 
> > > that model defines a second current in terms of
> a
> > > first. I(in,out) is not being defined as a
> branch
> > > relationship rather 
> > > I(out,0) is being defined in terms of I(in,0).
> > > two separate branchs, neither having to do with
> each
> > > other.. so I don't see any violation of KCL..
> > > Generally in the model I define the branch not
> the
> > > nodes.. 
> 
> > The KCL also applies to every possible collection
> of nodes in a 
> > conservative lumped element system.
> > 
> > > What I mean by flow signal_flow disciplines
> obeying
> > > the KCL, is that the sum of the current
> connections at
> > > a node will be 0.  so you can have N current
> > > discipline sources into a node, as long as there
> are
> > > N+1 connections (only 1 load) 
> > > without the electrical side to serve as a
> resolution
> > > function, I don't see how you could allow N+2 ..
> 
> > > Unless we arbitrarily say that every
> undertermined
> > > branch gets an equal flow.. 
> 
> > Now I see what you're getting at: if I have three
> instances all 
> > connected to the same node,
> > then if they are of a potential signal flow
> discipline, the value is
> > determined by just one of the three connections;
> if they are of a 
> > flow signal flow discipline, the value for one of
> the three should 
> > be determined from the other two.
> > 
> > As there is no energy conservation in a signal
> flow system, there is
> > no possibillity to say how currents will be
> distributed if only one 
> > of the three instances drives a current. It is an
> underdetermined 
> > system (1 known, 3 equations). So here is a
> duality: in the 
> > potential signal flow system there may be at most
> 1 source driving a
> > node, in the flow signal flow system there may be
> at most one sink 
> > collecting from a node.
> > 
> > Under these conditions I can agree that a flow
> signal-flow 
> > discipline adheres to the KCL. Should we then also
> mandate that 
> > directions be provided on all signal flow
> discipline ports so a 
> > check on the above restrictions can be performed?
> > 
> > The KCL does not apply to instances that contain
> sources in a signal
> > flow system: in a conservative system the KCL
> applies also in that case.
> > 
> > > Most simulators I know don't allow this on the
> > > potential side.. 
> > > if you have a loop of nodes, 0 ground, A, B, C,
> D, E,
> > > A
> > > and you define V(A,0), V(A,E) V(B,C) and V(C,D)
> > > (so V(A,B) and V(D,E) are undefined 
> > > KVL requires that the sum of voltages around the
> loop
> > > be 0, but there are two unknowns.. and one
> equation.
> 
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