RE: @above in analyses other than tran and dc sweep

Hi Mark,

Your remark on parallelization is a good one. A reasonable approach to improve performance of a DC sweep on a multi-threaded or multi-CPU system is break up the sweep in successive chunks and assign each of these chunks to a separate thread or core. However, the assumptions apparently underlying the DC sweep for circuits with hysteresis , but also the handling of global events for DC sweep in Verilog-A invalidate this approach! Verilog-AMS is essentially confined to single-threaded execution by the standard. Perhaps (... opening another can of worms ...) there's some room to reconsider some of the standard text in the light of multi-threaded execution.

Cheers,
Marq

-----Original Message-----
From: owner-verilog-ams@server.eda.org [mailto:owner-verilog-ams@server.eda.org] On Behalf Of Mark Zwolinski
Sent: Thursday 19 November 2009 11:50
To: Geoffrey.Coram
Cc: Ian Wilson; verilog-ams@eda.org
Subject: Re: @above in analyses other than tran and dc sweep



Geoffrey.Coram wrote:
> The existence of multiple dc operating points is generally
> swept under the rug.  The fact is, the results of the
> simulation of such a circuit can depend on the initial
> guess in Newton's method, or whether one is even using
> Newton or another solution method.
> 

I agree.


> So, you can either decide that @above has no meaning, because
> you expect your simulator to evaluate a dc sweep by randomly
> picking an order to evaluate the points, or you can be
> pragmatic about it and decide that every spice-like
> simulator I know of uses the previous sweep point as a
> starting point for the current one, and therefore @above
> could give you useful information.
> 

In practice, I agree. But...

> I suppose there's really no reason to assume that a simulator
> uses Newton's method to solve the circuit, in which case the
> fact that the ddx() operator requests the symbolic derivative
> is making some unwarranted assumption about how the simulator
> works; how dare we assume it needs a Jacobian?
> 

... this is the problem. Clause 8.2.2 states "Most circuit simulators
use the Newton-Raphson (NR) method to solve this system." To me that 
says "don't assume".

The point about the DC (or AC) sweep is that there is an unwritten 
assumption. That makes me uncomfortable. On this exact point, there may 
never be a problem because the pragmatic way to implement a sweep is to 
do it in order and there is no clear incentive to parallelize the algorithm.

The general point, however, is that Verilog-AMS (and VHDL-AMS) 
deliberately does not specify the algorithms used. That's OK. We know 
that in practice and for the foreseeable future, Verilog-AMS will be 
sitting on top of a SPICE-like (and probably SPICE-derived) simulator. 
OK, but there are assumptions. Some of those assumptions are not written 
down anywhere. I cannot find *any* written evidence (other, I guess, 
than source code) that a sweep is performed in a monotonic order.

The use of "can" or "may" might be correct according to the strict 
interpretation of the IEEE guidelines. In the context of clause 
5.10.3.2, there is an implicit assumption being made. But the next 
sentence makes the algorithm explicit:
"During a dc sweep, the above() function shall also generate an event 
when the expression crosses zero from below; however, the
step size of the dc sweep is not controlled to accurately resolve the 
crossing." Notice the use of the word "shall".

In other words, the algorithm is defined. This would seem to go against 
the philosophy of the LRM.


One other point. I did a Google search on "spice sweep hysteresis" and 
found this from "SPICE for power electronics and electric power" by M. 
H. Rashid, Hasan M. Rashid, p499 on Google Books: "The most common cause 
of failure of the DC sweep analysis is an attempt to analyze a circuit 
with regenerative feedback, such as a Schmitt trigger. The DC sweep is 
not appropriate for calculating the hysteresis of such circuits, because 
it is required to jump discontinuously from one solution to another at 
the crossover point." It's slightly off-topic, but they've clearly been 
burned by this. A DC sweep is not the right way to explore hysteresis!

Mark

> -Geoffrey
> 
> 
> Ian Wilson wrote:
>> Suppose we run two simulations at values a,b,c for some swept variable, x.
>>
>> Surely the results for the simulation with x=b should be the same 
>> regardless of
>> whether x had the value a or c in some previous simulation?
>>
>> I would expect all DC simulations of a circuit with hysteresis to 
>> produce the
>> same result (for the same parameter values), since there is no concept 
>> of a previous
>> state (unless you allow the simulator to start from the operating point 
>> calculated in some
>> other step).
>>
>> --ian
> 
> 

-- 
===================================================================
Professor Mark Zwolinski
Electronic Systems & Devices Group       Tel. (+44) (0)23 8059 3528
Electronics & Computer Science           Fax. (+44) (0)23 8059 2901
University of Southampton                Email.  mz@ecs.soton.ac.uk
Southampton SO17 1BJ, UK             http://www.ecs.soton.ac.uk/~mz

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