Page 85 - Mechatronic Systems Modelling and Simulation with HDLs

74 4 MODELLING IN HARDWARE DESCRIPTION LANGUAGES

END LOOP;
WAIT UNTIL rising_edge(clk); -- Wait for rising edge
q <= res; -- Signal assignment for output
END PROCESS;
END behaviour2;
Hardware description 4.7 Behavioural description of a multiplier on the basis of moving
and adding

Digital signal assignment

Up until now we have based our description of a signal assignment upon an intu-
itive understanding, which in some cases can be deceptive. This can be clarified by
looking at a simple inverter gate. The function of the inverter is quickly described.
However, in some cases this does not achieve the desired result. The inverter may
have a delay time of 100 picoseconds. If a pulse of one picosecond occurs at its
input then we would assume in the first approximation that this pulse would be
observed in the opposite polarity at the output 100 picoseconds later. However, this
is not physically correct because the pulse is much too short to effect a change
at the output. Before this has moved to a significant degree, the cause has disap-
peared again. In order to bring about this ‘inert’ behaviour it is necessary for each
signal assignment to evaluate the right-hand side correctly and to draw up a list of
current and future events. If necessary, the future events may have to be deleted
again before they are realised. This is also the case, for example, if the right-hand
side always produces an assignment with the same value, so that a formal assign-
ment yields no new information for the signal. In this case we can postpone the
assignment, so that no events without information content are produced. This task
and others are undertaken by the so-called signal driver.

4.5.4 Analogue modelling

Introduction

We can differentiate between three classical applications of analogue modelling,
see Vachoux and Berge [406]. Firstly, and self-evidently, it is implemented when
the system under investigation consists wholly or partially of analogue compo-
nents. But even when looking at digital systems, the consideration of an analogue
environment of the circuit may still be necessary. Finally, analogue effects, such
as signal delays or coupling capacitances, often cannot be disregarded especially
for digital high-speed circuits.
Again, the extremely different levels of abstraction can be represented. Thus, on
the purely behavioural level we can provide models based upon transfer functions
or differential equations. At a lower level of abstraction, so-called macromodels are
often used, which may represent the standard blocks of analogue circuit design, e.g.