Lecture Notes in Computer Science 3472

396 Axel Belinfante, Lars Frantzen, and Christian Schallhart
node COUNTER(val_init, val_incr : int; reset : bool)
returns (n : int);
let
n = val_init -> if reset then val_init else pre(n)+val_incr;
tel;
This example shows the two more fundamental time operators of Lustre 2 .The
first operator -> is the followed-by operator. If A and B have the respective
sequence of values a0, a1,... and b0, b1,... then A -> B declares the sequence
a0, b1, b2,.... Therefore, in the example, the flow of n starts with the first value
of val init.
The second time operator in the example is pre. GivenaflowA with the values
a0, a1,..., pre(A) is the flow with the values nil, a0, a1,.... So in the code above,
we find that if reset is true, then n is set to the current value of val init.
Otherwise n is set to the previous value of n plus the increment val incr. Two
simple applications of this node are the following two sequences.
even=COUNTER(0,2,false);
mod5=COUNTER(0,1,pre(mod5)=4);
The first sequence generates the even numbers, and the second cycles through
the numbers between 0 and 4. Note that the reset input is indeed fed with
another flow.
To approximate the position of an accelerating vehicle, we can use the following
two flows
speed=COUNTER(0,acceleration,false);
position=COUNTER(0,speed,false);
Note that speed used as input in the second statement is a flow which is changing
over time. E.g. if acceleration is the constant flow with value 4, then
speed would be the sequence 0,4,8,12,16,. . . , and position would start with
0,4,12,24,40,.. .
Testing Method
The construction of the test sequence generation is formally described in the
paper [dBORZ99]. Basically, a test sequence generator built by Lutess is based
on an environment description given in Lustre and a set of further (probabilistic)
2 Lustre also offers two other operators, namely when and current. These operators
allow the manipulation of the clock of a dataflow. Each dataflow in Lustre has an
associated clock which determines when a new value is added to the corresponding
flow. For example, a flow with the clock true, false, true,... would be expanded
by a new value every second cycle. The when operator allows to declare a sequence
which runs with a slower clock, while the current operator allows to interpolate a
flow with a slow clock such that it becomes accessible for recombination with faster
flows.