National Instruments AutoCode NI MATRIX User Manual
Page 31
Chapter 2
C Language Reference
2-14
ni.com
The operations within UCBs are controlled by the argument
INFO
, a pointer
to a structure of type
STATUS_RECORD
that is passed as part of the
argument list for each UCB (located in
sa_types
):
typedef struct STATUS_RECORD
{
RT_INTEGER ERROR;
RT_BOOLEAN INIT;
RT_BOOLEAN STATES;
RT_BOOLEAN OUTPUT;
}
RT_STATUS_RECORD;
The following example shows the general form of UCB equations for
AutoCode and indicates how the
INFO
status record pointer is used to
control the computations.
if (INFO->INIT) {
/* do user code initialization */
INFO->INIT = 0;
}
if (INFO->OUTPUTS) {
/* do output calculations having the general form:
Y = H(T,X,XD,U,RP,IP); */
}
if (INFO->STATES) {
/* do state update calculation with the general form:
XD = F(T,X,XD,U,RP,IP); */
}
When an error occurs within the UCB, set
INFO->ERROR
equal to some
nonzero integer value as an error return. Also, make sure that
INFO->INIT
is set to FALSE (0) at the end of the initialization cycle.
The process of linking handwritten UCBs (in
usr_dsp.c
) with an
AutoCode application (in
dsp.c
) is depicted in Figure 2-2. The
SystemBuild model in Figure 2-2 depicts the filter described by the
difference equation:
y(k) = –1/2 * y(k – 2) + 3 * u(k) + u(k – 2)
The UserCode Block provides the interface to the handwritten function
usr_dsp( )
implemented in the file
usr_dsp.c
. The
usr_dsp( )
function essentially implements the behavior of the same filter.