mdlInitializeSampleTimes (Writing S-Functions)

Specify the sample rates at which this S-function operates.

Syntax

void mdInitializeSampleTimes(SimStruct *S)

Arguments
S
Simstruct representing an S-function block.

Description

This method should specify the sample time and offset time for each sample rate at which this S-function operates via the following paired macros

ssSetSampleTime(S, sampleTimeIndex, sample_time)
ssSetOffsetTime(S, offsetTimeIndex, offset_time)

where sampleTimeIndex runs from 0 to one less than the number of sample times specified in mdlInitializeSizes via ssSetNumSampleTimes.

If the S-function operates at one or more sample rates, this method can specify any of the following sample time and offset values for a given sample time:

[CONTINUOUS_SAMPLE_TIME, 0.0]

[CONTINUOUS_SAMPLE_TIME, FIXED_IN_MINOR_STEP_OFFSET]

[discrete_sample_period, offset]

[VARIABLE_SAMPLE_TIME, 0.0]

The upper case values are macros defined in simstruc.h.

If the S-function operates at one rate, this method can alternatively set the sample time to one of the following sample/offset time pairs.

[INHERITED_SAMPLE_TIME, 0.0]

[INHERITED_SAMPLE_TIME, FIXED_IN_MINOR_STEP_OFFSET]

If the number of sample times is 0, Simulink assumes that the S-function inherits its sample time from the block to which it is connected, i.e., that the sample time is

[INHERITED_SAMPLE_TIME,  0.0]

This method can therefore return without doing anything.

Use the following guidelines when specifying sample times.

A continuous function that changes during minor integration steps should set the sample time to

[CONTINUOUS_SAMPLE_TIME, 0.0]

A continuous function that does not change during minor integration steps should set the sample time to

[CONTINUOUS_SAMPLE_TIME, FIXED_IN_MINOR_STEP_OFFSET]

A discrete function that changes at a specified rate should set the sample time to

[discrete_sample_period, offset]

where

discrete_sample_period > 0.0

and

0.0 <= offset < discrete_sample_period

A discrete function that changes at a variable rate should set the sample time to

[VARIABLE_SAMPLE_TIME, 0.0]

Simulink invokes mdlGetTimeOfNextVarHit function to get the time of the next sample hit for the variable step discrete task.

Note that VARIABLE_SAMPLE_TIME requires a variable step solver.

To operate correctly in a triggered subsystem or a periodic system, a discrete S-function should:

Specify a single sample time set to

[INHERITED_SAMPLE_TIME, 0.0]

Set the SS_DISALLOW_CONSTANT_SAMPLE_TIME simulation option in mdlInitializeSizes

Verify that it was assigned a discrete or triggered sample time in mdlSetWorkWidths:

if (ssGetSampleTime(S, 0) == CONTINUOUS_SAMPLE_TIME) {

ssSetErrorStatus(S,

"This block cannot be assigned a continuous sample time");

}

After propagating sample times throughout the block diagram, Simulink assigns the sample time

[INHERITED_SAMPLE_TIME, INHERITED_SAMPLE_TIME]

to discrete blocks residing in triggered subsystems.

If this function has no intrinsic sample time, it should set its sample time to inherited according to the following guidelines:

A function that changes as its input changes, even during minor integration steps, should set its sample time to

[INHERITED_SAMPLE_TIME, 0.0]

A function that changes as its input changes, but doesn't change during minor integration steps (i.e., held during minor steps) should set its sample time to

[INHERITED_SAMPLE_TIME, FIXED_IN_MINOR_STEP_OFFSET]

The S-function should use the ssIsSampleHit or ssIsContinuousTask macros to check for a sample hit during execution (in mdlOutputs or mdlUpdate). For example, if the block's first sample time is continuous, the function can use the following code-fragment to check for a sample hit.

if (ssIsContinuousTask(S,tid)) {
}

Note The function would get incorrect results if it used ssIsSampleHit(S,0,tid).

If the function wanted to determine if the third (discrete) task has a hit, it could use the following code-fragment.

if (ssIsSampleHit(S,2,tid) {
}

Languages

mdlInitializeConditions mdlInitializeSizes