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Memory

Output input from previous time step

  • Library:
  • Simulink / Discrete

Description

The Memory block holds and delays its input by one major integration time step. When placed in an iterator subsystem, it holds and delays its input by one iteration. This block accepts continuous and discrete signals. The block accepts one input and generates one output. Each signal can be a scalar, vector, matrix, or N-D array. If the input is non-scalar, the block holds and delays all elements of the input by the same time step.

You specify the block output for the first time step using the Initial condition parameter. Careful selection of this parameter can minimize unwanted output behavior. However, you cannot specify the sample time. This block’s sample time depends on the type of solver used, or you can specify to inherit it. The Inherit sample time parameter determines whether sample time is inherited or based on the solver.

Tip

Avoid using the Memory block when both these conditions are true:

  • Your model uses the variable-step solver ode15s or ode113.

  • The input to the block changes during simulation.

When the Memory block inherits a discrete sample time, the block is analogous to the Unit Delay block. However, the Memory block does not support state logging. If logging the final state is necessary, use a Unit Delay block instead.

Comparison with Similar Blocks

The Memory, Unit Delay, and Zero-Order Hold blocks provide similar functionality but have different capabilities. Also, the purpose of each block is different.

This table shows recommended usage for each block.

BlockPurpose of the BlockReference Examples
Unit DelayImplement a delay using a discrete sample time that you specify. The block accepts and outputs signals with a discrete sample time.
MemoryImplement a delay by one major integration time step. Ideally, the block accepts continuous (or fixed in minor time step) signals and outputs a signal that is fixed in minor time step.
Zero-Order HoldConvert an input signal with a continuous sample time to an output signal with a discrete sample time.

Each block has the following capabilities.

CapabilityMemoryUnit DelayZero-Order Hold
Specification of initial conditionYesYesNo, because the block output at time t = 0 must match the input value.
Specification of sample timeNo, because the block can only inherit sample time from the driving block or the solver used for the entire model.YesYes
Support for frame-based signalsNo YesYes
Support for state loggingNoYesNo

Bus Support

The Memory block is a bus-capable block. The input can be a virtual or nonvirtual bus signal subject to the following restrictions:

  • Initial condition must be zero, a nonzero scalar, or a finite numeric structure.

  • If Initial condition is zero or a structure, and you specify a State name, the input cannot be a virtual bus.

  • If Initial condition is a nonzero scalar, you cannot specify a State name.

For information about specifying an initial condition structure, see Specify Initial Conditions for Bus Signals.

All signals in a nonvirtual bus input to a Memory block must have the same sample time, even if the elements of the associated bus object specify inherited sample times. You can use a Rate Transition block to change the sample time of an individual signal, or of all signals in a bus. See Specify Bus Signal Sample Times and Bus-Capable Blocks for more information.

You can use an array of buses as an input signal to a Memory block. You can specify the Initial condition parameter with:

  • The value 0. In this case, all the individual signals in the array of buses use the initial value 0.

  • An array of structures that specifies an initial condition for each of the individual signals in the array of buses.

  • A single scalar structure that specifies an initial condition for each of the elements that the bus type defines. Use this technique to specify the same initial conditions for each of the buses in the array.

For details about defining and using an array of buses, see Combine Buses into an Array of Buses.

Ports

Input

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Input signal, specified as a scalar, vector, matrix, or N-D array. The input can be continuous or discrete, containing real, or complex values of any data type Simulink® supports.

Data Types: single | double | int8 | int16 | int32 | uint8 | uint16 | uint32 | Boolean | fixed point | enumerated | bus

Output

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Output is the input from the previous time step.

Data Types: single | double | int8 | int16 | int32 | uint8 | uint16 | uint32 | Boolean | fixed point | enumerated | bus

Parameters

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Main

Specify the output at the initial integration step. This value must be 0 when you do not use a built-in input data type. Simulink does not allow the initial output of this block to be inf or NaN.

Programmatic Use

Block Parameter: InitialCondition
Type: character vector
Values: scalar | vector
Default: '0'

Select to inherit the sample time from the driving block:

  • If the driving block has a discrete sample time, the block inherits the sample time.

  • If the driving block has a continuous sample time, selecting this check box has no effect. The sample time depends on the type of solver used for simulating the model.

When this check box is cleared, the block sample time depends on the type of solver used for simulating the model:

  • If the solver is a variable-step solver, the block sample time is continuous but fixed in minor time step: [0, 1].

  • If the solver is a fixed-step solver, the [0, 1] sample time converts to the solver step size after sample-time propagation.

Programmatic Use

Block Parameter: InheritSampleTime
Type: character vector
Values: 'off' | 'on'
Default: 'off'

Select to output the input during linearization and trim. This selection sets the block mode to direct feedthrough.

Selecting this check box can cause a change in the ordering of states in the model when using the functions linmod, dlinmod, or trim. To extract this new state ordering, use the following commands.

First compile the model using the following command, where model is the name of the Simulink model.

    [sizes, x0, x_str] = model([],[],[],'lincompile'); 

Next, terminate the compilation with this command.

  model([],[],[],'term'); 

The output argument, x_str, which is a cell array of the states in the Simulink model, contains the new state ordering. When passing a vector of states as input to the linmod, dlinmod, or trim functions, the state vector must use this new state ordering.

Programmatic Use

Block Parameter: LinearizeMemory
Type: character vector
Values: 'off' | 'on'
Default: 'off'

Select to linearize the Memory block to a unit delay when the Memory block is driven by a signal with a discrete sample time.

Programmatic Use

Block Parameter: LinearizeAsDelay
Type: character vector
Values: 'off' | 'on'
Default: 'off'

State Attributes

Use this parameter to assign a unique name to the block state. The default is ' '. When this field is blank, no name is assigned. When using this parameter, remember these considerations:

  • A valid identifier starts with an alphabetic or underscore character, followed by alphanumeric or underscore characters.

  • The state name applies only to the selected block.

This parameter enables State name must resolve to Simulink signal object when you click Apply.

For more information, see Apply Storage Classes to Individual Signal, State, and Parameter Data Elements (Simulink Coder).

Programmatic Use

Block Parameter: StateName
Type: character vector
Values: unique name
Default: ''

Select this check box to require that the state name resolves to a Simulink signal object.

Dependencies

To enable this parameter, specify a value for State name. This parameter appears only if you set the model configuration parameter Signal resolution to a value other than None.

Selecting this check box disables Code generation storage class.

Programmatic Use

Block Parameter: StateMustResolveToSignalObject
Type: character vector
Values: 'off' | 'on'
Default: 'off'

Choose a custom storage class package by selecting a signal object class that the target package defines. For example, to apply custom storage classes from the built-in package mpt, select mpt.Signal. Unless you use an ERT-based code generation target with Embedded Coder®, custom storage classes do not affect the generated code.

If the class that you want does not appear in the list, select Customize class lists. For instructions, see Target Class Does Not Appear in List of Signal Object Classes (Embedded Coder).

For information about storage classes, see Apply Storage Classes to Individual Signal, State, and Parameter Data Elements (Simulink Coder). For information about custom storage classes, see Apply Custom Storage Classes to Individual Signal, State, and Parameter Data Elements (Embedded Coder).

Programmatic Use

Block Parameter: StateSignalObject
Type: character vector
Values: 'Simulink.Signal' | '<StorageClass.PackageName>'
Default: 'Simulink.Signal'

Select state storage class for code generation.

Use Signal object class to select custom storage classes from a package other than Simulink.

Dependencies

To enable this parameter, specify a value for State name.

Programmatic Use

Block Parameter: StateStorageClass
Type: character vector
Values: 'Auto' | 'SimulinkGlobal' | 'ExportedGlobal' | 'ImportedExtern' | 'ImportedExternPointer' | 'Custom' | ...
Default: 'Auto'

Specify a storage type qualifier such as const or volatile.

Note

TypeQualifier will be removed in a future release. To apply storage type qualifiers to data, use custom storage classes and memory sections. Unless you use an ERT-based code generation target with Embedded Coder, custom storage classes and memory sections do not affect the generated code.

During simulation, the block uses the following values:

  • The initial value of the signal object to which the state name is resolved

  • Min and Max values of the signal object

For more information, see Data Objects.

Dependencies

To enable this parameter, set Code generation storage class to ExportedGlobal, ImportedExtern, ImportedExternPointer, or Model default. This parameter is hidden unless you previously set its value.

Programmatic Use

Block Parameter: RTWStateStorageTypeQualifier
Type: character vector
Values: '' | 'const' | 'volatile' | ...
Default: ''

Block Characteristics

Data Types

double | single | Boolean | base integer | fixed point | enumerated | bus

Direct Feedthrough

No

Multidimensional Signals

Yes

Variable-Size Signals

No

Zero-Crossing Detection

No

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

PLC Code Generation
Generate Structured Text code using Simulink® PLC Coder™.

Fixed-Point Conversion
Convert floating-point algorithms to fixed point using Fixed-Point Designer™.

Introduced before R2006a