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Unit Delay

Delay signal one sample period

  • Unit Delay block

Simulink / Discrete
HDL Coder / Discrete


The Unit Delay block holds and delays its input by the sample period you specify. When placed in an iterator subsystem, it holds and delays its input by one iteration. This block is equivalent to the z-1 discrete-time operator. The block accepts one input and generates one output. Each signal can be scalar or vector. If the input is a vector, the block holds and delays all elements of the vector by the same sample period.

You specify the block output for the first sampling period with the Initial conditions parameter. Careful selection of this parameter can minimize unwanted output behavior. You specify the time between samples with the Sample time parameter. A setting of -1 means the block inherits the Sample time.


The Unit Delay block errors out if you use it to create a transition between blocks operating at different sample rates. Use the Rate Transition 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

String Support

The Unit Delay block can accept and output string data type only if the block is configured for the default value of the Initial condition parameter (0).



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Input signal that the block delays by one sample period.

Data Types: half | single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | string | Boolean | fixed point | enumerated | bus | image


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Output signal that is the input delayed by one sample period.

Data Types: half | single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | string | Boolean | fixed point | enumerated | bus | image


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Specify the output of the simulation for the first sampling period, during which the output of the Unit Delay block is otherwise undefined.

Programmatic Use

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

Specify whether the block performs sample- or frame-based processing:

  • Columns as channels (frame based) — Treat each column of the input as a separate channel (frame-based processing).


    Frame-based processing requires a DSP System Toolbox™ license.

    For more information, see Sample- and Frame-Based Concepts (DSP System Toolbox).

  • Elements as channels (sample based) — Treat each element of the input as a separate channel (sample-based processing).

Use Input processing to specify whether the block performs sample- or frame-based processing. For more information about these two processing modes, see Sample- and Frame-Based Concepts (DSP System Toolbox).

Programmatic Use

Block Parameter: InputProcessing
Type: character vector
Values: 'Columns as channels (frame based)' | 'Elements as channels (sample based)'
Default: 'Elements as channels (sample based)'

Enter the discrete interval between sample time hits or specify -1 to inherit the sample time.

See also Specify Sample Time.

Programmatic Use

Block Parameter: SampleTime
Type: character vector
Value: real scalar
Default: '-1'

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 C Data Code Interface Configuration for Model Interface Elements (Simulink Coder).

Programmatic Use

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

Specify whether state names are required to resolve to signal objects. If selected, the software generates an error at run time if you specify a state name that does not match the name of a signal object.

Selecting this parameter disables the Code generation storage class parameter.


Enabled when you specify a value for the State name parameter and set the Signal resolution model configuration parameter to a value other than None.

Programmatic Use

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

Block Characteristics

Data Types

Boolean | bus | double | enumerated | fixed point | half | image | integer | single | string

Direct Feedthrough


Multidimensional Signals


Variable-Size Signals


Zero-Crossing Detection


a Ports of this block have different direct feedthrough characteristics.

Extended Capabilities

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

Fixed-Point Conversion
Design and simulate fixed-point systems using Fixed-Point Designer™.

Version History

Introduced before R2006a