Sample and Hold

Sample and hold input signal

• Library:

• DSP System Toolbox / Signal Operations

  DSP System Toolbox HDL Support / Signal Operations

• 

Description

The Sample and Hold block acquires the input at the signal port whenever it receives a trigger event at the trigger port (marked by ). The block then holds the output at the acquired input value until the next triggering event occurs.

Ports

Input

In — Signal port

scalar | vector | matrix

The signal port can accept data in the form of a scalar, vector, or matrix.

Dependencies

This port is named In<Lo> when you select the Latch (buffer) input parameter.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | Boolean | fixed point

Trigger — Trigger port

scalar

The trigger input must be a sample-based scalar with sample rate equal to the input frame rate at the signal port. You specify the trigger event using the Trigger type parameter.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | Boolean | fixed point

Output

Port_1 — Sample and hold value

scalar | vector | matrix

Sample and hold output, returned as a scalar, vector, or a matrix. The block acquires input at the signal port whenever it receives a trigger event at the trigger port. The block then holds the acquired data until the next triggering event occurs.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | Boolean | fixed point

Parameters

Trigger type — Type of trigger

Rising edge (default) | Falling edge | Either edge

The type of event that triggers the block to acquire the input signal:

• Rising edge –– The trigger input rises from a negative value or zero to a positive value.

• Falling edge –– The trigger input falls from a positive value or zero to a negative value.

• Either edge –– The trigger input either rises from a negative value or zero to a positive value or falls from a positive value or zero to a negative value.

Initial condition — Block output prior to first trigger event

0 (default) | scalar | vector | matrix

Specify the block's output before the first trigger event using the Initial condition parameter. When the acquired input is an M-by-N matrix, the Initial condition can be an M-by-N matrix or a scalar repeated across all elements of the matrix. When the input is a length-M unoriented vector, the Initial condition can be a length-M row or column vector, or a scalar to be repeated across all elements of the vector.

Latch (buffer) input — Latch buffer input

off (default) | on

If you select the Latch (buffer) input check box, the block outputs the value of the input from the previous time step until the next triggering event occurs. To use this block in a loop, select this check box.

Model Examples

Sample and Hold a Signal

Shows the effect of different trigger events on output of the Sample and Hold block.

Block Characteristics

Data Types	Boolean | double | fixed point | integer | single
Direct Feedthrough	no
Multidimensional Signals	no
Variable-Size Signals	no
Zero-Crossing Detection	no

Extended Capabilities

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

HDL Code Generation

Generate Verilog and VHDL code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL code for the Sample and Hold block is generated as a Triggered Subsystem. Similar restrictions apply to both blocks. See Using Triggered Subsystems for HDL Code Generation (HDL Coder).

HDL Block Properties

For HDL block property descriptions, see HDL Block Properties: General (HDL Coder).

Best Practices

When using the Sample and Hold block in models targeted for HDL code generation, consider the following:

• For synthesis results to match Simulink^® results, drive the trigger port with registered logic (with a synchronous clock) on the FPGA.

• It is good practice to put a unit delay on the output signal. Doing so prevents the code generator from inserting extra bypass registers in the HDL code.

• The use of triggered subsystems, such as the Sample and Hold block, can affect synthesis results in the following ways:

  • In some cases, the system clock speed can drop by a small percentage.

  • Generated code uses more resources, scaling with the number of triggered subsystem instances.

Restrictions

The Sample and Hold block must meet the following conditions:

• The DUT (i.e., the top-level subsystem for which code is generated) must not be the Sample and Hold block.

• The trigger signal must be a scalar.

• The data type of the trigger signal must be either boolean or ufix1.

• The output of the Sample and Hold block must have an initial value of 0.

• The input, output, and trigger signal of the Sample and Hold block must run at the same rate. If one of the input or the trigger signals is an output of a Signal Builder block, see Using the Signal Builder Block (HDL Coder) for how to match rates.

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