DSP Blockset

Stack

Store inputs into a LIFO register.

Library

Signal Management / Buffers

Description

The Stack block stores a sequence of input samples in a LIFO (last in, first out) register. The register capacity is set by the Stack depth parameter, and inputs can be scalars, vectors, or matrices.

The block pushes the input at the In port onto the top of the stack when a trigger event is received at the Push port. When a trigger event is received at the Pop port, the block pops the top element off the stack and holds the Out port at that value. The last input to be pushed onto the stack is always the first to be popped off.

A trigger event at the optional Clr port (enabled by the Clear input check box) empties the stack contents. If Clear output port on reset is selected, then a trigger event at the Clr port empties the stack and sets the value at the Out port to zero. This setting also applies when a disabled subsystem containing the Stack block is re-enabled; the Out port value is only reset to zero in this case if Clear output port on reset is selected.

When two or more of the control input ports are triggered at the same time step, the operations are executed in the following order:

1. Clr
2. Push
3. Pop

The rate of the trigger signal must be a positive integer multiple of the rate of the data signal input. The triggering event for the Push, Pop, and Clr ports is specified by the Trigger type pop-up menu, and can be one of the following:

• Rising edge -- Triggers execution of the block when the trigger input does one of the following:
• Rises from a negative value to a positive value or zero
• Rises from zero to a positive value, where the rise is not a continuation of a rise from a negative value to zero (see the following figure)
  
  
• Falling edge -- Triggers execution of the block when the trigger input does one of the following:
• Falls from a positive value to a negative value or zero
• Falls from zero to a negative value, where the fall is not a continuation of a fall from a positive value to zero (see the following figure)
  
  
• Either edge -- Triggers execution of the block when the trigger input is a Rising edge or Falling edge (as described above).
• Non-zero sample -- Triggers execution of the block at each sample time that the trigger input is not zero.

Note    When running simulations in the Simulink MultiTasking mode, sample-based trigger signals have a one-sample latency, and frame-based trigger signals have one frame of latency. Thus, there is a one-sample or one-frame delay between the time the block detects a trigger event, and when it applies the trigger. For more information on latency and the Simulink tasking modes, see Excess Algorithmic Delay (Tasking Latency) and the topic on the Simulation Parameters dialog box in the Simulink documentation.

The Push full stack parameter specifies the block's behavior when a trigger is received at the Push port but the register is full. The Pop empty stack parameter specifies the block's behavior when a trigger is received at the Pop port but the register is empty. The following options are available for both cases:

• Ignore -- Ignore the trigger event, and continue the simulation.
• Warning -- Ignore the trigger event, but display a warning message in the MATLAB command window.
• Error -- Display an error dialog box and terminate the simulation.

The Push full stack parameter additionally offers the Dynamic reallocation option, which dynamically resizes the register to accept as many additional inputs as memory permits. To find out how many elements are on the stack at a given time, enable the Num output port by selecting the Output number of stack entries option.

Examples

Example 1

The table below illustrates the Stack block's operation for a Stack depth of 4, Trigger type of Either edge, and Clear output port on reset enabled. Because the block triggers on both rising and falling edges in this example, each transition from 1 to 0 or 0 to 1 in the Push, Pop, and Clr columns below represents a distinct trigger event. A 1 in the Empty column indicates an empty buffer, while a 1 in the Full column indicates a full buffer.

In	Push	Pop	Clr	Stack	Out	Empty	Full	Num
1	0	0	0		0	1	0	0
2	1	0	0		0	0	0	1
3	0	0	0		0	0	0	2
4	1	0	0		0	0	0	3
5	0	0	0		0	0	1	4
6	0	1	0		5	0	0	3
7	0	0	0		4	0	0	2
8	0	1	0		3	0	0	1
9	0	0	0		2	1	0	0
10	1	0	0		2	0	0	1
11	0	0	0		2	0	0	2
12	1	0	1		0	0	0	1

Note that at the last step shown, the Push and Clr ports are triggered simultaneously. The Clr trigger takes precedence, and the stack is first cleared and then pushed.

Example 2

The dspqdemo demo provides an example of the related Queue block.

Dialog Box

Stack depth

The number of entries that the LIFO register can hold.

Trigger type

The type of event that triggers the block's execution. The rate of the trigger signal must be a positive integer multiple of the rate of the data signal input. Tunable only in simulation (not tunable in Real-Time Workshop external mode or in the Simulink Performance Tools Accelerator).

Push full stack

Response to a trigger received at the Push port when the register is full. Inputs to this port must have the same built-in data type as inputs to the Pop and Clr input ports.

Pop empty stack

Response to a trigger received at the Pop port when the register is empty. Inputs to this port must have the same built-in data type as inputs to the Push and Clr input ports. Tunable.

Empty stack output

Enable the Empty output port, which is high (1) when the stack is empty, and low (0) otherwise.

Full stack output

Enable the Full output port, which is high (1) when the stack is full, and low (0) otherwise. The Full port remains low when Dynamic reallocation is selected from the Push full stack parameter.

Output number of stack entries

Enable the Num output port, which tracks the number of entries currently on the stack. When inputs to the In port are double-precision values, the outputs from the Num port are double-precision values. Otherwise, the outputs from the Num port are 32-bit unsigned integer values.

Clear input

Enable the Clr input port, which empties the stack when the trigger specified by the Trigger type is received. Inputs to this port must have the same built-in data type as inputs to the Push and Pop input ports.

Clear output port on reset

Reset the Out port to zero (in addition to clearing the stack) when a trigger is received at the Clr input port. Tunable.

Supported Data Types

• Double-precision floating point
• Single-precision floating point
• Fixed-point
• Custom data types
• Boolean -- The block accepts Boolean inputs to the Push, Pop, and Clr ports. The block may output Boolean values at the Out and Full ports depending on the input data type, and whether Boolean support is enabled or disabled, as described in Effects of Enabling and Disabling Boolean Support. To learn how to disable Boolean output support, see Steps to Disabling Boolean Support.
• 8-, 16-, and 32-bit signed integers
• 8-, 16-, and 32-bit unsigned integers

To learn how to convert to the above data types in MATLAB and Simulink, see Supported Data Types and How to Convert to Them.

See Also

Buffer	DSP Blockset
Delay Line	DSP Blockset
Queue	DSP Blockset

Also see Buffers for a list of all the blocks in the Buffers library.

Spectrum Scope

Standard Deviation