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 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 rises from a negative value to zero or a positive value, or from zero to a positive value.
• Falling edge triggers execution of the block when the trigger input falls from a positive value to zero or a negative value, or from zero to a negative value.
• Either edge triggers execution of the block when either a rising or falling edge (as described above) occurs.

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.

Push full stack

Response to a trigger received at the Push port when the register is full.

Pop empty stack

Response to a trigger received at the Pop port when the register is empty. 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.

Clear input

Enable the Clr input port, which empties the stack when the trigger specified by the Trigger type is received.

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.

See Also

Buffer	DSP Blockset
Delay Line	DSP Blockset
Queue	DSP Blockset

Spectrum Scope

Standard Deviation