DSP Blockset

Queue

Store inputs in a FIFO register.

Library

Signal Management / Buffers

Description

The Queue block stores a sequence of input samples in a FIFO (first in, first out) register. The register capacity is set by the Register size parameter, and inputs can be scalars, vectors, or matrices.

The block pushes the input at the In port onto the end of the queue when a trigger event is received at the Push port. When a trigger event is received at the Pop port, the block pops the first element off the queue and holds the Out port at that value. The first input to be pushed onto the queue 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 queue contents. If Clear output port on reset is selected, then a trigger event at the Clr port empties the queue and sets the value at the Out port to zero. This setting also applies when a disabled subsystem containing the Queue block is reenabled; 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 onto full register parameter specifies the block's behavior when a trigger is received at the Push port but the register is full. The Pop empty register 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 onto full register 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 queue at a given time, enable the Num output port by selecting the Output number of register entries option.

Examples

Example 1

The table below illustrates the Queue block's operation for a Register size 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 queue, while a 1 in the Full column indicates a full queue.

In	Push	Pop	Clr	Queue	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		2	0	0	3
7	0	0	0		3	0	0	2
8	0	1	0		4	0	0	1
9	0	0	0		5	1	0	0
10	1	0	0		5	0	0	1
11	0	0	0		5	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 queue is first cleared and then pushed.

Example 2

The dspqdemo demo provides another example of Queue operation.

Dialog Box

Register size

The number of entries that the FIFO register can hold.

Trigger type

The type of event that triggers the block's execution.

Push onto full register

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

Pop empty register

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

Empty register output

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

Full register output

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

Output number of register entries

Enable the Num output port, which tracks the number of entries currently on the queue.

Clear input

Enable the Clr input port, which empties the queue 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 queue) when a trigger is received at the Clr input port. Tunable.

See Also

Buffer	DSP Blockset
Delay Line	DSP Blockset
Stack	DSP Blockset

QR Solver

Random Source