HDL FIFO

Stores sequence of input samples in first in, first out (FIFO) register

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Libraries:
HDL Coder / HDL RAMs

Description

The HDL FIFO block stores a sequence of input samples in a first in, first out (FIFO) register. The data written first into the FIFO register comes out first. The block implementation resembles the FIFO unit in hardware platforms in terms of functionality and behavior.

The HDL FIFO block uses the Simple Dual Port RAM block internally. You can use the HDL FIFO block to generate HDL code that maps to RAM in most FPGAs.

Ports

Input

In — Data input signal

scalar

Data input signal to the FIFO block. When you write data into the FIFO, the newest data is pushed to the end of the FIFO register. The block pushes subsequent data entries below this entry.

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

Push — Write control signal

scalar

When this port receives a value of 1, the block pushes the input at the In port onto the end of the FIFO register.

Data Types: Boolean

Pop — Read control signal

scalar

When this port receives a value of 1, the block pops the first element off the FIFO register and holds the Out port at that value.

Data Types: Boolean

Note

If two or more of the control input ports are triggered in the same time step, the pop operation executes first, followed by the push operation.

rst — Reset control signal

scalar

When the reset port receives a value of 1, it resets the Empty, Full, and Num outputs of the HDL FIFO block.

Dependencies

To enable this port, select the block parameter Local reset port.

Data Types: Boolean

Output

Out — Data output signal

scalar

Data output signal from the FIFO block. When you read data from the FIFO, the data that you wrote first into the FIFO register comes off the FIFO and is held at the output.

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

Empty — Data output signal

scalar

Control signal output from the FIFO that becomes 1 when the FIFO register is empty and does not contain data entries.

Dependencies

To enable this port, Show empty register indicator port (Empty) must be selected.

Data Types: Boolean

Full — Data output signal

scalar

Control signal output from the FIFO that becomes 1 when the FIFO register is full and cannot take more data entries.

Dependencies

To enable this port, Show full register indicator port (Full) must be selected.

Data Types: Boolean

Num — Number of entries

scalar

Number of data entries that are currently inside the FIFO register. Num increments by 1 for every data that you write into the FIFO. Num decrements by 1 for every data that you read from the FIFO.

Dependencies

To enable this port, Show number of entries register port (Num) must be selected.

Data Types: double

Parameters

Register size — Number of entries

10 (default)

Specify the number of entries that the FIFO register can hold. The default value for Register size is 10. The minimum value for Register size is 4.

Programmatic Use

Block parameter: fifo_size

Type: character vector

Value: An integer greater than or equal to four

Default: '10'

Mode — Operation mode

'Classic' (default) | 'FWFT'

Specify the operation mode of the FIFO. By default, the Mode is set to Classic. You see that the block icon displays Classic FIFO. You can change the Mode to FWFT. When you change the Mode, the block icon displays FWFT FIFO. By using the FWFT mode, you can lookahead and see the first word written to the FIFO register without placing a read request. The FWFT mode is especially useful when you apply the back-pressure with AXI4-Stream interfaces.

Programmatic Use

Block parameter: mode

Type: character vector

Value: 'Classic' | 'FWFT' |

Default: 'Classic'

The ratio of output sample time to input sample time — Sample rate ratio

1 (default)

Specify the ratio of output sample time to input sample time. The default ratio is 1, which means that the inputs In and Push, and outputs Out and Pop, run at the same sample rate. The inputs and outputs can run at different sample times. Use a positive integer or 1/N, where N is a positive integer. For example, if you enter 1/2, the output sample time is half the input sample time, or the outputs run faster. The Full, Empty, and Num signals run at the faster rate.

Programmatic Use

Block parameter: ratio

Type: character vector

Value: An integer greater than or equal to one

Default: '1'

Push onto full register — Overflow condition

'Warning' (default) | 'Ignore' | 'Error'

Specify how you want the block to respond when you write to a FIFO that is full. The default is Warning.

Programmatic Use

Block parameter: push_msg

Type: character vector

Value: 'Warning' | 'Ignore' | 'Error'

Default: 'Warning'

Pop onto empty register — Underflow condition

'Warning' (default) | 'Ignore' | 'Error'

Specify how you want the block to respond when you read from a FIFO that is empty. The default is Warning.

Programmatic Use

Block parameter: pop_msg

Type: character vector

Value: 'Warning' | 'Ignore' | 'Error'

Default: 'Warning'

Show empty register indicator port (Empty) — Optional empty port

on (default) | off

Specify whether to enable the Empty output port. This port outputs a 1 when the FIFO register is empty and 0 when the FIFO contains one or more data entries.

Programmatic Use

Block parameter: show_empty

Type: character vector

Value: 'on' | 'off'

Default: 'on'

Show full register indicator port (Full) — Optional full port

on (default) | off

Specify whether to enable the Full output port. This port outputs a 1 when the FIFO register is full.

Programmatic Use

Block parameter: show_full

Type: character vector

Value: 'on' | 'off'

Default: 'on'

Show num register indicator port (Num) — Optional num port

on (default) | off

Specify whether to enable the Num output port. This port outputs the number of data entries that are currently available in the FIFO queue.

Programmatic Use

Block parameter: show_num

Type: character vector

Value: 'on' | 'off'

Default: 'on'

Local reset port — Optional reset port

off (default) | on

Specify whether to insert an additional rst input port.

Programmatic Use

Block parameter: rst_port

Type: character vector

Value: 'on' | 'off'

Default: 'off'

Algorithms

FIFO Write Operation

This figure displays the FIFO write operation. The Push input port acts as the enable signal for the write operation. This signal is denoted by the write_en signal in the figure.

When the write_en signal is 0, the block does not write data to the FIFO and the empty flag is asserted.

When the write_en becomes 1, the block pushes the din signal at input port In to the end of the FIFO register in the next time step. The Num signal indicates the number of data entries in the FIFO register. Every time you write data into the FIFO, the Num signal increments by 1. At time step 12, write_en is 1. At the next time step 13, data is written to the FIFO. Num signal increments by 1 and the empty flag is de-asserted.

This FIFO uses the default register size of 10. When the Num signal becomes equal to the Register size at time step 49, the Full signal is asserted. After the Full signal becomes 1, if you try to write more entries into the FIFO, the block generates a warning.

Classic FIFO Read Operation

This figure displays the FIFO read operation. The Pop input port acts as the enable signal for the read operation. This signal is denoted by the read_en signal in the figure.

When the read_en signal is 0, the block output dout is 0. When the read_en signal becomes 1 at time step 50, the dout signal outputs the oldest entry in the FIFO in the next time step 51. The Full flag is de-asserted and the Num signal decrements by 1 starting from time step 51 as you read data from the FIFO.

When the Num signal becomes equal to 0, the Empty signal is asserted. After the Empty signal becomes 0, if you try to read more entries from the FIFO, the block generates a warning.

FWFT FIFO Read Operation

This figure displays the FIFO read operation. The Pop input port acts as the enable signal for the read operation. This signal is denoted by the read_en signal in the figure.

By default, the HDL FIFO works in the Classic mode. You can also use a first-word-fall-through (FWFT) mode for the FIFO. In the Block Parameters dialog box, specify the Mode as FWFT.

In the FWFT mode, the write operation works in the same way as the Classic mode. The FWFT mode differs from the Classic mode when you perform a read operation. In the Classic mode, after you place a read request or input a 1 to the Pop port, the data becomes available at the FIFO output in the next clock cycle. In the FWFT mode, the first word you write to the FIFO falls through to the output and is available at the output signal Out.

In the figure, though read-en becomes 1 at time step 50, the FIFO read the first word dout at time step 15. You can use this capability to look ahead and see the first word that has been written to the FIFO.

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 Architecture

This block has one default HDL architecture.

HDL Block Properties

General
ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline.
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline.
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline.
RAMDirective	Specify whether to map RAM blocks in your design to the memory blocks, such as distributed RAMs, block RAMs, or UltraRAM, on the target FPGA. The default value is empty string. See also RAMDirective.

Version History

Introduced in R2014a