Week 4 - (9/22/20)

• Read: Chapters 7 & 8 of Textbook: Free Range VHDL

  • 7. Using VHDL for Sequential Circuits

    • Dives into the use of Sequential Circuits for memory storage in VHDL via Flip-Flops for single bit storage

    • Bit storage occurs when a function can have multiple different possible outcomes but an output value is only specified for some of them.

      • When the function processes data for which there is no specified output, the previous data is "remembered", thus: data storage

  • 8. Finite State Machine Design Using VHDL

    • Finite State Machines (FSMs) are a system that can be in exactly one state, from a finite set of states, at any one time.

      • FSMs can change from one state to another in response to a change in their inputs (changes are called transitions)

    • Moore vs. Mealy FSM

      • Moore FSMs' output values depend only on the current state

      • Mealy FSMs' output values determined by current state and values of inputs

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• Watched: Video from Ben Eater

  • Describes D Flip-Flop and Latch

• Worked through various GHDL examples:

  • 4-to-1 multiplexer

  • 1-to-4 demultiplexer

4-to-1 multiplexer example (followed from: All About FPGA)

• First create mux.vhdl file with multiplexer functionality

• Next create the mux_tb.vhdl file which tests the functionality by verifying each case for the inputs

• In the terminal, use ghdl (with flags '-s', '-a', '-e', '-r') to check the syntax of the file, analyze the file, to elaborate the unit in the file (and create its executable) and to run it

• Next, run 'ghdl -r mux_tb --vcd=mux.vcd' which will create the gtkwave compatible file to view to functionality on a waveform generator

  • mux_tb is name of the entity, not the file

A multiplexer has multiple inputs with one output, in this case 4 inputs, 2 selection lines, and 1 output

1-to-4 demultiplexer example (followed from: All About FPGA)

• First create demux.vhdl file with multiplexer functionality

• Next create the demux_tb.vhdl file which tests the functionality by verifying each case for the inputs

• In the terminal, use ghdl (with flags '-s', '-a', '-e', '-r') to check the syntax of the file, analyze the file, to elaborate the unit in the file (and create its executable) and to run it

• Next, run 'ghdl -r demux_tb --vcd=demux.vcd' which will create the gtkwave compatible file to view to functionality on a waveform generator

  • note: demux_tb is name of the entity, not the file

A demultiplexer has one input with multiple output, in this case 1 input, 2 selection lines, and 4 outputs