Reading the Mealy 101 Detector and 8-to-1 MUX Sources

Starting from source

I reopened the stored mealy.vhd, mealy_tb.vhd, and mux_8to1.vhd files from my 2024 coursework. I focused on the entity, architecture, and stimulus that I had written instead of the Vivado cache and generated executables.

Finding 101 with three states

The transition path uses three states. st0 has no useful prefix, st1 means the latest bit is 1, and st2 means the latest two bits are 10. A 1 in st2 completes 101 and asserts dout. Returning to st1 preserves that final 1 as the start of an overlapping pattern.

Tracing 01010101

The stored testbench deasserts reset and applies 01010101 at 20 ns intervals. Following the table by hand completes 101 at the fourth, sixth, and eighth input positions. I did not rerun GHDL or Vivado simulation while writing this note, so these are source-level expectations rather than a new waveform result.

Writing a standalone MUX

The original mux_8to1.vhd instantiates a dec_3to8 component and combines decoded lines with AND and OR gates. The decoder source was not present in the stored bundle I reviewed. I therefore rewrote the same truth table as a dependency-free study example.

with sel select
  y <= a when "000",
       b when "001",
       c when "010",
       d when "011",
       e when "100",
       f when "101",
       g when "110",
       h when others;

What I will check next

On the next run I will place present_state, next_state, and dout beside the input at each clock edge. For the MUX, I will exercise all select values on the standalone version first and compare the structural version only after recovering its decoder source.

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