Modeling Features

Modeling Features

Forked Transitions

Forks let one transition condition feed several branch conditions. This avoids duplicating the common part of an expression on every outgoing transition.

Without a fork, a state like START may need repeated logic:

START: begin
  if (rdy && error)
    nextstate = ERROR;
  else if (rdy && !error)
    nextstate = FINISHED;
end

With a fork, rdy is written once on the transition into the fork. The branch transitions choose the final destination:

START: begin
  if (rdy) begin
    if (error)
      nextstate = ERROR;
    else
      nextstate = FINISHED;
  end
end

Fork priority rules:

• The transition into the fork controls where expanded fork transitions sit relative to other transitions leaving the original source state.
• Outgoing fork branches are sorted by their own priority.
• Lower numeric priority values are tested first.
• equation == 1 is treated as the default/fallback branch.
• If the incoming transition has a priority, generated branches keep that priority with a tiny stable ordering offset.

Transition Priorities

Every transition has a priority attribute. The HDL backend uses that attribute to sort transitions from the same source before emitting the if/else decision tree. Lower numeric values are tested first.

Fizzim 2.0 keeps priority mostly out of the way:

• The default priority is 1000.
• A transition with only the default/implied priority does not show a priority label on the canvas.
• The priority row appears immediately after equation in the transition property editor and property inspector.
• When a transition source has multiple outgoing transitions, Fizzim can fill inherited/default priorities automatically so the ordering is explicit.
• If you manually edit a transition priority, that value becomes local to that transition and the automatic priority pass preserves it.
• Manual priorities are still checked by lint. Use unique integer priorities from 0 through 10000 for transitions that leave the same source.

This behavior is GUI-side metadata management. The Perl backend still consumes the same saved priority attribute it always used for RTL ordering.

State Groups

State groups let several real states share transition or output behavior. A state group is drawn as a rounded rectangle around member states. Only one level of grouping is supported.

A group is a diagram and code-generation convenience, not a real encoded state. The generated FSM still uses the original child states.

State group behavior:

• Shared state outputs are inherited by child states only when the child state leaves that output blank.
• A transition out of a state group is expanded into one transition from each child state.
• A transition into a state group resolves to a real child state using the group's Default entry setting.
• The default entry child is drawn with a bold outline.

State group exits intentionally have priority over transitions authored inside child states. This lets a group define a common escape condition that exits the group even when a child transition is also true.

The mental model is:

1. Resolve state-group entry destinations.
2. Expand state-group exits to each child state.
3. Expand forks by combining incoming and outgoing equations.
4. Sort transitions from each real state: state-group exits first, lower priority first, unconditional fallbacks last, then transition name for deterministic output.

Transition Actions

Transition actions let a regdp output be assigned directly on a transition. This is useful for one-cycle pulses that are awkward to express as destination state behavior.

Example intent:

S1 -- rdy / pulse_done = 1 --> S2

This avoids encoding the pulse in S2 using a nextstate == S2 expression.

Transition actions have higher priority than state-authored assignments for the same registered output on that transition path.

Legacy Modeling Concepts

The original Fizzim tutorial covers several modeling features that still matter for compatibility and advanced use: statebit, comb, and regdp output types; HEROS and one-hot encoding; implied_loopback versus default_state_is_x; custom backend options; comments; parameters; includes; and warning controls.

See Legacy Fizzim Concepts for the compact reference.

Debug State Names

Simulation debug state names are guarded from synthesis:

`ifndef SYNTHESIS
reg [2047:0] statename;
...
`endif

The default width reserves 256 ASCII characters. For grouped states, debug names use GROUP.CHILD format.