FSM handling

The fsm command identifies, extracts, optimizes (re-encodes), and re-synthesizes finite state machines. It again is a macro that calls a series of other commands:

Listing 32 Passes called by fsm
# Identify and extract FSMs:

# Basic optimizations:

# Expanding to nearby gate-logic (if called with -expand):

# Re-code FSM states (unless called with -norecode):

# Print information about FSMs:

# Export FSMs in KISS2 file format (if called with -export):

# Map FSMs to RTL cells (unless called with -nomap):

See also fsm - extract and optimize finite state machines.

The algorithms used for FSM detection and extraction are influenced by a more general reported technique [STGR10].

FSM detection

The fsm_detect pass identifies FSM state registers. It sets the \fsm_encoding = "auto" attribute on any (multi-bit) wire that matches the following description:

  • Does not already have the \fsm_encoding attribute.

  • Is not an output of the containing module.

  • Is driven by single $dff or $adff cell.

  • The \D-Input of this $dff or $adff cell is driven by a multiplexer tree that only has constants or the old state value on its leaves.

  • The state value is only used in the said multiplexer tree or by simple relational cells that compare the state value to a constant (usually $eq cells).

This heuristic has proven to work very well. It is possible to overwrite it by setting \fsm_encoding = "auto" on registers that should be considered FSM state registers and setting \fsm_encoding = "none" on registers that match the above criteria but should not be considered FSM state registers.

Note however that marking state registers with \fsm_encoding that are not suitable for FSM recoding can cause synthesis to fail or produce invalid results.

FSM extraction

The fsm_extract pass operates on all state signals marked with the (\fsm_encoding != "none") attribute. For each state signal the following information is determined:

  • The state registers

  • The asynchronous reset state if the state registers use asynchronous reset

  • All states and the control input signals used in the state transition functions

  • The control output signals calculated from the state signals and control inputs

  • A table of all state transitions and corresponding control inputs- and outputs

The state registers (and asynchronous reset state, if applicable) is simply determined by identifying the driver for the state signal.

From there the $mux-tree driving the state register inputs is recursively traversed. All select inputs are control signals and the leaves of the $mux-tree are the states. The algorithm fails if a non-constant leaf that is not the state signal itself is found.

The list of control outputs is initialized with the bits from the state signal. It is then extended by adding all values that are calculated by cells that compare the state signal with a constant value.

In most cases this will cover all uses of the state register, thus rendering the state encoding arbitrary. If however a design uses e.g. a single bit of the state value to drive a control output directly, this bit of the state signal will be transformed to a control output of the same value.

Finally, a transition table for the FSM is generated. This is done by using the ConstEval C++ helper class (defined in kernel/consteval.h) that can be used to evaluate parts of the design. The ConstEval class can be asked to calculate a given set of result signals using a set of signal-value assignments. It can also be passed a list of stop-signals that abort the ConstEval algorithm if the value of a stop-signal is needed in order to calculate the result signals.

The fsm_extract pass uses the ConstEval class in the following way to create a transition table. For each state:

  1. Create a ConstEval object for the module containing the FSM

  2. Add all control inputs to the list of stop signals

  3. Set the state signal to the current state

  4. Try to evaluate the next state and control output

  5. If step 4 was not successful:

    • Recursively goto step 4 with the offending stop-signal set to 0.

    • Recursively goto step 4 with the offending stop-signal set to 1.

  6. If step 4 was successful: Emit transition

Finally a $fsm cell is created with the generated transition table and added to the module. This new cell is connected to the control signals and the old drivers for the control outputs are disconnected.

FSM optimization

The fsm_opt pass performs basic optimizations on $fsm cells (not including state recoding). The following optimizations are performed (in this order):

  • Unused control outputs are removed from the $fsm cell. The attribute \unused_bits (that is usually set by the opt_clean pass) is used to determine which control outputs are unused.

  • Control inputs that are connected to the same driver are merged.

  • When a control input is driven by a control output, the control input is removed and the transition table altered to give the same performance without the external feedback path.

  • Entries in the transition table that yield the same output and only differ in the value of a single control input bit are merged and the different bit is removed from the sensitivity list (turned into a don’t-care bit).

  • Constant inputs are removed and the transition table is altered to give an unchanged behaviour.

  • Unused inputs are removed.

FSM recoding

The fsm_recode pass assigns new bit pattern to the states. Usually this also implies a change in the width of the state signal. At the moment of this writing only one-hot encoding with all-zero for the reset state is supported.

The fsm_recode pass can also write a text file with the changes performed by it that can be used when verifying designs synthesized by Yosys using Synopsys Formality.