10 tips for writing a clear state machine in Verilog: A UART transmitter example.

FPGAs for Beginners

6 chapters7 takeaways10 key terms5 questions

Overview

This video explains how to write clear and maintainable state machines in Verilog, using a UART transmitter as a practical example. It emphasizes organization, readability, and robust design principles. Key takeaways include separating combinatorial and sequential logic, using descriptive naming, and anchoring state transitions to facilitate debugging and modification. The presenter advocates for a two-stage process for state transitions (next state determination and state registration) and demonstrates how to drive output signals based on the current state, ensuring clarity and preventing common design pitfalls.

How was this?

Save this permanently with flashcards, quizzes, and AI chat

Chapters

Load data into an intermediate buffer when input data is valid and the system is ready.
This buffer is necessary because input valid signals might only last for one clock cycle.
Clear variable naming is crucial for understanding calculations, like baud rate and counter sizes, based on parameters.

Properly buffering input data ensures that the transmission process doesn't lose data if the input signal is transient, leading to a more reliable communication system.

Storing the input data in an intermediate variable because the 'valid' signal might go low the next clock cycle, preventing data loss.

Use two counters: one for the baud rate clock cycles and one for data bits.
A neat trick is to reset counters on a state change rather than explicitly checking against a maximum value.
This reset mechanism, tied to the state machine, allows for easier adjustments to state durations without complex counter logic modifications.

Smart counter design, especially resetting on state changes, simplifies state machine logic and makes it more adaptable to timing adjustments, reducing potential errors.

Resetting the baud rate counter automatically when the state changes, instead of explicitly checking if the counter has reached its maximum value.

The transmit buffer is shifted on every clock cycle, regardless of the current state, to prepare data bits.
The data counter increments only on the baud rate clock cycles, ensuring correct timing for each data bit.
This separation of concerns allows data shifting to happen continuously while bit timing is managed by the baud rate clock.

Synchronizing data shifting with the baud rate clock ensures that each bit is transmitted at the correct interval, which is fundamental for successful serial communication.

Using a 'baud_rate_done' signal to enable the increment of the data counter only once per baud rate clock cycle.

The state machine is asynchronous, with the 'next_state' signal driven combinatorially.
Every state must have a defined transition, using an if-else if structure, with a default 'next_state = current_state' to prevent latches.
State transitions are clearly defined based on conditions like 'baud_rate_done' or 'data_done'.

An asynchronous state machine with clearly defined transitions allows for immediate response to conditions and simplifies the logic for determining the next operational phase.

A transition from the 'data' state to the 'parity' state occurs when both the data counter and baud rate counter are done.

State machines operate in two stages: determining the 'next_state' and then registering it as the 'current_state'.
The 'next_state' logic is combinatorial, preparing for the upcoming state.
The 'current_state' logic is sequential, capturing the determined 'next_state' on the clock edge, creating a predictable transition.

This two-stage approach separates the decision-making from the actual state change, making state transitions predictable and easier to debug by allowing observation of the 'next_state' before it becomes the 'current_state'.

The 'next_state' logic calculates where to go next, and then on the next clock cycle, this 'next_state' becomes the 'current_state'.

Output signals (like TX data) are driven based on the 'current_state' using a synchronous process.
The 'ready' signal is typically asserted only in the 'idle' state, indicating the system is prepared to accept new data.
Clarity in code is paramount, especially for complex designs, to ensure maintainability and reduce errors.

Clearly defining output behavior based on the current state ensures the system functions as intended and provides a clear indicator of its operational status to external components.

In the 'idle' state, the TX output is set to the idle value (high), and the 'ready' signal is asserted.

Key takeaways

1Prioritize clear, descriptive naming for all variables and states to enhance code readability and understanding.
2Separate combinatorial logic (next state determination) from sequential logic (state registration) for predictable state transitions.
3Use intermediate buffers for input data to handle transient signals and prevent data loss.
4Resetting counters on state transitions simplifies logic and makes timing adjustments easier.
5Anchor state machine logic to a framework that allows states to be easily separated and modified without cascading issues.
6Drive output signals based on the 'current_state' in a synchronous process for predictable behavior.
7Thoroughly cover all possible state transitions in combinatorial logic to avoid inferring unwanted latches.

Key terms

State MachineVerilogUART TransmitterBaud RateClock RateSynchronous ProcessCombinatorial LogicSequential LogicState TransitionIntermediate Buffer

Test your understanding

1Why is it important to buffer input data in a state machine, even if the input signal is only valid for one clock cycle?
2How does resetting a counter on a state change simplify state machine design compared to checking against a maximum value?
3What is the purpose of the two-stage process (next state determination and state registration) in state machine transitions?
4Explain the difference between combinatorial and sequential logic in the context of a Verilog state machine.
5How can descriptive naming conventions contribute to the maintainability of complex Verilog state machines?