Hardware vs Software Control
The choice between implementing control in fixed hardware or in software trades determinism and speed against flexibility and complexity.
Two Ways to Build a Controller
A control function can be realized as dedicated hardware logic or as software running on a processor. Hardware implementations, in FPGAs or ASICs, fix the behavior in circuitry. Software implementations, on microcontrollers or CPUs, express behavior as instructions. Both compute the same control law; they differ in timing, flexibility, and how their correctness is guaranteed.
Where Hardware Wins
- Deterministic timing set by a clock, with nanosecond latency and negligible jitter
- True parallelism: many operations complete every clock cycle
- Independence from operating-system and scheduling effects
- Suitability for protection logic that must act faster than any software loop
Where Software Wins
- Flexibility: changing behavior is editing code, not re-synthesizing logic
- Easier complex math, including floating point and iterative algorithms
- Richer development, debugging, and logging tooling
- Lower effort for supervisory logic, sequencing, and communication
The Timescale Split
The prevailing pattern is not to choose one but to place each function where its timescale demands. The fastest, simplest, most safety-critical functions go into hardware: inner feedback, fast filtering, and interlocks. Slower, more complex, and more frequently changed functions go into software: setpoint scheduling, optimization, operator interfaces. The interface between them must be defined so the software cannot delay the hardware's guaranteed timing.
Verification Differences
The two also differ in how they are validated. Hardware timing is proven by static timing analysis of the synthesized logic against the clock. Software timing is proven by WCET analysis and schedulability proofs. Safety-critical designs sometimes implement a function in both, using simple independent hardware as a backstop to more capable software, so that a software fault cannot defeat the last line of protection.