Technical note
What deterministic FPGA latency buys a control loop
Ask how fast a controller is and most people quote an average: a loop rate, a clock speed, a benchmark. For a feedback loop that is the wrong question. What matters is not how fast the controller usually responds, but whether it responds at the same time every time. Fast on average is not the same as on time.
Why timing is a stability problem, not a speed problem
Every control loop has a delay between measuring the plant and acting on it. That delay eats phase margin, the cushion that keeps the loop stable as you push its bandwidth up. A fixed, known delay you can design around: you account for it and set your gains accordingly. A delay that varies from cycle to cycle, jitter, you cannot. You have to leave margin for the worst case, which means detuning the loop for the cases that were fine. The jitter quietly caps how much performance you are allowed to use.
Where the jitter comes from
On a general-purpose processor, the control code shares the machine with everything else. Interrupts arrive and get serviced in an order that depends on what else is happening. Caches hit or miss depending on history. An operating system scheduler decides when your task runs. Each of these is usually small, and each is variable. The result is a controller that is quick most of the time and occasionally late, with no way to know in advance which cycle will be the late one.
What an FPGA changes
On an FPGA the control law is not software waiting its turn; it is logic laid down in hardware. It executes in a fixed number of clock cycles, the same number on every cycle, with no interrupts, no cache, and no scheduler to introduce surprises. The latency is not just low, it is deterministic: cycle-accurate and identical run to run. That has three practical consequences:
- Higher usable bandwidth. With the jitter gone, you can push the loop closer to its real limit instead of detuning for a worst case that no longer exists.
- Actions locked to the switching cycle. Sampling the current at exactly the right instant, and timing gate events precisely, are things you can only do reliably when your timing does not wander. This is what makes tight current control and clean soft switching possible.
- Deterministic protection. An overcurrent response that always fires within a known, short, fixed time is worth far more than one that is usually fast.
None of this means an FPGA is always the right answer; plenty of drives are perfectly happy on a microcontroller. But when a loop needs high bandwidth, precise switching-cycle timing, or hard guarantees on response time, deterministic latency stops being a nice-to-have and becomes the thing that makes the design work. It is the part general embedded shops most often cannot deliver, and the part we build routinely.
Loop bandwidth capped by timing you cannot pin down?
Moving the hot path to deterministic FPGA logic is often the unlock. A diagnostic sprint tells you whether it is. Start one.