PID loop rate

Summary¶

The PID loop rate is how many times per second the flight controller reads the gyroscope, runs the PID calculation, and sends new motor commands. Betaflight on the H7A3-SLIM runs at 8 000 Hz — 40× faster than the Nyquist minimum for the highest useful manoeuvre frequency. This is not excess: each step in the gyro-to-motor chain introduces delay, and lower delay per cycle allows higher PID gains with the same stability margin. Higher P gain means better disturbance rejection. Better disturbance rejection means the drone holds attitude more precisely in wind. The H7's hardware FPU makes this possible without compromising processor headroom for the RPM filter, GPS parsing, and blackbox logging running simultaneously.

Concept¶

Why faster than Nyquist¶

The Nyquist-Shannon theorem states that to faithfully capture a signal of frequency F, you must sample at ≥ 2F. The highest meaningful control frequency for drone attitude manoeuvres is approximately 100 Hz — so a 200 Hz loop rate would satisfy Nyquist. Betaflight runs at 8 000 Hz — 40× this minimum. Why?

In a control system, delay is the enemy of stability. Every processing step between gyro measurement and motor command takes time, and that accumulated delay reduces the phase margin of the PID controller. Lower phase margin means the PID gain must be set lower to avoid oscillation. Lower P gain means slower disturbance rejection and less precise attitude hold.

At 1 000 Hz, each loop cycle takes 1 ms. At 8 000 Hz, each cycle takes 125 µs. The PID gain that is stable at 8 000 Hz would cause oscillation at 1 000 Hz because the 8× longer cycle latency reduces the phase margin below the stability threshold. Higher loop rate does not change the control physics — it changes the achievable gain, and higher gain means better performance.

The full processing chain¶

At each 125 µs loop cycle, the H7A3-SLIM executes this sequence:

Gyro read via DMA (~13 µs): The DMA controller transfers 16 bytes from the ICM-42688-P over SPI at 10 MHz without CPU involvement. The CPU set up the transfer and receives an interrupt when data arrives.

RPM filter (~20 µs): eRPM telemetry from the four ESCs is used to calculate current motor frequencies and harmonics. Thirty-six notch filter coefficients are updated and the raw gyro data is filtered.

Dynamic notch filter (~10 µs): A sliding spectrum estimate identifies high-amplitude noise peaks not corresponding to motor frequencies and places additional notch filters at them.

PID calculation (~15 µs): Three independent loops (roll, pitch, yaw) each compute P, I, and D terms. The hardware FPU executes floating-point multiply-accumulate in a single clock cycle — PID arithmetic is effectively free.

Motor mixing (~5 µs): Three PID outputs are combined with base throttle through the mixer matrix to produce four motor commands.

DShot600 transmission (~26 µs): Four 16-bit DShot packets are transmitted simultaneously via hardware timer peripherals.

Total: ~90 µs. This leaves 35 µs of slack in the 125 µs window for lower- priority tasks: GPS parsing, OSD frame generation, UART communication, and blackbox logging.

Why the H7A3 is necessary¶

The F4 processor could run the PID loop at 8 000 Hz but not the RPM filter simultaneously — it lacked the CPU headroom. The H7A3 runs both with processor headroom to spare. This is not marketing — it is the specific reason why the RPM filter is available without trade-offs on the H7A3 and required dropping loop rate to 4 000 Hz on F4. See → rpm-filter.

Reference¶

H7A3-SLIM processing headroom at 8 kHz: - PID + RPM filter + dynamic notch: ~84 µs per cycle - Remaining headroom: ~41 µs (33% of loop window) - Used for: GPS, OSD, UART, blackbox

DShot600 bit rate: 600 kbit/s — transmits one 16-bit packet in ~26 µs. All four motor packets transmitted simultaneously via independent hardware timer channels.

Procedure¶

Verify loop rate in Betaflight¶

In Betaflight Configurator → Configuration tab → PID Loop: - Gyro update frequency: 8 kHz - PID loop frequency: 8 kHz

If the H7A3-SLIM shows less than 8 kHz, check that RPM filter (bidirectional DShot) is enabled — the filter requires F7/H7 class processor. Disable RPM filter temporarily to confirm 8 kHz is achievable, then re-enable.

Rationale¶

Understanding loop rate as a consequence of control system latency requirements — rather than as a marketing specification — changes how builders interpret build anomalies. If a build oscillates at a gain setting that should be stable on H7A3, the first diagnostic question is whether the loop rate is actually achieving 8 kHz or has been throttled by a conflicting process. If props-in arming fails on a new build, checking DShot and loop rate is the correct starting point. Loop rate is not set-and-forget — it is the timing substrate that every other flight controller behaviour depends on.

Connections¶

requires: - flight-controller-hardware - closed-loop-control - rpm-filter related: - imu-gyroscope - imu-filter-tuning - dshot-protocol - pid-derivative-term - resonance-filtering leads_to: - rpm-filter - pid-tuning-rate-profile - blackbox-analysis