Fix setting pid denom

[haslinghuis]

• Fixes CRAZYBEEF4SX1280 fixed DSHOT300 and 4kHz PID Loop when Bidirectional DShot is enabled on Betaflight 4.5 #13534

[github-actions]

Do you want to test this code? You can flash it directly from Betaflight Configurator:

• Simply put #13535 (this pull request number) in the Select commit field of the Configurator firmware flasher tab (you need to Enable expert mode, Show release candidates and Development).

WARNING: It may be unstable. Use only for testing!

[haslinghuis]

pid_denom is reset here again:

betaflight/src/main/config/config.c

Lines 719 to 732 in 7c8a552

	} else {
	const float pidLooptime = samplingTime * pidConfig()->pid_process_denom;
	if (motorConfig()->dev.useDshotTelemetry) {
	motorUpdateRestriction *= 2;
	}
	if (pidLooptime < motorUpdateRestriction) {
	uint8_t minPidProcessDenom = motorUpdateRestriction / samplingTime;
	if (motorUpdateRestriction / samplingTime > minPidProcessDenom) {
	// if any fractional part then round up
	minPidProcessDenom++;
	}
	minPidProcessDenom = constrain(minPidProcessDenom, 1, MAX_PID_PROCESS_DENOM);
	pidConfigMutable()->pid_process_denom = MAX(pidConfigMutable()->pid_process_denom, minPidProcessDenom);
	}

[sanderpuh]

I've tested it by adding #13535 as commit on flashing Betaflight 4.5 dev and for me it works!

After enabling bidirectional DShot in the configurator, I am now still able to change the esc/motor protocol and I can lower the PID Loop frequency. Changes are persistent after pressing save, thank you! Tested it on 2 builds, both had the same issue and both are now fixed.

[ledvinap]

@haslinghuis : the test for 4k loop is not necessary?

[haslinghuis]

On the test target pid_denom is set to 2 after enabling dshot telemetry.

[ledvinap]

betaflight/src/main/config/config.c

Lines 726 to 729 in 7c8a552

	if (motorUpdateRestriction / samplingTime > minPidProcessDenom) {
	// if any fractional part then round up
	minPidProcessDenom++;
	}

strange way to write 'ceil()'

[ledvinap]

On the test target pid_denom is set to 2 after enabling dshot telemetry.

yes, that is a bug. But it will also remove test to prevent denom==1.

MIN(2, pid_denom) will fix the obvious bug. The 4k test may be from old CPUs and not necessary

[ledvinap]

fixes the bug

[ledvinap]

@haslinghuis : 4000/3600 is pretty arbitrary, maybe you can keep 4000. I mixed it up with 4k PID loop time ..
(#12509)