RPMCapture: add hardcoded rpm processing

Such that we can continue development on this part.
The implementation was already used before porting things into the RPM capture module.

src/drivers/rpm_capture/RPMCapture.cpp

@@ -99,13 +99,34 @@ void RPMCapture::Run()
 		return;
 	}
 
+	hrt_abstime now = hrt_absolute_time();
+	const hrt_abstime period = _hrt_timestamp - _hrt_timestamp_prev;
+
 	pwm_input_s pwm_input{};
-	pwm_input.timestamp = hrt_absolute_time();
-	pwm_input.period = _hrt_timestamp - _hrt_timestamp_prev;
+	pwm_input.timestamp = now;
+	pwm_input.period = period;
 	pwm_input.error_count = _error_count;
 	_hrt_timestamp_prev = _hrt_timestamp;
 	_value_processed.store(true);
 	_pwm_input_pub.publish(pwm_input);
+
+	float rpm_raw{0.f};
+
+	if ((1 < period) && (period < 1_s)) {
+		// 1'000'000 / [us] -> pulses per second * 60 -> pulses per minute
+		rpm_raw = 60.f * 1e6f / (static_cast<float>(period) * 1.f);
+	}
+
+	const float dt = math::constrain((now - _timestamp_last_update) * 1e-6f, 0.01f, 1.f);
+	_timestamp_last_update = now;
+	_rpm_filter.setParameters(dt, 0.5f);
+	_rpm_filter.update(rpm_raw);
+
+	rpm_s rpm{};
+	rpm.timestamp = now;
+	rpm.rpm_raw = rpm_raw;
+	rpm.rpm_estimate = _rpm_filter.getState();
+	_rpm_pub.publish(rpm);
 }
 
 int RPMCapture::gpio_interrupt_callback(int irq, void *context, void *arg)

src/drivers/rpm_capture/RPMCapture.hpp

@@ -34,12 +34,14 @@
 #pragma once
 
 #include <drivers/drv_hrt.h>
+#include <lib/mathlib/math/filter/AlphaFilter.hpp>
 #include <px4_arch/micro_hal.h>
 #include <px4_platform_common/module.h>
 #include <px4_platform_common/px4_work_queue/ScheduledWorkItem.hpp>
 #include <uORB/Publication.hpp>
 #include <uORB/Subscription.hpp>
 #include <uORB/topics/pwm_input.h>
+#include <uORB/topics/rpm.h>
 
 using namespace time_literals;
 
@@ -71,9 +73,13 @@ private:
 	int _channel{-1};
 	uint32_t _rpm_capture_gpio{0};
 	uORB::Publication<pwm_input_s>	_pwm_input_pub{ORB_ID(pwm_input)};
+	uORB::Publication<rpm_s>	_rpm_pub{ORB_ID(rpm)};
 
 	hrt_abstime _hrt_timestamp{0};
 	hrt_abstime _hrt_timestamp_prev{0};
 	uint32_t _error_count{0};
 	px4::atomic<bool> _value_processed{true};
+
+	hrt_abstime _timestamp_last_update{0}; ///< to caluclate dt
+	AlphaFilter<float> _rpm_filter;
 };