add oneshot mode capability

change fmu to task

increase fmu_servo task priority to max and enable true oneshot

use lowest FMU priority which minimizes jitter

constrain oneshot updates to control group 0 events

src/modules/px4iofirmware/mixer.cpp

@@ -75,6 +75,9 @@ static volatile bool should_arm_nothrottle = false;
 static volatile bool should_always_enable_pwm = false;
 static volatile bool in_mixer = false;
 
+static bool new_fmu_data = false;
+static uint64_t last_fmu_update = 0;
+
 extern int _sbus_fd;
 
 /* selected control values and count for mixing */
@@ -133,6 +136,11 @@ mixer_tick(void)
 
 		/* this flag is never cleared once OK */
 		r_status_flags |= PX4IO_P_STATUS_FLAGS_FMU_INITIALIZED;
+
+		if (system_state.fmu_data_received_time > last_fmu_update) {
+			new_fmu_data = true;
+			last_fmu_update = system_state.fmu_data_received_time;
+		}
 	}
 
 	/* default to failsafe mixing - it will be forced below if flag is set */
@@ -301,6 +309,15 @@ mixer_tick(void)
 		for (unsigned i = 0; i < PX4IO_SERVO_COUNT; i++) {
 			r_page_actuators[i] = FLOAT_TO_REG(outputs[i]);
 		}
+
+		if (new_fmu_data) {
+			new_fmu_data = false;
+
+			if (up_pwm_get_oneshot_mode()) {
+				up_pwm_force_update();
+			}
+		}
+
 	}
 
 	/* set arming */

src/modules/px4iofirmware/px4io.c

@@ -235,6 +235,7 @@ int
 user_start(int argc, char *argv[])
 {
 	/* configure the first 8 PWM outputs (i.e. all of them) */
+	up_pwm_set_oneshot_mode(true);
 	up_pwm_servo_init(0xff);
 
 #if defined(CONFIG_HAVE_CXX) && defined(CONFIG_HAVE_CXXINITIALIZE)

src/modules/px4iofirmware/registers.c

@@ -325,6 +325,10 @@ registers_set(uint8_t page, uint8_t offset, const uint16_t *values, unsigned num
 		system_state.fmu_data_received_time = hrt_absolute_time();
 		r_status_flags |= PX4IO_P_STATUS_FLAGS_RAW_PWM;
 
+		if (up_pwm_get_oneshot_mode()) {
+			up_pwm_force_update();
+		}
+
 		break;
 
 	/* handle setup for servo failsafe values */