HAL_PX4: support OneShot on PX4

this greatly lowers output latency

libraries/AP_HAL_PX4/RCOutput.cpp

@@ -18,6 +18,12 @@ extern const AP_HAL::HAL& hal;
 
 using namespace PX4;
 
+/*
+  enable RCOUT_DEBUG_LATENCY to measure output latency using a logic
+  analyser. AUX6 will go high during the cork/push output. 
+ */
+#define RCOUT_DEBUG_LATENCY 0
+
 void PX4RCOutput::init()
 {
     _perf_rcout = perf_alloc(PC_ELAPSED, "APM_rcout");
@@ -151,6 +157,10 @@ void PX4RCOutput::set_freq(uint32_t chmask, uint16_t freq_hz)
     if (freq_hz > 400) {
         freq_hz = 400;
     }
+    if (freq_hz == 400) {
+        // remember mask for fast channels
+        _fast_channel_mask = chmask;
+    }
     uint32_t primary_mask = chmask & ((1UL<<_servo_count)-1);
     uint32_t alt_mask = chmask >> _servo_count;
     if (primary_mask && _pwm_fd != -1) {
@@ -369,7 +379,7 @@ void PX4RCOutput::_publish_actuators(void)
     }
 }
 
-void PX4RCOutput::_timer_tick(void)
+void PX4RCOutput::_send_outputs(void)
 {
     uint32_t now = AP_HAL::micros();
 
@@ -431,4 +441,38 @@ update_pwm:
 
 }
 
+void PX4RCOutput::cork()
+{
+#if RCOUT_DEBUG_LATENCY
+    hal.gpio->pinMode(55, HAL_GPIO_OUTPUT);
+    hal.gpio->write(55, 1);
+#endif
+    _corking = true;
+}
+
+void PX4RCOutput::push()
+{
+#if RCOUT_DEBUG_LATENCY
+    hal.gpio->pinMode(55, HAL_GPIO_OUTPUT);
+    hal.gpio->write(55, 0);
+#endif
+    _corking = false;
+    if (_output_mode == MODE_PWM_ONESHOT) {
+        // run timer immediately in oneshot mode
+        _send_outputs();
+        if (_fast_channel_mask != 0) {
+            // this is a crude way of triggering immediate output
+            set_freq(_fast_channel_mask, 400);
+            set_freq(_fast_channel_mask, 0);
+        }
+    }
+}
+
+void PX4RCOutput::_timer_tick(void)
+{
+    if (_output_mode != MODE_PWM_ONESHOT) {
+        _send_outputs();
+    }
+}
+
 #endif // CONFIG_HAL_BOARD

libraries/AP_HAL_PX4/RCOutput.h

@@ -28,7 +28,13 @@ public:
         _esc_pwm_min = min_pwm;
         _esc_pwm_max = max_pwm;
     }
+    void cork();
+    void push();
 
+    void    set_output_mode(enum output_mode mode) override {
+        _output_mode = mode;
+    }
+    
     void _timer_tick(void);
 
 private:
@@ -55,9 +61,13 @@ private:
     orb_advert_t _actuator_armed_pub = NULL;
     uint16_t _esc_pwm_min = 0;
     uint16_t _esc_pwm_max = 0;
+    uint16_t _fast_channel_mask;
 
     void _init_alt_channels(void);
     void _publish_actuators(void);
     void _arm_actuators(bool arm);
     void set_freq_fd(int fd, uint32_t chmask, uint16_t freq_hz);
+    bool _corking;
+    enum output_mode _output_mode = MODE_PWM_NORMAL;
+    void _send_outputs(void);
 };