AP_HAL_AVR: accept a much shorted sync pulse width on RCInput

this should fix issues with OpenLRSng default settings

libraries/AP_HAL_AVR/RCInput.h

@@ -8,6 +8,14 @@
 #define AVR_RC_INPUT_NUM_CHANNELS 11
 #define AVR_RC_INPUT_MIN_CHANNELS 5     // for ppm sum we allow less than 8 channels to make up a valid packet
 
+/*
+  mininum pulse width in microseconds to signal end of a PPM-SUM
+  frame. This value is chosen to be smaller than the default 3000 sync
+  pulse width for OpenLRSng. Note that this is the total pulse with
+  (typically 300us low followed by a long high pulse)
+ */
+#define AVR_RC_INPUT_MIN_SYNC_PULSE_WIDTH 2700
+
 class AP_HAL_AVR::APM1RCInput : public AP_HAL::RCInput {
 public:
     /**

libraries/AP_HAL_AVR/RCInput_APM1.cpp

@@ -32,7 +32,7 @@ void APM1RCInput::_timer4_capt_cb(void) {
         pulse_width = icr4_current - icr4_prev;
     }
 
-    if (pulse_width > 8000) {
+    if (pulse_width > AVR_RC_INPUT_MIN_SYNC_PULSE_WIDTH*2) {
         // sync pulse detected.  Pass through values if at least a minimum number of channels received
         if( channel_ctr >= AVR_RC_INPUT_MIN_CHANNELS ) {
             _num_channels = channel_ctr;

libraries/AP_HAL_AVR/RCInput_APM2.cpp

@@ -32,7 +32,7 @@ void APM2RCInput::_timer5_capt_cb(void) {
         pulse_width = icr5_current - icr5_prev;
     }
 
-    if (pulse_width > 8000) {
+    if (pulse_width > AVR_RC_INPUT_MIN_SYNC_PULSE_WIDTH*2) {
         // sync pulse detected.  Pass through values if at least a minimum number of channels received
         if( channel_ctr >= AVR_RC_INPUT_MIN_CHANNELS ) {
             _num_channels = channel_ctr;