Main loop - call yield - yield can call SerialEvent

Comment out some of the main programs debug print statements plus add yield call to main loop.

yield can now call SerialEvent.

Put hack in that if the user has not overwritten the SerialEvent function, the weak linked one will turn off calling itself...

teensy4/startup.c

 	tempmon_init();
 
 	while (millis() < 300) ; // wait at least 300ms before calling user code
-	printf("before C++ constructors\n");
+	//printf("before C++ constructors\n");
 	__libc_init_array();
-	printf("after C++ constructors\n");
-	printf("before setup\n");
+	//printf("after C++ constructors\n");
+	//printf("before setup\n");
 	setup();
-	printf("after setup\n");
+	//printf("after setup\n");
 	while (1) {
 		//printf("loop\n");
 		loop();
+		yield();
 	}
 }
 

teensy4/usb_inst.cpp

 #endif
 
 #endif // F_CPU
-
+uint8_t usb_enable_serial_event_processing = 1;
 void serialEvent() __attribute__((weak));
-void serialEvent() {}
+void serialEvent() {usb_enable_serial_event_processing = 0;}

teensy4/usb_serial.h

 extern volatile uint8_t usb_cdc_line_rtsdtr;
 extern volatile uint8_t usb_cdc_transmit_flush_timer;
 extern volatile uint8_t usb_configuration;
+extern uint8_t usb_enable_serial_event_processing;
 #ifdef __cplusplus
 }
 #endif

teensy4/yield.cpp

 	if (running) return; // TODO: does this need to be atomic?
 	running = 1;
 
+	// USB Serail - Add hack to minimize impact...
+	if (usb_enable_serial_event_processing && Serial.available()) serialEvent();
+
 	// Current workaround until integrate with EventResponder.
 	HardwareSerial::processSerialEvents();