common: add note on Cube pull-up resistors

Author: peterbarker

common/source/docs/common-thecubeorange-overview.rst

@@ -474,6 +474,18 @@ The Cube connector pin assignments
 
 Analog/PWM RSSI Input is pin 103
 
+.. note::
+
+   CubeOrange has strong protection on many of its pins to prevent damage from ESD and other source of electrical noise.  Part of this protection involves a 40kΩ resistor, used to pull signal pins up to Vcc.  These pull-up resistors are present on the servo pin headers and on CTS/RTS pins.
+
+   This has implications for pins to be used as GPIO; the internal STM32 pull-down resistors are unable to overcome this 40kΩ resistor, so if a pin must be used as a "pull-up to assert" input additional circuitry will be required.
+
+   Best practice would be to configure the pin floating (default state
+after setting mode), and then:
+
+   * If you have an input device that transitions between Floating or (3.3-5V) and driven to ground with less than 10K resistor, you don't need to place any resistor, it will work out of the box.
+   * If you have an input device that transition between High level (3.3-5V) and floating, then recommendation is to add a 10K resistor to pull down.
+
 
 Cubepilot Ecosystem
 ===================