Adding an OK button to the DiscoEasy200
I wanted to be able to use the filament change feature of the marlin firmware (M600 gcode). But it currently requires a LCD panel which I don’t have.
Since the M108 gcode doesn’t work correctly, I decided to add a push button used as an ‘OK button’ and modify the firmware to handle it.
The MKS Base 1.2 motherboard of the DiscoEasy200 has 2 free analog pins (A11 and A12): 
By wiring a simple push button and a 10kΩ pull-up resistor to A11 
you get a pin value set to HIGH when the button is not pressed, and to LOW when it is pressed.
The wiring can be tested with the M43 gcode if the firmware is built with the following define in Marlin/Configuration_adv.h :
#define PINS_DEBUGGING
Issuing M43 dumps the pins status. When the button is not pressed we get :
M43
PIN: 0 Port: E0 RXD protected
PIN: 1 Port: E1 TXD protected
PIN: 2 Port: E4 CASE_LIGHT_PIN protected
...............
PIN: 62 Port: K0 (A 8) Z_ENABLE_PIN protected
PIN: 63 Port: K1 (A 9) <unused/unknown> Analog in = 923 Input = 1
PIN: 64 Port: K2 (A10) <unused/unknown> Analog in = 769 Input = 1
PIN: 65 Port: K3 (A11) <unused/unknown> Analog in = 1023 Input = 1
...............
PIN: 69 Port: K7 (A15) TEMP_1_PIN Analog in = 1023
ok P15 B3
So we have a high digital value when button is not pressed. The when we issue M43 again with the button held down we have a 0 digital value :
M43
PIN: 0 Port: E0 RXD protected
PIN: 1 Port: E1 TXD protected
PIN: 2 Port: E4 CASE_LIGHT_PIN protected
...............
PIN: 62 Port: K0 (A 8) Z_ENABLE_PIN protected
PIN: 63 Port: K1 (A 9) <unused/unknown> Analog in = 922 Input = 1
PIN: 64 Port: K2 (A10) <unused/unknown> Analog in = 769 Input = 1
PIN: 65 Port: K3 (A11) <unused/unknown> Analog in = 0 Input = 0
PIN: 66 Port: K4 (A12) BEEPER_PIN Output = 0
PIN: 67 Port: K5 (A13) TEMP_0_PIN protected
PIN: 68 Port: K6 (A14) TEMP_BED_PIN protected
PIN: 69 Port: K7 (A15) TEMP_1_PIN Analog in = 1023
ok P15 B3
So the code to test button is pressed is as simple as :
digitalRead(65) == LOW
