The pin of a button or an RPM sensor cannot be used

Stops itRangefinder, optical flow, camera, OSD and other devices

What ArduPilot says:

BTN_PINN=N, set SERVON_FUNCTION=-1
RPMN_PIN not set
RPMN_PIN=N invalid
RPM: Failed to attach to pin N
RPMN_PIN=N, set SERVON_FUNCTION=-1
BTN_PINN=N invalid

A button input (BTN_ENABLE 1, pins BTN_PIN1 to BTN_PIN4) or an RPM sensor read from a pin (RPMn_TYPE 1 or 2) names a pin ArduPilot cannot read. “not set”: RPMn_PIN is still -1. “invalid”: this board has no GPIO (a general-purpose pin) with that number. “set SERVOn_FUNCTION=-1”: the pin is a servo/motor output that still works as an output. “RPM: Failed to attach to pin”: the pin exists, but its pulses cannot be counted.

What to do, most likely first

The pin is one of the servo/motor outputs and has not been turned into a GPIO.
CheckThe message names the output: “set SERVO9_FUNCTION=-1” is output 9.
FixSet that SERVOn_FUNCTION to -1 (GPIO) and reboot: outputs become GPIO pins only at start-up.
The RPM comes from the ESCs, not from a sensor on a pin, and RPMn_TYPE is wrong.
CheckTypes 1 and 2 are only for a hall or optical sensor wired to a pin.
FixSet RPMn_TYPE to 5 (ESC telemetry), or to 0 if you do not use the RPM library at all.
The pin number belongs to another board, or the button input is not used at all.
FixUse a GPIO number from your own board's documentation, or set BTN_ENABLE to 0 (and the unused pins to -1).
“Failed to attach”: another pin with the same number on a different port already uses that interrupt line.
FixMove the sensor to another GPIO and set RPMn_PIN to it.

The parameter list calls RPMn_TYPE 1 “Not Used”, but the code treats it exactly like 2 (GPIO), pin check included.

Parameters to look at

Parameter	What it is
`BTN_ENABLE`	Enable button reporting
`BTN_PIN1`	First button Pin
`BTN_PIN2`	Second button Pin
`RPM1_TYPE`	RPM type
`RPM1_PIN`	Input pin number
`RPM2_TYPE`	RPM type
`RPM2_PIN`	Input pin number
`SERVO9_FUNCTION`	Servo output function

Where to do it:Board & Wiring Planner

The code that sends it

BTN_PINN=N, set SERVON_FUNCTION=-1

libraries/AP_Button/AP_Button.cpp AP_Button::arming_checks() · Open in ArduPilot Copter-4.7.1

RPMN_PIN not set

libraries/AP_RPM/AP_RPM.cpp AP_RPM::arming_checks() · Open in ArduPilot Copter-4.7.1

RPMN_PIN=N invalid

libraries/AP_RPM/AP_RPM.cpp AP_RPM::arming_checks() · Open in ArduPilot Copter-4.7.1

RPM: Failed to attach to pin N

libraries/AP_RPM/RPM_Pin.cpp AP_RPM_Pin::update() · Open in ArduPilot Copter-4.7.1

And 2 more messages, sent from the same places.