docs: Add a "hardware pin type" section to the FAQ

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/FAQ.md

@@ -19,10 +19,11 @@ Frequently asked questions
 16. [When I set "restart_method=command" my AVR device just hangs on a restart](#when-i-set-restart_methodcommand-my-avr-device-just-hangs-on-a-restart)
 17. [Will the heaters be left on if the Raspberry Pi crashes?](#will-the-heaters-be-left-on-if-the-raspberry-pi-crashes)
 18. [How do I convert a Marlin pin number to a Klipper pin name?](#how-do-i-convert-a-marlin-pin-number-to-a-klipper-pin-name)
-19. [How do I cancel an M109/M190 "wait for temperature" request?](#how-do-i-cancel-an-m109m190-wait-for-temperature-request)
+19. [Do I have to wire my device to a specific type of micro-controller pin?](#do-i-have-to-wire-my-device-to-a-specific-type-of-micro-controller-pin)
-20. [Can I find out whether the printer has lost steps?](#can-i-find-out-whether-the-printer-has-lost-steps)
+20. [How do I cancel an M109/M190 "wait for temperature" request?](#how-do-i-cancel-an-m109m190-wait-for-temperature-request)
-21. [Why does Klipper report errors? I lost my print!](#why-does-klipper-report-errors-i-lost-my-print)
+21. [Can I find out whether the printer has lost steps?](#can-i-find-out-whether-the-printer-has-lost-steps)
-22. [How do I upgrade to the latest software?](#how-do-i-upgrade-to-the-latest-software)
+22. [Why does Klipper report errors? I lost my print!](#why-does-klipper-report-errors-i-lost-my-print)
+23. [How do I upgrade to the latest software?](#how-do-i-upgrade-to-the-latest-software)
 
 ### How can I donate to the project?
 
@@ -386,6 +387,41 @@ support these custom pin numbers - check Marlin's fastio headers (see
 above) to translate these pin numbers to their standard hardware
 names.
 
+### Do I have to wire my device to a specific type of micro-controller pin?
+
+It depends on the type of device and type of pin:
+
+ADC pins (or Analog pins): For thermistors and similar "analog"
+sensors, the device must be wired to an "analog" or "ADC" capable pin
+on the micro-controller. If you configure Klipper to use a pin that is
+not analog capable, Klipper will report a "Not a valid ADC pin" error.
+
+PWM pins (or Timer pins): Klipper does not use hardware PWM by default
+for any device. So, in general, one may wire heaters, fans, and
+similar devices to any general purpose IO pin. However, fans and
+output_pin devices may be optionally configured to use `hardware_pwm:
+True`, in which case the micro-controller must support hardware PWM on
+the pin (otherwise, Klipper will report a "Not a valid PWM pin"
+error). Note that hardware PWM is currently only supported on the avr,
+atsam, samd21, and linux micro-controllers.
+
+IRQ pins (or Interrupt pins): Klipper does not use hardware interrupts
+on IO pins, so it is never necessary to wire a device to one of these
+micro-controller pins.
+
+SPI pins: When using hardware SPI it is necessary to wire the pins to
+the micro-controller's SPI capable pins. However, most devices can be
+configured to use "software SPI", in which case any general purpose IO
+pins may be used.
+
+I2C pins: When using I2C it is necessary to wire the pins to the
+micro-controller's I2C capable pins.
+
+Other devices may be wired to any general purpose IO pin. For example,
+steppers, heaters, fans, Z probes, servos, LEDs, common hd44780/st7920
+LCD displays, the Trinamic UART control line may be wired to any
+general purpose IO pin.
+
 ### How do I cancel an M109/M190 "wait for temperature" request?
 
 Navigate to the OctoPrint terminal tab and issue an M112 command in