I am using Gen7-ARM and Teacup and would like to add a PWM-controlled fan to my printer.

I managed to add a new device/fan in the board.h, using one of the SPI pins as output (in my case PIO0_11). This does make the pin controllable via M106 as intended, but unfortunately only as on/off state. There's no PWM, regardless of the respective parameter/setting in the board.h. I am not sure how the PWM is implemented in the LPC1114 but the extruder heater/ PIO0_10 pin is using pwm, so the functionality should be there.
Is there an easy way to do this or would I need to code my own solution?

The timer from PIO0_11 is CT32B0. And this is the timer for the stepper. So you have 2 options. Use another pin with available pwm or you can try the experimental branch which has software pwm.

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Triffid Hunter's Calibration Guide

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Most important Gcode.

I didn't think of trying a different pin. PIO0_9 works perfectly.