vcgencmd "measure_clock" is really helpful in determining Pi Zero PWM clock. After reboot it is 0, after enabling and different settings it gives the current clock. For 1 not 9.6MHz but 5KHz gets selected, that seems to be the reason why tc_lib was not able to determine the period:

`pi@raspberrypi01:~ $ vcgencmd measure_clock pwm`

frequency(25)=0

pi@raspberrypi01:~ $ gpio pwmc 0

gpio: clock must be between 0 and 4096

pi@raspberrypi01:~ $ gpio pwmc 4096

gpio: clock must be between 0 and 4096

pi@raspberrypi01:~ $ gpio pwmc 1

pi@raspberrypi01:~ $ vcgencmd measure_clock pwm

frequency(25)=5000

pi@raspberrypi01:~ $ gpio pwmc 2

pi@raspberrypi01:~ $ vcgencmd measure_clock pwm

frequency(25)=9600000

pi@raspberrypi01:~ $ gpio pwmc 3

pi@raspberrypi01:~ $ vcgencmd measure_clock pwm

frequency(25)=6400000

pi@raspberrypi01:~ $ gpio pwmc 4

pi@raspberrypi01:~ $ vcgencmd measure_clock pwm

frequency(25)=4800000

pi@raspberrypi01:~ $ gpio pwmc 4095

pi@raspberrypi01:~ $ vcgencmd measure_clock pwm

frequency(25)=5000

pi@raspberrypi01:~ $ gpio pwmc 3590

pi@raspberrypi01:~ $ vcgencmd measure_clock pwm

frequency(25)=5000

pi@raspberrypi01:~ $ gpio pwmc 3589

pi@raspberrypi01:~ $ vcgencmd measure_clock pwm

frequency(25)=6000

pi@raspberrypi01:~ $

Hermann.