i2C-6ULX-B位置 i2S-6UB Pinout
J4.26 PWM7
J4.28 PWM8
这里配置周期为100kHz，占空比为50%。

$ cd /sys/class/pwm/pwmchip1
$ echo 0 > export
$ echo 10000 > pwm0/period
$ echo 5000  > pwm0/duty_cycle
$ echo 1       > pwm0/enable