attach probe of the oscilloscope too the
3.3v & PWM3 like below:

          jack
    ------|  |---|    |-|    |----
    |o o 3.3v    |eth1| |eth0|
    |o o
    |o o
    |o o PWM3
    |o o

type commands below to rt-thread console:
    msh /> pwm_enable pwm0 3
    msh /> pwm_set pwm0 3 140 90

turn off pwm:
    msh /> pwm_disable pwm0 3
Signed-off-by: NDu Huanpeng <548708880@qq.com>