pwm.py

import os.path


class PWMException(Exception):
    pass


class PWM():
    SYSFS_PWM_PATH_BASE = "/sys/class/pwm/pwmchip0/"
    SYSFS_PWM_EXPORT_PATH = "/sys/class/pwm/pwmchip0/export"
    SYSFS_PWM_UNEXPORT_PATH = "/sys/class/pwm/pwmchip0/unexport"

    def __init__(self, channel):
        self.channel = channel
        self.channel_path = self.SYSFS_PWM_PATH_BASE + "pwm{}/".format(self.channel)
        self.is_initialized = False
        self.is_enabled = False
        self._freq = 1  # Hz        


    def __enter__(self):
        self.initialize()
        return self

    def __exit__(self, *args):
        self.deinitialize()

    def deinitialize(self):
        try:
            if self.is_enabled:
                self.set_period(1)
                self.disable()
            with open(self.SYSFS_PWM_UNEXPORT_PATH, "a") as pwm_unexport:
                pwm_unexport.write(str(self.channel))
        except FileNotFoundError:
            pass  # skip it since the error must result from initialize() - self.SYSFS_PWM_PATH_BASE or self.SYSFS_PWM_EXPORT_PATH not found
        finally:
            pass

    def initialize(self):
        if not os.path.exists(self.SYSFS_PWM_PATH_BASE):
            raise OSError(f"pwm module wasn't loaded ({self.SYSFS_PWM_PATH_BASE} not found)")

        if not os.path.exists(self.channel_path):
            with open(self.SYSFS_PWM_EXPORT_PATH, "a") as pwm_export:
                pwm_export.write(str(self.channel))

        self.is_initialized = True

    def enable(self):
        with open(self.channel_path + "enable", "w") as pwm_enable:
            pwm_enable.write("1")
            self.is_enabled = True

    def disable(self):
        with open(self.channel_path + "enable", "w") as pwm_enable:
            pwm_enable.write("0")
            self.is_enabled = False

    def set_period(self, freq: float):
        # freq in Hz
        
        if not self.is_initialized:
            raise PWMException("PWM not initialized. Call initialize first")

        if freq < 0.1:
            raise PWMException("Frequency must be >= 0.1 on Raspberry Pi!")

        self._freq = freq
        
        self._period_ns = int(1e9/freq)
        with open(self.channel_path + "period",  "w") as pwm_period:
            pwm_period.write(str(self._period_ns))

    def set_duty_cycle(self, duty_cycle: float) -> None:
        """
        a value between 0 and 100
        0 represents always low.
        100 represents always high.
        """        

        if not self.is_initialized:
            raise PWMException("PWM not initialized. Call initialize first")
    
        if not (0 <= duty_cycle <= 100):
            raise PWMException("Duty cycle must be between 0 and 100 (inclusive).")
            
        dc = int(self._period_ns * duty_cycle / 100)

        with open(self.channel_path + "duty_cycle", "w") as pwm_duty:
            pwm_duty.write(str(dc))