How to use Rpi python GPIO pins in PWM output mode to create PWM signals?

Question

I was trying to understand PWM when I ran across this example:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BOARD)
GPIO.setup(12, GPIO.OUT)

p = GPIO.PWM(12, 0.5)
p.start(1)
input('Press return to stop:')   
p.stop()
GPIO.cleanup()

I tried it out and thee LED blinks every 2 sekunds. However, I simply cannot wrap my head around why. If I understand everything correctly, we specify a duty cycle of 100%, so I thought that we supply 100% of the time the LED with power. I dont see how the frequency of 0.5 Hz plays a rule in this case.

I would really appreciate if somebody could explain to me, why we still see the LED blinking.

Cheers

Answer 1

If you don't want it to blink @0.5Hz DON'T try to run it at 0.5Hz - leave it at the default 1kHz.

Answer 2

p.start(1) starts RPi.GPIO PWM with a 1% dutycycle.

You need to use p.start(100) if you want 100%.

https://sourceforge.net/p/raspberry-gpio-python/wiki/PWM/

Answer 3

Question

How to do python PWM?

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Answer

Your program is not very clearly commented, You might like to compare and contrast a similar python PWM segment, in my debugged, complete, copy-paste-run program.

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# *** GPIO PWM Mode Setup and PWM Output ***

def setGpioPinPwmMode(gpioPinNum, frequency):
    pwmPinObject = GPIO.PWM(gpioPinNum, frequency)
    return pwmPinObject

def pwmPinChangeFrequency(pwmPinObject, frequency):
    pwmPinObject.ChangeFrequency(frequency)
    return

def pwmPinChangeDutyCycle(pwmPinObject, dutyCycle):
    pwmPinObject.ChangeDutyCycle(dutyCycle)
    return

def pwmPinStart(pwmPinObject):
    initDutyCycle = 50
    pwmPinObject.start(initDutyCycle)
    return

def pwmPinStop(pwmPinObject):
    pwmPinObject.stop()
    return

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Notes

1. My program is written for controlling a toy servo. But you don't need a servo to test. The main point is to understand how to set, start, and stop the PWM pin, which is connected to a servo or a LED. If you use a LED instead of a servo, you should see the LED goes very dim if duty cycle is very small, and full bright if big duty cycle.

2. Rpi python is running a toy servo at 50Hz PWM. It might go crazy if you ask it to run so ridiculously low a frequency of 0.5Hz. Rpi might complain that 0.5 is not a frequency value!

3. If you set frequency higher than 25Hz, we stupid human eyes are not fast to catch, so you see that in my servo tests I need to use a oscilloscope to watch the PWM signal blinking in "slow motion".

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References

(1) How can Rpi move a Servo motor using a GPIO pin in PWM mode?, RpiSE 2019may11, Viewed 2k times

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Appendices

Appendix A - Rpi python program to PWM run a servo (or blink a LED ridiculously fast)

# Servo_test32 tlfong01 2019may12hkt1506 ***
# Raspbian stretch 2019apr08, Python 3.5.3
# Reference: How can Rpi move a Servo motor using a GPIO pin in PWM mode?, RpiSE 2019may11,  Viewed 2k times
# https://raspberrypi.stackexchange.com/questions/98467/how-can-rpi-move-a-servo-motor-using-a-gpio-pin-in-pwm-mode

import RPi.GPIO as GPIO
from time import sleep

# *** GPIO Housekeeping Functions ***

def setupGpio():
    GPIO.setmode(GPIO.BCM)
    GPIO.setwarnings(False)
    return

def cleanupGpio():
    GPIO.cleanup()
    return

# *** GPIO Input/Output Mode Setup and High/Low Level Output ***

def setGpioPinLowLevel(gpioPinNum):
    lowLevel = 0
    GPIO.output(gpioPinNum, lowLevel)
    return

def setGpioPinHighLevel(gpioPinNum):
    highLevel = 1
    GPIO.output(gpioPinNum, highLevel)
    return

def setGpioPinOutputMode(gpioPinNum):
    GPIO.setup(gpioPinNum, GPIO.OUT)
    setGpioPinLowLevel(gpioPinNum)
    return

# *** GPIO PWM Mode Setup and PWM Output ***

def setGpioPinPwmMode(gpioPinNum, frequency):
    pwmPinObject = GPIO.PWM(gpioPinNum, frequency)
    return pwmPinObject

def pwmPinChangeFrequency(pwmPinObject, frequency):
    pwmPinObject.ChangeFrequency(frequency)
    return

def pwmPinChangeDutyCycle(pwmPinObject, dutyCycle):
    pwmPinObject.ChangeDutyCycle(dutyCycle)
    return

def pwmPinStart(pwmPinObject):
    initDutyCycle = 50
    pwmPinObject.start(initDutyCycle)
    return

def pwmPinStop(pwmPinObject):
    pwmPinObject.stop()
    return

# *** Test Functions ***

def setHighLevelGpioPin18():
    print('  Begin setHighLevelGpioPin18, ...')
    gpioPinNum   = 18
    sleepSeconds =  2    
    setupGpio()
    setGpioPinOutputMode(gpioPinNum)
    setGpioPinHighLevel(gpioPinNum)
    sleep(sleepSeconds)
    cleanupGpio()
    print('  End setHighLevelGpioPin18, ...\r\n')
    return

def setPwmModeGpioPin18():
    print('  Begin setPwmModeGpioPin18, ...')
    
    gpioPinNum   =   18
    sleepSeconds =   10
    frequency    = 1000
    dutyCycle    =   50

    setupGpio()
    setGpioPinOutputMode(gpioPinNum)
    
    pwmPinObject = setGpioPinPwmMode(gpioPinNum, frequency)
    pwmPinStart(pwmPinObject)
    pwmPinChangeFrequency(pwmPinObject, frequency)
    pwmPinChangeDutyCycle(pwmPinObject, dutyCycle)
    sleep(sleepSeconds)
    pwmPinObject.stop()
    cleanupGpio()   

    print('  End   setPwmModeGpioPin18, ...\r\n')

    return

# *** Main ***

print('Begin testing, ...\r\n')
setHighLevelGpioPin18()
setPwmModeGpioPin18()
print('End   testing.')

# *** End of program ***

'''
Sample Output - 2019may12hkt1319
>>> 
 RESTART: /home/pi/Python Programs/Python_Programs/test1198/servo_test31_2019may1201.py 
Begin testing, ...

  Begin setHighLevelGpioPin18, ...
  End setHighLevelGpioPin18, ...

  Begin setPwmModeGpioPin18, ...
  End   setPwmModeGpioPin18, ...

End   testing.
>>> 

>>> 


'''

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