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I am attempting to set an analog voltage control signal with a Rasberry Pi 3 B+.

My Python 3 script uses RPi.GPIO to output PWM across a range of duty cycles to simulate analog output---without the use of any additional hardware. The signal I wish to output ranges between 0.221 and 0.310 Volts.

Examples on the web usually show a call to initialize the PWM at 100 by p = GPIO.PWM(OUT_PIN1, 100);. Ideally, I would increase this. My code (seen below) runs through a range of PWM values and I can observe voltage changes using a multimeter on the pins.

It appears that at a frequency of 100, the values are slightly high but at a value of 200, the values are much higher than expected and then erratic. Ideally, I would run at the maximum frequency to have as many values between 0.221 and 0.310 as possible but with a frequency of 100 it appears I only have increments of 0.033V and so only ~2-3 actual levels between the two extremes.

Is 100 the maximum value for RPi or should I disregard the multimeter at these high frequencies?

import RPi.GPIO as GPIO
import time

# Cycle Through a range of simulated analog output voltages using PWM

print('GPIO Version', GPIO.VERSION)

print('Setting GPIO to BOARD Mode...\n')
GPIO.setmode(GPIO.BOARD)

OUT_PIN1 = 33 # PWM PIN

OUTPUT_VOLTAGE = 3.3

MIN_VOLTAGE = 0.221
MAX_VOLTAGE = 0.310

BASE_DUTY = MIN_VOLTAGE / OUTPUT_VOLTAGE * 100.0
MAX_DUTY = MAX_VOLTAGE / OUTPUT_VOLTAGE * 100.0

MAX_INC = 5

print('PWM Setup ...')
GPIO.setup(OUT_PIN1, GPIO.OUT)
p = GPIO.PWM(OUT_PIN1, 100);
p.start(0)


print('Running')
while 1:
    for k in range(MAX_INC + 1):
        newDuty = (MAX_DUTY - BASE_DUTY)/MAX_INC*k + BASE_DUTY
        print(k,'  Changing to Duty Cycle:', newDuty, '%', '    Target: ', newDuty/100*3.3, 'V')
        p.ChangeDutyCycle(newDuty)
        time.sleep(3)
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Software timed PWM will always be erratic, especially under (CPU) load.

Perhaps have a look at (my) pigpio which uses hardware timing to generate 250 steps at 800Hz (500 steps @ 400Hz, 1000 steps @ 200 Hz).

http://abyz.me.uk/rpi/pigpio/python.html#set_PWM_dutycycle http://abyz.me.uk/rpi/pigpio/python.html#set_PWM_frequency

Alternatively use one of the hardware PWM pins which will do a million steps at 250Hz.

http://abyz.me.uk/rpi/pigpio/python.html#hardware_PWM

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