Control PWM duration with python

Question

I need to control the PWM module on the raspberry pi. I’m now familiar with pigpio, and with hardware_PWM. However, I did not find anywhere where I can control the total duration of the PWM. Example: turn off after 50 clock cycles. Also, what would be the most accurate way in doing so using python?

My pulses can vary in duty cycle, and are between 0-1khz. I tried sing the software pwm but it couldn’t get to 1khz, but when I tried using the hardware_pwm it was honestly perfect. But I can’t control it.

• tlfong01: For general python PWM programming, the following might help: (1) raspberrypi.stackexchange.com/questions/99315/… (2) raspberrypi.stackexchange.com/questions/102269/… (3) raspberrypi.stackexchange.com/questions/102115/… (4) raspberrypi.stackexchange.com/questions/99408/…, / to continue, ...
• tlfong01: For precise PWM timing, I recommend PCM9685: "RPi4B PCA9685 PWM Controlling Servo Problem": (1) raspberrypi.stackexchange.com/questions/99315/…
• tlfong01: you are welcome. Cheers
• George: I will look into these and see if it's a feasible option. I really wanted to try and use the existing hardware if possible to not add more elements to my circuit. But if all fails, I will have to look into these options. Thank you
• George: Thank you. I'm going to see if the waves option that joan suggested is going to work. I'll get back to the post in a few.

Answer 1

If the pulses aren't very fast and you don't mind jitter you could do it in software. Untested Python.

def send_PWM(GPIO, pulses, frequency, dutycycle):
 # dutycycle expected to be in range 0.0 to 1.0
 duration = 1.0 / frequency
 on_period = duration * dutycycle
 off_period = duration - on_period
 for i in range(pulses):
 pi.write(GPIO, 1)
 time.sleep(on_period)
 pi.write(GPIO, 0)
 time.sleep(off_period)

If the pulses are fast (say the 100 kHz or less region) and you don't like jitter you could use a wave chain.

There are various examples of using waves at http://abyz.me.uk/rpi/pigpio/examples.html

Here is an example using waves and wave chains.

#!/usr/bin/env python
import time
import pigpio
def tx_pulses(pi, GPIO, frequency, num, dutycycle=0.5):
 assert 1 <= frequency <= 500000
 assert 1 <= num <= 65535
 assert 0.0 <= dutycycle <= 1.0
 duration = int(1000000/frequency)
 on_micros = int(duration * dutycycle)
 num_low = num % 256
 num_high = num // 256
 wf = []
 # on off time
 wf.append(pigpio.pulse(1<<GPIO, 0, on_micros))
 wf.append(pigpio.pulse( 0, 1<<GPIO, duration - on_micros))
 pi.wave_add_generic(wf)
 wid = pi.wave_create()
 if wid >= 0:
 pi.wave_chain([255, 0, wid, 255, 1, num_low, num_high])
 while pi.wave_tx_busy():
 time.sleep(0.01)
 pi.wave_delete(wid)
pi = pigpio.pi()
if not pi.connected:
 exit()
GPIO=19
pi.set_mode(GPIO, pigpio.OUTPUT)
tx_pulses(pi, GPIO, 100, 25) # 25 pulses @ 100 Hz 50% duty
tx_pulses(pi, GPIO, 1000, 250, 0.1) # 250 pulses @ 1000 Hz 10% duty
tx_pulses(pi, GPIO, 5000, 2391, 0.03) # 2391 pulses @ 5000 Hz 3% duty
pi.stop()

• George: My pulses can vary in duty cycle, and are between 0-1khz. I tried sing the software pwm but it couldn’t get to 1khz, but when I tried using the hardware_pwm it was honestly perfect. But I can’t control it. The wave chain looks neat since I can looo it for an x amount, I’ll need to look more into that. It says it disables the hardwsre_PWM tho
• joan: Yes, waves use PWM and PCM so no audio from them. When I get time I will add a wave chain example to the answer.