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I'm trying to flash a LED at 27kHz for a project and using the general GPIO the fastest I can get is around 3kHz. Any library's that would be suitable for this?

# Import the libraries
import RPi.GPIO as GPIO
import time

Set the GPIO mode


GPIO.setmode(GPIO.BCM)


Set the LED pin as output


LED_PIN = 18
GPIO.setup(LED_PIN, GPIO.OUT)


Create a PWM object


pwm = GPIO.PWM(LED_PIN, 27000) # 27 kHz frequency


Start the PWM with 0 duty cycle


pwm.start(0)


Define a function to read a value from a text file


def read_value():
    # Open the file in read mode
    with open("value.txt", "r") as f:
        # Read the first line
        line = f.readline()
        # Try to convert it to a float
        try:
            value = float(line)
        except ValueError:
            # If the conversion fails, return None
            value = None
    # Return the value
    return value


Define a loop to flash the LED


def flash_led():
    # Get the value from the file
    value = read_value()
    # If the value is valid, change the duty cycle
    if value is not None and 0 <= value <= 100:
        pwm.ChangeDutyCycle(value)
    # Wait for 0.01 seconds
    time.sleep(0.01)


Run the loop until interrupted by keyboard or system


try:
    while True:
        flash_led()
except (KeyboardInterrupt, SystemExit):
    # Stop the PWM and clean up the GPIO
    pwm.stop()
    GPIO.cleanup()
Stuart Brody
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2 Answers2

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This is frankly bizarre code which uses RPi.GPIO software PWM which more or less by definition can't do anything "precise" and it unclear what output you are expecting to do with the "value".

It would be possible, using hardware PWM, to generate a precise output (but the actual values possible are constrained by the clock) and you can not get an exact 27kHz signal. (Using the fractional divider may get closer but I have never seen any code using this.)

Most models (pre Pi4) have 19.2MHz clock so a 711 divider would give 27.004kHz. You would have to choose a combination of PWM range and divider to give the output you want. e.g. DIVIDER = 15 & RANGE = 128 gives a 15 * 128 => 1920 divider and thus 10kHz.

For 711 use 9 * 79 or 3 * 237.

My pi-gpio (c) & pi_gpio (python) would permit this.

pigpio has hardware timed PWM and should be able to produce similar but only a small set of pre-defined frequencies, pigpio supports arbitrary waveform generation which may be better for your purpose and can generate fixed frequency clock outputs up to 30MHz.

The kernel pwm overlay may also be a candidate (although I have never tested its ability to generate arbitrary frequencies - it is usually used with much lower frequencies) https://raspberrypi.stackexchange.com/a/143644/8697

Milliways
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you'd be better off using a dedicated timer circuit (e.g using a 555 ic or similar) as the Pi isn't suitable for this type of application as the operating system can get busy doing other things.

Bra1n
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