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i'm looking for a solution using a mcp23017 gpio-expander with raspberry pi as led-dimmer, but every 4-5sec there is a short flickering. i carried out that flickering is also there if i use gpio directly (comment/uncomment relevant parts in code if u try it) i cant use rpi.gpio software-pwm or pi-blaster because it is not usable via i2c, if you have a solution for getting this packages ready for i2c it would also be great i think the problem is somewhere in adressing GPIO, but i dont get it

#!/usr/bin/python
# -*- coding: utf-8 -*-

# uncomment line 14-20 for using I2C and comment line 24-35, switch for using GPIO directly

import smbus
import time
import RPi.GPIO as GPIO

liste = [4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0]
# print liste #debugging ...
periodendauer = 0.001 # means 1000Hz

# # to send data to mcp23017
# b = smbus.SMBus(1) # 0 indicates /dev/i2c-0, muss auf 1 stehen (für rev2)
# while True:
    # for values in liste:
        # b.write_byte_data(0x20,0x14,values) #send data via smbus(I2C) to mcp23017
        # # print values #debugging only
        # time.sleep(periodendauer)


# to send data direct to gpio-pin
GPIO.cleanup()
GPIO.setmode(GPIO.BOARD)
GPIO.setup(7, GPIO.OUT)
while True:
    for values in liste:
        if values > 0:
            values = True
        else:
            values = False
        GPIO.output(7,values)
        # print values #debugging only
        time.sleep(periodendauer)
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edit: this code is also available via github github.com/herrxyz/software-pwm-raspberry-python/blob/master/… –  herr_xyz Aug 20 '14 at 9:44

1 Answer 1

up vote -1 down vote

Based on the comments I rewrited my answer for your question.

I simplified your application with removing the I2C part and the comments and dropped the sleep function. I did that to show you how unreliable is the Raspbery Pi for this kind of precise timing in the way you want to use it. What I added to your code is a time measuring at the start and the end of the for loop, so it is now measuring a whole cycle processing of the "liste" array not the length of an individual "values".

#!/usr/bin/python
# -*- coding: utf-8 -*-

import time
import RPi.GPIO as GPIO
import sys

liste = [4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0]

GPIO.setmode(GPIO.BOARD)
GPIO.setup(7, GPIO.OUT)
while True:
    ms = time.time() * 1000.0
    for values in liste:
        if values > 0:
            values = True
        else:
            values = False
        GPIO.output(7,values)
    me = time.time() * 1000.0 - ms
    sys.stdout.write("\r{0:4.4f}".format(me)),
    sys.stdout.flush()

I have a Banana Pi at home hat have the same GPIO outputs, but please run it on a Raspberry, you will have the same result (maybe with longer cycle times). For me the results were between 4-5ms with several 6ms length pulses and sometimes it went over 10ms. That's why you have flickering led.

For an I2C based solution I would suggest a dedicated I2C PWM driver board to produce a smooth PWM signal such as the PCA9685 from NXP.

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This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. –  fredtantini Jan 28 at 11:57
    
Thanks @fredtantini, I edited the post to be more relevant to the original question. –  KZD76 Feb 1 at 19:03

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