Raspbery Pi Analog Input with MCP3008

adafruit_mcp3008.py

#!/usr/bin/env python

# Written by Limor "Ladyada" Fried for Adafruit Industries, (c) 2015
# This code is released into the public domain

import time
import os
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
DEBUG = 1

# read SPI data from MCP3008 chip, 8 possible adc's (0 thru 7)
def readadc(adcnum, clockpin, mosipin, misopin, cspin):
        if ((adcnum > 7) or (adcnum < 0)):
                return -1
        GPIO.output(cspin, True)

        GPIO.output(clockpin, False)  # start clock low
        GPIO.output(cspin, False)     # bring CS low

        commandout = adcnum
        commandout |= 0x18  # start bit + single-ended bit
        commandout <<= 3    # we only need to send 5 bits here
        for i in range(5):
                if (commandout & 0x80):
                        GPIO.output(mosipin, True)
                else:
                        GPIO.output(mosipin, False)
                commandout <<= 1
                GPIO.output(clockpin, True)
                GPIO.output(clockpin, False)

        adcout = 0
        # read in one empty bit, one null bit and 10 ADC bits
        for i in range(12):
                GPIO.output(clockpin, True)
                GPIO.output(clockpin, False)
                adcout <<= 1
                if (GPIO.input(misopin)):
                        adcout |= 0x1

        GPIO.output(cspin, True)
        
        adcout >>= 1       # first bit is 'null' so drop it
        return adcout

# change these as desired - they're the pins connected from the
# SPI port on the ADC to the Cobbler
SPICLK = 18
SPIMISO = 23
SPIMOSI = 24
SPICS = 25

# set up the SPI interface pins
GPIO.setup(SPIMOSI, GPIO.OUT)
GPIO.setup(SPIMISO, GPIO.IN)
GPIO.setup(SPICLK, GPIO.OUT)
GPIO.setup(SPICS, GPIO.OUT)

# 10k trim pot connected to adc #0
potentiometer_adc = 0;

last_read = 0       # this keeps track of the last potentiometer value
tolerance = 5       # to keep from being jittery we'll only change
                    # volume when the pot has moved more than 5 'counts'

while True:
        # we'll assume that the pot didn't move
        trim_pot_changed = False

        # read the analog pin
        trim_pot = readadc(potentiometer_adc, SPICLK, SPIMOSI, SPIMISO, SPICS)
        # how much has it changed since the last read?
        pot_adjust = abs(trim_pot - last_read)

        if DEBUG:
                print "trim_pot:", trim_pot
                print "pot_adjust:", pot_adjust
                print "last_read", last_read

        if ( pot_adjust > tolerance ):
               trim_pot_changed = True

        if DEBUG:
                print "trim_pot_changed", trim_pot_changed

        if ( trim_pot_changed ):
                set_volume = trim_pot / 10.24           # convert 10bit adc0 (0-1024) trim pot read into 0-100 volume level
                set_volume = round(set_volume)          # round out decimal value
                set_volume = int(set_volume)            # cast volume as integer

                print 'Volume = {volume}%' .format(volume = set_volume)
                set_vol_cmd = 'sudo amixer cset numid=1 -- {volume}% > /dev/null' .format(volume = set_volume)
                os.system(set_vol_cmd)  # set volume

                if DEBUG:
                        print "set_volume", set_volume
                        print "tri_pot_changed", set_volume

                # save the potentiometer reading for the next loop
                last_read = trim_pot

        # hang out and do nothing for a half second
        time.sleep(0.5)

I'm assuming that

adcout >>= 1 # first bit is 'null' so drop it

is a erroneously commented correction for the mistake mentioned by ChickenProp above

Why aren't you using the SPI hardware? raspi-config can enable it, so it's not hard. It's easy to use from C, and I HATE C. Bit banging SPI doesn't teach anyone how to use SPI, IMHO.

Has anyone ported the code to use the SPI interface?
Thank,
Ralf

Hey guys!
take a look at my library that uses SpiDev to communicate with the chip.

Reviving this because I think the issue ChickenProp raised in 2012 still remains. Reading 12 bits in that loop is indeed 1 too many, and a right shift or, equivalently, division by 2 is misleading, as is the comment that this is a "null" bit. As someone learning about SPI, this mysterious shift confused me for 2 solid days. The empty bit is already on the bus after the command, as ChickenProp states, so you only need to read out 11 bits: 1 null and 10 valid ADC bits. Sure, you can read the additional bit and shift it out after, but what's the point?

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Can you make the print statements compatible with Python 2.7/3.x?