Reading values using MCP3002

Question

I'm trying to use MCP3002 for the first time, and can't seem to get the basics to work

I've been following this guide and am pretty certain that all my connections are right (updated photo below).

Running the program below, I can't make sense of the figures i'm getting - using the RPis 3.3V and with this as the potentiometer, I get either 1024 or 0 values irrespective of the position of the potentiometer.

I have added a led to check that the potentiometer works. I've tested all the wires to make sure the crimping is ok.

What am I getting wrong and/or how can I try and troubleshoot to look for any potential issues?

Code:

# botbook_mcp3002.py - read analog values from mcp3002
# (c) BotBook.com - Karvinen, Karvinen, Valtokari

# Installing spidev: 
#  sudo apt-get update
#  sudo apt-get -y install git python-dev
#  git clone https://github.com/doceme/py-spidev.git
#  cd py-spidev/
#  sudo python setup.py install

import spidev   # installation help in botbook_mcp3002.py comments
import time

def readAnalog(device = 0,channel = 0):
    assert device in (1, 0)
    assert channel in (1, 0)
    #open spi
    spi = spidev.SpiDev()
    #spi.open(0, device)
    spi.open(0, device)
    """
    Protocol start bit (S), sql/diff (D), odd/sign (C), MSBF (M)
    Use leading zero for more stable clock cycle
    0000 000S DCM0 0000 0000 0000
    Sending 3 8bit packages so xpi.xfer2 will return the same amount.
    start bit = 1
    sql/diff = 1 SINGLE ENDED MODE  (2 channel mode) 
    odd/sign = channel 0/1
    MSBF = 0
    """
    command = [1, (2 + channel) << 6, 0]
    #2 + channel shifted 6 to left
    #10 or 11 << 6 = 1000 0000 or 1100 0000
    reply = spi.xfer2(command)

    """
    Parse right bits from 24 bit package (3*8bit)
    We need only data from last 2 bytes.
    And there we can discard last two bits to get 10 bit value 
    as MCP3002 resolution is 10bits
    Discard reply[0] byte and start from reply[1] where our data starts
    """ 
    value = reply[1] & 31   
    #31 = 0001 1111 with & operation makes sure that we have all data from XXXX DDDD and nothing more. 0001 is for signed in next operation.
    value = value << 6  #Move to left to make room for next piece of data.
    #000D DDDD << 6 = 0DDD DD00 0000
    #Now we get the last of data from reply[2]
    value = value + (reply[2] >> 2)
    #Here we discard last to bits
    #DDDD DDXXX >> 2 = 00DD DDDD
    #0DDD DD00 0000 + 00DD DDDD = 0DDD DDDD DDDD
    spi.close()
    return value


def main():
    #read channel 0 on device 0
    value = readAnalog(0, 0)
    print(value)
    #time.sleep(10)

if __name__ == "__main__":
    main()

Answer 1

To fault find: Remove the connection to the potentiometer and get two identical resistors (say between 4.7k and 47k). Connect the data in line to each, with the other ends of the resistors connected to 0v and 5v. This should give you a 2.5v voltage in on the analog line in. It should read around 512. If still not, switch to the other data in line (in case the code is reading the other one). It would help also if you have a voltmeter to measure the voltage on the input line.

If that all works step through attaching the potentiometer. But get rid of the LED. You definitely need a pull up (or down) resistor on the other side so that you get a changing voltage.

Also, have you enabled the SPI in raspi-config?

pi@raspberrypi:~ $ ls -l /dev/spi*
pi@raspberrypi:~ $ lsmod | grep spi

Enable it in raspi-config, doesn't appear to need a reboot with the latest HW/OS.

pi@raspberrypi:~ $ sudo raspi-config
pi@raspberrypi:~ $ ls -l /dev/spi*
crw-rw---- 1 root spi 153, 0 Jun 16 09:00 /dev/spidev0.0
crw-rw---- 1 root spi 153, 1 Jun 16 09:00 /dev/spidev0.1
pi@raspberrypi:~ $ lsmod | grep spi
spidev                 16384  0
spi_bcm2835            16384  0

I think the method in the doc you reference may be old, try the above.