Reading analog-in on BeagleBone, avoiding "segmentation fault" error

Question

I am new to the Beaglebone, and was moving along at a pretty good click until this strange problem popped-up. I spent all weekend trying to figure it out before posting here.

I am trying to read analog input using multiple sensors from a beaglebone with Angstrom 3.2.5+ and I have a problem that I think might be a hardware issue.

Using this Python script and Alexander Hiam's PyBBIO I have been able to test a photoresistor with success (the value responds to more and less incidental light):

# Import PyBBIO library:
from bbio import *

# Create a setup function:
def setup(): pass # Nothing to do here

# Create a main function:
def loop():

  print "-----"

  print "AIN0 [P9.39] - " + str(analogRead(A0))
  print "AIN1 [P9.40] - " + str(analogRead(A1))
  print "AIN2 [P9.37] - " + str(analogRead(A2))
  print "AIN3 [P9.38] - " + str(analogRead(A3))
  print "AIN4 [P9.35] - " + str(analogRead(A4))
  print "AIN5 [P9.36] - " + str(analogRead(A5))
  print "AIN6 [P9.33] - " + str(analogRead(A6))
  print "AIN7 [P9.39] - " + str(analogRead(A7))

  delay(500)

# Start the loop:
run(setup, loop)

But, any other analog-in (AINx(2-7)) I read with the script above always shows the same value of the photo resistor I have plugged-into AIN1

And, possibly related, I am unable to read any of the AINx(1-7) using cat with an error

cat /sys/bus/platform/devices/tsc/ain1
Segmentation fault

Any ideas? I am really stuck. Thanks in advance.

PS: More info...

My circuit consists of a 10k resistor and a photoresistor. My pin connections are:

• Header P9 Pin 32 (VDD_ADC(1.8V)) to one end of photocell
• Header P9 Pin 34 (GNDA_ADC) to the other end of the resistor
• Header P9 Pin 39 (AIN0) to the other side of the photocell along with one end of pull-down resistor

Here is an image which matches my own circuit:

The results from all of the AINx(1-7)'s and then some...

# cat /sys/bus/platform/devices/tsc/ain0 
cat: /sys/bus/platform/devices/tsc/ain0: No such file or directory 
# cat /sys/bus/platform/devices/tsc/ain1 
Segmentation fault 
# cat /sys/bus/platform/devices/tsc/ain2 
Segmentation fault 
# cat /sys/bus/platform/devices/tsc/ain3 
Segmentation fault 
# cat /sys/bus/platform/devices/tsc/ain4 
Segmentation fault 
# cat /sys/bus/platform/devices/tsc/ain5 
Segmentation fault 
# cat /sys/bus/platform/devices/tsc/ain6 
Segmentation fault 
# cat /sys/bus/platform/devices/tsc/ain7 
Segmentation fault 
# cat /sys/bus/platform/devices/tsc/ain8 
Segmentation fault 
# cat /sys/bus/platform/devices/tsc/ain9 
cat: /sys/bus/platform/devices/tsc/ain9: No such file or directory

UPDATE 1

I edited the analog_test_simple.py file to show all ADC ports on the BeagleBone. This is the result from the shell with no sensors plugged-in.

-----
AIN0 [P9.39] - 3948
AIN1 [P9.40] - 4016
AIN2 [P9.37] - 4002
AIN3 [P9.38] - 3934
AIN4 [P9.35] - 4001
AIN5 [P9.36] - 3935
AIN6 [P9.33] - 3968
AIN7 [P9.39] - 4005
-----
AIN0 [P9.39] - 3946
AIN1 [P9.40] - 4005
AIN2 [P9.37] - 3945
AIN3 [P9.38] - 3957
AIN4 [P9.35] - 4018
AIN5 [P9.36] - 3945
AIN6 [P9.33] - 3967
AIN7 [P9.39] - 3955
...

And if I plug the sensor back in I get a change in value, but again, they are all the same:

-----
AIN0 [P9.39] - 2836
AIN1 [P9.40] - 2836
AIN2 [P9.37] - 2837
AIN3 [P9.38] - 2831
AIN4 [P9.35] - 2840
AIN5 [P9.36] - 2837
AIN6 [P9.33] - 2837
AIN7 [P9.39] - 2837
-----
AIN0 [P9.39] - 2834
AIN1 [P9.40] - 2834
AIN2 [P9.37] - 2829
AIN3 [P9.38] - 2825
AIN4 [P9.35] - 2826
AIN5 [P9.36] - 2817
AIN6 [P9.33] - 2815
AIN7 [P9.39] - 2815
...

Answer 1

Your pulldown resistor between AIN0 and AGND should not exceed 1K Ohm. ADC input current is very limited (2 uA). To make sure that your voltage divider circuit is not influencing the ADC input circuit you should provide for enough current through your voltage divider resistor, therefore lowering its value. At most 1K ohm is recommended.

Answer 2

OK, the answer is that there are two separate problems, both of which are caused by using the PyBBIO library. Read on...

PROBLEM 1: Inconsistent/incorrect sensor values

A suggestion in the comments on my question prompted me to add a delay to my original PyBBIO-based script. Here it is without the sensor:

# python PyBBIO/examples/analog_test_simple.py
-----
AIN0 [P9.39] - 3955
AIN1 [P9.40] - 4015
AIN2 [P9.37] - 4012
AIN3 [P9.38] - 3950
AIN4 [P9.33] - 3998
AIN5 [P9.36] - 4007
AIN6 [P9.35] - 3984
-----
AIN0 [P9.39] - 4007
AIN1 [P9.40] - 3953
AIN2 [P9.37] - 4005
AIN3 [P9.38] - 3941
AIN4 [P9.33] - 3956
AIN5 [P9.36] - 3934
AIN6 [P9.35] - 3946
...

And here are the results after I plug-in the sensor:

# python PyBBIO/examples/analog_test_simple.py
-----
AIN0 [P9.39] - 2888
AIN1 [P9.40] - 2894
AIN2 [P9.37] - 2899
AIN3 [P9.38] - 2907
AIN4 [P9.33] - 2914
AIN5 [P9.36] - 2912
AIN6 [P9.35] - 2914
-----
AIN0 [P9.39] - 2912
AIN1 [P9.40] - 2905
AIN2 [P9.37] - 2901
AIN3 [P9.38] - 2891
AIN4 [P9.33] - 2896
AIN5 [P9.36] - 2890
AIN6 [P9.35] - 2896
...

You can see that plugging-in the sensor still affects all the values, regardless of the delay.

After thinking about this a bit I decided to try to repeat the results with a script without using PyBBIO. I wrote a new Python file to test and print the reading from each analog-in pin on the BeagleBone using the Python os module. The file uses os.system('cat ...') to print the values, and includes a short delay between each. Here is the code:

import os,time
i = 1

while(True):
    os.system("cat /sys/bus/platform/devices/tsc/ain" + str(i))
    print " - AIN" + str(i-1)
    if i==7: 
        i=1
        time.sleep(.5)
        print "------"
    else:
        i+=1    
    time.sleep(.1)

Here are the results without any components plugged-into the BeagleBone:

# python analog-test-all-pins.py
------
3943 - AIN0
3819 - AIN1
3955 - AIN2
2018 - AIN3
2093 - AIN4
3583 - AIN5
3658 - AIN6
------
3947 - AIN0
3746 - AIN1
3959 - AIN2
2034 - AIN3
2123 - AIN4
3547 - AIN5
3644 - AIN6
...

And here are the results with the photoresistor circuit above plugged-into AIN0:

# python analog-test-all-pins.py
------
2915 - AIN0
3347 - AIN1
3556 - AIN2
1478 - AIN3
1602 - AIN4
2393 - AIN5
2402 - AIN6
------
2913 - AIN0
3337 - AIN1
3560 - AIN2
1487 - AIN3
1606 - AIN4
2350 - AIN5
2489 - AIN6

And here are the results with the photo resistor circuit data lead plugged-into AIN2:

# python analog-test-all-pins.py
------
3939 - AIN0
3792 - AIN1
2881 - AIN2
1986 - AIN3
2089 - AIN4
3462 - AIN5
3543 - AIN6
------
3923 - AIN0
3791 - AIN1
2866 - AIN2
1960 - AIN3
2055 - AIN4
3528 - AIN5
3615 - AIN6
...

AIN3:

# python analog-test-all-pins.py
------
3951 - AIN0
3764 - AIN1
3933 - AIN2
2899 - AIN3
2134 - AIN4
3606 - AIN5
3737 - AIN6
------
3960 - AIN0
3823 - AIN1
3957 - AIN2
2910 - AIN3
2118 - AIN4
3635 - AIN5
3645 - AIN6
...

And here are the results if I plug the photoresistor data lead back into AIN0 and plug a TMP36 temperature sensor into AIN3.

# python analog-test-all-pins.py
------
2866 - AIN0
3340 - AIN1
3540 - AIN2
885 - AIN3
1591 - AIN4
2348 - AIN5
2389 - AIN6
------
2872 - AIN0
3339 - AIN1
3551 - AIN2
884 - AIN3
1560 - AIN4
2383 - AIN5
2434 - AIN6
...

And just to confirm that the sensors are being read without affecting the other pins, here is the TMP36 on AIN5

# python analog-test-all-pins.py
------
2897 - AIN0
3338 - AIN1
3557 - AIN2
1464 - AIN3
1578 - AIN4
888 - AIN5
2459 - AIN6
------
2901 - AIN0
3344 - AIN1
3561 - AIN2
1460 - AIN3
1579 - AIN4
889 - AIN5
2395 - AIN6
...

This leads me to conclude that there is a issue with the PyBBIO library that is causing this. It is also causing the second problem, though I have no idea how...

PROBLEM 2: Segmentation fault error

The problem I had with the Segmentation fault error is due to something in the PyBBIO library. For example, if I restart the BeagleBoard and run:

# cat /sys/bus/platform/devices/tsc/ain1
3953
# cat /sys/bus/platform/devices/tsc/ain2
3818

I get values which match the above output from the Python script using the os module.

But if I run any PyBBIO script which accesses AIN pins (examples of which are above) and then run the cat again I get the error:

# cat /sys/bus/platform/devices/tsc/ain1
Segmentation fault
# cat /sys/bus/platform/devices/tsc/ain2
Segmentation fault

If I restart the board (unplug and plug-in the power) I can use cat again:

# cat /sys/bus/platform/devices/tsc/ain1
2890
# cat /sys/bus/platform/devices/tsc/ain2
3366

I also tried using Matt Richardson's mrBBIO library instead, with this file:

from mrbbio import *

def setup():
    #pinMode("P9.36", INPUT)
    pass

def loop():

    print "-----"

    for key, value in analogPinDef.items():
        # There is currently an error in mrBBIO, as the analogPinDef dict points to:
        # ain[0-6] in /sys/devices/platform/tsc/ while it should point at: ain[1-7]
        if key=="P9.39": 
            print analogRead(key) + " - " + value
            delay(100)

    delay(1000)

run (setup,loop)

to get this:

# python mrbbio/analog-test-all-pins.py
-----
2005 - ain4
3636 - ain6
3812 - ain2
2114 - ain5
3872 - ain3
3950 - ain1
-----
2002 - ain4
3530 - ain6
3787 - ain2
2059 - ain5
3895 - ain3
3952 - ain1
...
Cleaning up. Unexporting the following pins:

And I found it does not mess with my ability to get the results with cat -- possibly because it exports the pins at the end.

# cat /sys/bus/platform/devices/tsc/ain1
3960
# cat /sys/bus/platform/devices/tsc/ain2
3830