BeagleBone Black Debian 8 ADC voltage readings incorrect

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1

We just purchased a group of BeagleBone Black Rev. C boards. We’re trying to get the ADC working properly. We use a Adafruit BeagleBone Proto cape. We have a pressure sensor connected to pin P9_39 (AIN0). The pressure sensor produces millivoltage readings. We only sample the reading hourly in production so we’re not using continuous buffered input. This setup worked accurately with the previous BBB Rec C boards using Debian 7. I can’t seem to get an accurate reading under Debian 8 though. I assume I’m not configuring the AIN overlay correctly. I get readings but they are not correct. The values are more than double what I’m reading by using a digital multimeter on P9_39 (202mv digital multimeter 464mv AIN0). If I move the cape from the new Debian 8 BBB Rev C to an earlier Debian 7 BBB Rev C the readings are correct: same cape, same sensor, same power supply.

I’ve tried 3 new BBB Rev C and get the same result

I’ve tried both these two tutorials to build ADC into the overlay. Both tutorial produce working overlays with identical, incorrect readings on AIN0.

https://www.teachmemicro.com/beaglebone-black-adc/ ← specific to Ubuntu 16 but I gave it a whirl

https://gist.github.com/matthewmcneely/bf44655c74096ff96475

I tried loading a number of distribution provided dtb’s, disabling HDMI and Audio. In addition, I downloaded Robert Nelsons dtb-rebuilder and compiled new dtbo’s using the stanza’s suggested in the tutorials above : https://github.com/RobertCNelson/dtb-rebuilder

I’ve tried building overlays using the stock out of the box kernel and after running apt dist-upgrade. In each case I can see the overlay loaded but the values I read are always the same and incorrect.

/boot/uEnv.txt

#dtb=am335x-boneblack.dtb
dtb=am335x-boneblack-overlay.dtb
cape_disable=bone_capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN
#cape_enable=bone_capemgr.enable_partno=
cmdline=coherent_pool=1M quiet cape_universal=enable

cat /sys/devices/platform/bone_capemgr/slots

0: PF---- -1

1: PF---- -1

2: PF---- -1

3: PF---- -1

4: P-O-L- 0 Override Board Name,00A0,Override Manuf,univ-emmc

5: P-O-L- 1 Override Board Name,00A0,Override Manuf,BB-ADC

uname -a : Linux beaglebone 4.1.15-ti-rt-r43 #1 SMP PREEMPT RT Thu Jan 21 20:13:58 UTC 2016 armv7l GNU/Linux

2

We just purchased a group of BeagleBone Black Rev. C boards. We’re trying
to get the ADC working properly. We use a Adafruit BeagleBone Proto cape.
We have a pressure sensor connected to pin P9_39 (AIN0). The pressure
sensor produces millivoltage readings. We only sample the reading hourly in
production so we’re not using continuous buffered input. This setup worked
accurately with the previous BBB Rec C boards using Debian 7. I can’t seem
to get an accurate reading under Debian 8 though. I assume I’m not
configuring the AIN overlay correctly. I get readings but they are not
correct. The values are more than double what I’m reading by using a
digital multimeter on P9_39 (202mv digital multimeter 464mv AIN0). If I
move the cape from the new Debian 8 BBB Rev C to an earlier Debian 7 BBB Rev
C the readings are correct: same cape, same sensor, same power supply.

I've tried 3 new BBB Rev C and get the same result

You can install 3.8.13 in Debian 8..

The easiest way is just:

cd /opt/scripts/tools/
sudo ./update_kernel.sh --bone-channel --stable
sudo reboot

and you'll have 3.8.13-boneX installed..

https://elinux.org/BeagleBoardDebian#Install_Latest_Kernel_Image

I’ve tried both these two tutorials to build ADC into the overlay. Both
tutorial produce working overlays with identical, incorrect readings on
AIN0.

Beaglebone Black ADC: Reading Analog Voltages | Microcontroller Tutorials <- specific to Ubuntu 16
but I gave it a whirl

Enabling ADC (AIN0 - AIN6) on BBB for newer Linux kernels that do not support device tree overlays · GitHub

I tried loading a number of distribution provided dtb's, disabling HDMI and
Audio. In addition, I downloaded Robert Nelsons dtb-rebuilder and compiled
new dtbo's using the stanza's suggested in the tutorials above :
GitHub - RobertCNelson/dtb-rebuilder

I’ve tried building overlays using the stock out of the box kernel and after
running apt dist-upgrade. In each case I can see the overlay loaded but the
values I read are always the same and incorrect.

/boot/uEnv.txt

#dtb=am335x-boneblack.dtb
dtb=am335x-boneblack-overlay.dtb
cape_disable=bone_capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN
#cape_enable=bone_capemgr.enable_partno=
cmdline=coherent_pool=1M quiet cape_universal=enable

cat /sys/devices/platform/bone_capemgr/slots

0: PF---- -1

1: PF---- -1

2: PF---- -1

3: PF---- -1

4: P-O-L- 0 Override Board Name,00A0,Override Manuf,univ-emmc

5: P-O-L- 1 Override Board Name,00A0,Override Manuf,BB-ADC

uname -a : Linux beaglebone 4.1.15-ti-rt-r43 #1 SMP PREEMPT RT Thu Jan 21
20:13:58 UTC 2016 armv7l GNU/Linux

Yuck "4.1.x"...

Regards,

3

Thanks Robert. I’m running the 3.8.13-bone86 kernel now. I apologize because this questions is probably below your pay grade but…do I have to compile a custom overlay to get ADC working on this kernel? I’m confused because our legacy BBB are running 3.8.13-bone70 and the uEnv.txt is simply this:
#dtb=
cmdline=quiet init=/lib/systemd/systemd

##Example
#cape_disable=capemgr.disable_partno=
cape_enable=capemgr.enable_partno=BB-UART4

Somehow ADC is working. When I use that uEnv.txt on kernel 3.8.13-bone86 i get the dreaded "RuntimeError: Unable to setup ADC system. Possible causes are:

  • A cape with a conflicting pin mapping is loaded
  • A device tree object is loaded that uses the same name for a fragment: helper"
    when using the AdaFruit BBIO library.
4

Thanks Robert. I'm running the 3.8.13-bone86 kernel now. I apologize
because this questions is probably below your pay grade but...do I have to
compile a custom overlay to get ADC working on this kernel? I'm confused
because our legacy BBB are running 3.8.13-bone70 and the uEnv.txt is simply

Hi Steven,

So a lot with 3.8.13, i honestly just don't remember. :wink:

this:
#dtb=
cmdline=quiet init=/lib/systemd/systemd

##Example
#cape_disable=capemgr.disable_partno=
cape_enable=capemgr.enable_partno=BB-UART4

Just tested this:

cape_enable=capemgr.enable_partno=BB-UART4,BB-ADC

debian@beaglebone:~$ uname -r
3.8.13-bone80
debian@beaglebone:~$ cat /sys/devices/bone_capemgr.9/slots
0: 54:PF---
1: 55:PF---
2: 56:PF---
3: 57:PF---
4: ff:P-O-L Bone-LT-eMMC-2G,00A0,Texas Instrument,BB-BONE-EMMC-2G
5: ff:P-O-L Bone-Black-HDMI,00A0,Texas Instrument,BB-BONELT-HDMI
7: ff:P-O-L Override Board Name,00A0,Override Manuf,BB-UART4
8: ff:P-O-L Override Board Name,00A0,Override Manuf,BB-ADC
debian@beaglebone:~$ cat /sys/bus/iio/devices/iio\:device0/*_raw
3687
2991
3486
1993
1419
1569
1680
3839

Somehow ADC is working. When I use that uEnv.txt on kernel 3.8.13-bone86 i
get the dreaded "RuntimeError: Unable to setup ADC system. Possible causes
are:
  - A cape with a conflicting pin mapping is loaded
  - A device tree object is loaded that uses the same name for a fragment:
helper"
when using the AdaFruit BBIO library.

Regards,

5

I guess you remembered! You’re incredible…truly. That worked…and I get the same incorrect voltage as I did when using Debian 8. Perhaps this isn’t a kernel issue? I guess this is a hardware issue? Again I tested the voltage at the P9_39 pin with a multimeter. Meter reads 200mv ADC raw input on AIN0 is 465mv.

6

Hi Steven,

Was the 465 value directly from reading the "raw" value?

cat /sys/bus/iio/devices/iio\:device0/*_raw

465 * (1.8V / 4096) = 204.3mV

Regards,

7

Yes that is a raw value.