[MinnowBoard] Controlling SPI lines for getting data from ADC via SPI on MinnowBoard Turbot

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[MinnowBoard] Controlling SPI lines for getting data from ADC via SPI on MinnowBoard Turbot


I'm trying to get data from ADS8318 ADC hooked up to MinnowBoard Turbot (fw
0.95) running Ubuntu 14.04 3.19.0-25-generic. According to the documentation
of the ADC (https://www.ti.com/lit/ds/symlink/ads8318.pdf) I should modify
the SPI lines in specific order and timing to initiate the conversion and
then read the resulting sample via SPI. The ADC is used in daisy chain mode.

I'm able to access the SPI device at /dev/spidev0.0 using the
low-speed-spidev board file, but it seems that the GPIO pins corresponding
to the SPI lines cannot be accessed, when SPI is in use. When trying to
export the SPI pins (such as CS in pin 476), I get the following error:

echo 476 > /sys/class/gpio/export
bash: echo: write error: Invalid argument

and dmesg shows:

byt_gpio INT33FC:00: pin 66 cannot be used as GPIO.

I've tested that, e.g., the onboard d2 led can be accessed with GPIO 360, so
the issue seems to be specifically with the SPI pins. I also tried disabling
LPSS SPI Support in BIOS to enable access to the SPI GPIO pins, but this
prevents me from using the SPI device.

The issue has been discussed earlier, but no solution is given at the end of
the thread:

Should it be possible to use the ADC via SPI with MinnowBoard Turbot? How
should the SPI lines be accessible for initiating the ADC sampling? I'm not
aware of any special SPI control messages to control the ADC functionality
by sending the control messages via SPI, as seems to be the case for some
other ADCs.

I'm new to embedded systems in general, so any pointers to even the most
obvious things I might be missing would be highly appreciated.

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