GPIO 1 28 on Debian

After booting Debian for Beaglebone Black I looked at P9 pin 12 (GPIO 1 28) on an oscilloscope, their is a positive 65ms wide positive pulse that repeats approximately every second.

Is this pin not set up as a GPIO pin?

I tried

echo 60 > /sys/class/gpio/export

However when I tried

ls -al /sys/class/gpio

after this gpio60 did not show up.

It sounds like the line is configured to be a clock signal, perhaps it is in MODE 6 (mcasp0_aclkr_mux3)?

What dtb/overlays are you using when booting?