In continuous mode, the ADC driver can be triggered using a GPIO.
(For details on continuous mode: http://beagleboard-gsoc13.blogspot.co.uk/2013/07/sampling-analogue-signals-using-adc-on.html)
GPIO numbering on the BBB can be a bit tricky due to the multiple banks.
If you don't know what I'm talking about, see
Now that we know which GPIO to use for triggering your ADC, we try to look at how to connect the gpio to the IIO driver.
As usual, the IIO subsystem is cryptic. Diving deep into IIO code is very tricky. I'm not that good yet I guess.
I've asked on the iio list. Could be something really simple i'm missing..
Have tried googling for a couple of hours.