Hi klingberg!
For I2C, it’s normal to send a command to the device and then do the read. For example, a command could be to return the current soil moisture and then you’d read a few bytes back that would be the soil moisture.
The API for talking to devices is normally found in something called a datasheet that you can download from the manufacturer. I took a quick look at the soil sensor link. From what I can tell, Adafruit has a microcontroller on the soil sensor board and they programmed it themselves with something called the seesaw firmware.
I could not find a datasheet or API guide that lists commands from Adafruit, but they do have Python code that “documents” the interface. See the moisture_read function in seesaw.py.
Reading the code, it looks like the function sends something related to _TOUCH_BASE and _TOUCH_CHANNEL_OFFSET and then reads into buf (which is 2 bytes):
self.read(_TOUCH_BASE, _TOUCH_CHANNEL_OFFSET, buf, .005)
Going to the read function, it looks like it writes _TOUCH_BASE, _TOUCH_CHANNEL_OFFSET to the device, sleeps 0.005 seconds and then reads 2 bytes.
Translating this to Elixir, I’d try:
I2C.write(ref, 0x36, <<0x0f, 0x10>>)
Process.sleep(5)
{:ok, <<value::little-size(16)>> = I2C.read(ref, 0x36, 2)
_TOUCH_BASE = 0x0f and _TOUCH_CHANNEL_OFFSET = 0x10
Based on Adafruit’s code, it looks like they retry if value > 4095.
Hope this helps!
