It is in the "older" FCOMs : In AUTO position on ground, the Transponder does not transmit anything. It starts transmitting in AUTO as soon as one engine is started. When the selector is set to ON, on ground, even with both engines stopped, the transponder is transmitting assigned squawk code and callsign as inserted on the INIT page of the MCDU. Verified it the other day with a cooperative Ground Controller.
Thus, on many big airports, as per notams or Jeppesen Airport plates, pilots are required to make themselves "visible" to the GMC radar, by switching the transponder to on, as they might be pushed back for a while before they'd be allowed to start the engines, hence, just before push back set the transponder to ON and as soon as you have an engine above the oil pressure red line ( information from the maintenance guys which I could not verify ) you can select the transponder back to AUTO. Now, I stand corrected whether the newer A320s in AUTO transponding mode transmit either the squawk or the callsign only, on ground with engines shut down, when the selector is ON it transmits both : the callsign and the squawk code. Hope this helps, Cheers and safe flying, SA.