@pbfarmer: I'm currently testing my Vegas at 1258 core (OT setting) @ 807mV and they seem to hold for now (except for two weaker cards that won't have anything less than 850mV even at low clocks).
EDIT: 807 didn't cut it. It mined as I changed it on the fly, but would not initialise at this voltage. 815 seems to work (both init and mining) - Couple of hours going, so far so good.
EDIT2: 815 held for a few hours but ended up crashing in the end. Reducing core clocks further doesn't seem to increase stability, maybe there really is a balance to be found.
Yeah, this is a tough nut to crack - esp for those couple salty cards you inevitably run into. I'm kinda figuring out that you have to tune this a bit backwards from the normal process - stability seems to be all about mem (SOC) speed + cn_config. Core speed tuning on it's own doesn't do much - cclocks only real importance seems to be in determining which cn_config you can use. This is roughly what i'm seeing:
L28+28: >= 1150mhz
L26+26: 1050mhz - 1150mhz
L22+22: 950mhz - 1050mhz (not sure about this one...)
L18+18: 850mhz - 950?mhz
The relationship isn't cleanly linear - i've seen cn_config 'holes' where for instance, in one case L26+26 is fine, L24+24 is crap, L22+22 is good again. And in other cases, I've run into tight couplings, where even reducing cn_config incrementally caused instability.
I'm tempted to just run all my vegas like i do for ethash - 850 (core-p0) / 1107 (mem-p3) / L18+18 - and then tune voltage as needed. It's straightforward (no acg, everything fixed except voltage,) low power, and i still get 18.5kh/s out of it.
EDIT: cn_config numbers are for V64 - maybe scale down by 4 for V56?
I just don't understand how you guys are able to pull off <850mV voltage at the said memclk . My one and only MSI Reference Vega64 at the very same setting is only stable above 870mV.