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Does VSTBY need to be connected on MCF52258 if we're not using it?

Question asked by DAVE BRYANT on Apr 4, 2013
Latest reply on Apr 5, 2013 by DAVE BRYANT

If we're not using the RTC and we don't need to retain SRAM through a short-term power-down, do we need to connect VSTBY to anything?  In our last design, we just tied VSTBY to Vcc, which probably didn't accomplish much since VSTBY will go away at the same time as Vcc.  On this design, we want to lay it out for an RTC, in case we decide to use it, but if not, I'd like to leave VSBTY floating.  I measured some voltage on VSTBY when it was disconnected, so it appears that the micro is powering it internally.  In case someone tells me that they want RAM power through a Vcc glitch, is it ok to use a 1N914 didoe between that pin and Vcc, with a storage cap between the diode and VSTBY?  I assume so, since we'll still be above the 1.8V min voltage spec for the pin.  The main thing I'm looking for is whether we'll have problems with the RAM if we leave VSTBY unconnected.  I also heard that some datasheets for other ColdFire parts recommend tying VSTBY to ground.  That seems counterintuitive to me, but if that's what you say needs to be done, so be it.


I found a post from 2008 on this subject, but noone ever responded to it.