Suspend Power Cheat

Andy Green andy at
Thu Feb 14 18:24:47 CET 2008

Hash: SHA1

Hi folks -

I was musing about the SDRAM on the 1.8V rail and some of the CPU rails
so it can come back up maybe are all we are interested in.  We're not
really interested in IO_3V3, despite it is pulling 1mA and we didn't
find out why.

What happens if when entering suspend, right at the end we do one of the
following (IO_3V3 consumption is 3.3mW at 3.3V of total 6mW):

 - Set IO_3V3 to 3.0V (max saving  300uW / 6mW =  5%)
 - Set IO_3V3 to 2.7V              600uW       = 10%
 - Set IO_3V3 to 1.8V             1500uW       = 25%
 - Set IO_3V3 to 1.2V             2100uW       = 35%

I had a quick look around and we should survive the first two, maybe the
next two.  For the CPU only the CPU IO takes IO_3V3, the logic should be
powered at 1.2V during this time.  If we also provide 1.2V, we shouldn't
make any bad current flow.

Obviously this can affect WLAN module state which is supplied by IO_3V3,
although GSM doesn't seem to use IO_3V3 any may be fine with it.

What do you think?

- -Andy
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