STM32F401 Flex Module

I have added the 32KHz crystal on both sizes. This should save a tiny amount of power and allow for RTC operation on the larger board design. The USB BiPower board's new OTG design includes a more efficient i2c bus for communicating with the charge detection and negotiating OTG (yes, OTG is in this uc, but size and cost of a charge pump over a separate OTG negotiator is negligible). I'll be swapping some of the traces to work more efficiently with i2c and better impedance control on the USB traces.

The boards still need more testing, but they are programmable.  I'll be testing more functions of the configured buses for both small and large layouts.

It appears that STM32F401 might be unofficially being updated.  The STM32F401 has yet to become generally available and the new STM32F411 is coming out with on first glance drop in pin replacement.  It is lower current and higher frequency, so this should be a good thing.  We are talking to our ST sales rep and will try to get samples.

We have shrunk the module by going 4 layer.  You can find the new smaller version available in the 'small' directory on github.  The large version hasn't changed much, though we did redo it's layout in an attempt to see if we could add more components on the same board size.  To shrink any further, we will need to switch to the much smaller WLCSP 3x3mm version.