Does the Arduino pro-micro 3.3v use an 8MHz crystal or a 16MHz crystal with a clock divisor of 2? I am trying to convert some 5v Leonardo-compatible boards to run at a lower 3-3.7v voltage, so that I can power my project using LiPo batteries. 

As far as I can tell, the default Leonardo settings (and pro-micro settings) will not run at this voltage. When I power the boards (which expect 5v) from a lithium battery, typically the system does not start at all. My two theories are that either the 16 Mhz xtal oscillator does not start resonating, or that the brown-out detection of the boards precludes booting at the lower voltage. 

I would like to modify the boards to run on lower power. Since the pro-micro exists in a 3.3v variant, I presume there is some way to get the Atmega32u4 chip working at lower voltages. I have reviewed the Atmega32u4 datasheet and schematics from sparkfun, as well as examined the fuse bit differences between the 3.3v and 5v versions of the pro micro. As far as I can tell, the only difference in configuration is the brown-out detection. The clock source settings are configured for a crystal between 8Mhz and 16Mhz, so this could be the same for both boards. 

I wasn't quite able to understand the build system for the Catarina bootloader, which might give hints as to how the clock is handled. It seems like both the 3.3v and 5v boards bootloaders are built from the same source, but with a different "CPU frequency" variable. However, there are quite substantial differences in the .hex file outputs and I wondering if there is something fancier going on. 

As far as I can tell, it's possible to get an 8MHz clock on the Atmega32u4 either by using a 16MHz xtal with a clock divisor of 2, or using an 8MHz xtal with a clock divisor of 1. 

Since the pro-micro 3.3v is the "mainstream" supported board using the Atmega32u4 at a lower voltage, I would like to configure my project to be compatible. Which means I need to build in a compatible clock source, if possible. 

Does anyone know more details of the pro-micro 3.3v/8Mhz configuration that might help clarify how to set this up?
On the schematic of the Sparkfun Pro Micro 3.3V/8 Mhz (here) a resonator of 16 MHz is drawn. But also is stated there: "Board is marked with combination of resonator frequency and regulator voltage". From this you can deduct that the 3.3V/8MHz and the 5V/16MHz versions don't have the same resonator and that the 3.3V/8MHz version most probably has a 8MHz resonator.