I dont really see the header file for the functions of the STM chip that the board uses.
I’m not clear on what device you have or what Serial pins you want to use, but here’s a bunch of Docs that might help you sort it out on your own 
0.5.x
https://docs.particle.io/reference/firmware/electron/#begin--1
0.6.0-rc.1
https://github.com/spark/docs/blob/prerelease/firmware/firmware.md#begin-1
If that doesn’t help, please ask a specific question about what you are trying to do and we’ll try to point you in the right direction 
2 Likes