The datasheet (section 5) explains it the best that I could even try to.
Basically, the first six bits of register CNF1 are used for the Baud Rate Prescaler (BRP) which generates the Time Quantum (TQ). The time quantum is just a time duration that we will add together to create the Bit Time.
The Bit Time is composed up of:
SyncSeg with a fixed duration of one time quantum
PropSeg with a configurable duration of one to eight time quantum
PhaseSeg1 (PS1) with a configurable duration of one to eight time quantum
PhaseSeg2 (PS2) with a configurable duration of one to eight time quantum
Section 5.3 shows a few crucial requirements about the relationship of PropSeg, PS1, PS2, TDelay, and SJW.
PropSeg + PS1 >= PS2
PropSeg + PS1 >= Tdelay
PS2 > SJW
So for a BRP of 15 and using a 16MHz crystal, you end up with a 2 microsecond TQ.
At a baudrate of 31.25k, you would have a Bit Time of 32 microseconds.
Dividing the Bit Time by the TQ gives the total number of TQ needed to create the baudrate.
So, I ended up making a spreadsheet to calculate the bits…