MC9S12XD256CAL Datasheet, PDF (336/1348 Pages) Freescale Semiconductor, Inc – This specification describes the function of the clocks and reset generator (CRG).

Chapter 7 Enhanced Capture Timer (S12ECT16B8CV2)
When PACN1 overflows from 0x00FF to 0x0000, the interrupt flag PBOVF in PBFLG is set.
Full count register access will take place in one clock cycle.
NOTE
A separate read/write for high byte and low byte will give a different result
than accessing them as a word.
When clocking pulse and write to the registers occurs simultaneously, write
takes priority and the register is not incremented.
MC9S12XDP512 Data Sheet, Rev. 2.21
336
Freescale Semiconductor