Using LPM_COUNTER carry out to stop wraparound

Well it comes from a knowledge of how counters work. If you've ever had the pleasure of working with 74 series logic devices like the 74LS163 for example you would probably know. But I agree it isn't actually very clear from the description.

Think about a 3 bit counter. How would you design one with D type flip flops? Try to do it on paper and you will see.

000

001 -> carry into bit 1

010

011 -> carry into bit 1 and 2

100

101 -> carry into bit 1

110

111 -> carry into bit 1, 2 and 3 (if bit 3 added separately, but this is a 3 bit counter)