old: CCOMPARE timer triggers when CCOUNT increase to equal to CCOMPARE, then ISR will increase integer of "_xt_tick_divisor"
to CCOMPARE and wait for next interrupt triggering
now: CCOMPARE timer triggers when CCOUNT increase to equal to CCOMPARE, then ISR will reset CCOUNT to be 0 and reset CCOMPARE
to be integer of "_xt_tick_divisor", then wait for next interrupt triggering
Using the new method, we may get the CCOUNT value without considing if it has overflowed.
System running microseconds = g_os_ticks * microseconds per tick + CCOUNT.
