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dongheng b061230056 feat(esp8266): refactor CCOMPARE timer and system time by microseconds
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.
2019-08-19 14:03:20 +08:00
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