CTxIn::SEQUENCE_LOCKTIME_GRANULARITY

In order to use the same number of bits to encode roughly the same wall‐clock duration, and because blocks are naturally limited to occur every 600s on average, the minimum granularity for time‐based relative lock‐time is fixed at 512 seconds. Converting from CTxIn::nSequence to seconds is performed by multiplying by 512 = 2ˆ9, or equivalently shifting up by 9 bits.