Worth Read #8

clock stretching
During master-slave interaction, if the slave is busy with other activity, master has to be informed to wait, that is,
If a slave cant receive or transmit another complete byte of data until it performs its activity, it holds the SCL(serial clock line) low to force the master into wait state, this phenomenon is called "clock stretching".
Data transfer continues when the slave is ready for another byte of data and releases the SCL line.