As the capacitance of the memory cells depletes its charge, the refresh process is used to recharge it line by line. The refresh rate depends on the number of rows and is 1 kHz, 2 kHz, 4 kHz or 8 kHz. The refresh time is approx. 15 ms. During the refresh, the central processing unit has no access to the data stored in the DRAM.
There are special DRAMs that perform the refresh themselves, independently of the central processing unit( CPU) or the external supply. These memory chips with self-refresh mode are used in laptops and notebooks.