A method and apparatus for selectively causing each
bank of a number of banks of DRAMs of a
DRAM memory card to enter into the self-refresh mode without affecting the operation of any other
bank. In the computer
system incorporating the SIMM or
DIMM type
DRAM cards, each
bank of memory on each card has a RAS
signal specific to that specific bank. One or more CAS signals are supplied across all of the memory banks, on all cards. Thus, each
memory bank is accessed separately for a read / write operation by the RAS becoming active before the CAS becomes active; and refresh takes place by the CAS
signal becoming active before the RAS
signal becomes active. The number of
clock cycles or refresh cycles between active RAS signals to each
memory bank are counted. If RAS does not become active for N
clock or refresh cycles, a signal is provided within each respective
memory bank and that memory bank will immediately, or preferably after M additional
clock or refresh cycles enter self-refresh mode without affecting the operation of any other bank. At the same time, the
memory controller counts cycles of RAS inactivity for each
DRAM bank it controls. A signal is also provided to a register to require a double read / write on the next active read / write cycle to that bank, for reactivating that bank from the self-refresh mode when RAS signal specific to that bank becomes active while CAS is inactive.