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An extract on #returnjourney

An asynchronous DRAM chip has power connections, some number of address inputs (typically 12), and a few (typically one or four) bidirectional data lines. There are four active-low control signals: RAS, the Row Address Strobe. The address inputs are captured on the falling edge of RAS, and select a row to open. The row is held open as long as RAS is low. CAS, the Column Address Strobe. The address inputs are captured on the falling edge of CAS, and select a column from the currently open row to read or write. WE, Write Enable. This signal determines whether a given falling edge of CAS is a read (if high) or write (if low). If low, the data inputs are also captured on the falling edge of CAS. OE, Output Enable. This is an additional signal that controls output to the data I/O pins. The data pins are driven by the DRAM chip if RAS and CAS are low, WE is high, and OE is low. In many applications, OE can be permanently connected low (output always enabled), but it can be useful when connecting multiple memory chips in parallel. This interface provides direct control of internal timing. When RAS is driven low, a CAS cycle must not be attempted until the sense amplifiers have sensed the memory state, and RAS must not be returned high until the storage cells have been refreshed. When RAS is driven high, it must be held high long enough for precharging to complete. Although the DRAM is asynchronous, the signals are typically generated by a clocked memory controller, which limits their timing to multiples of the controller's clock cycle.

Classic asynchronous DRAM is refreshed by opening each row in turn. The refresh cycles are distributed across the entire refresh interval in such a way that all rows are refreshed within the required interval. To refresh one row of the memory array using RAS Only Refresh, the following steps must occur: The row address of the row to be refreshed must be applied at the address input pins. RAS must switch from high to low. CAS must remain high. At the end of the required amount of time, RAS must return high. This can be done by supplying a row address and pulsing RAS low; it is not necessary to perform any CAS cycles. An external counter is needed to iterate over the row addresses in turn.