Everything You Always Wanted to Know About SDRAM (Memory): But Were Afraid to Ask

SDRAM can, in many ways, be best described as a simple state machine (Figure 2) which is either idle, active, or precharging one or more open banks. As with any machine, transition from one state to another requires a minimum wait time before the system is ready to respond to subsequent requests to do additional work. These delays have a major impact on SDRAM read and write performance, and more importantly, performance of the system as a whole.

Since SDRAM memory cells are really just miniature capacitors, the charge they contain will dissipate away naturally over time due to many factors that can influence the leakage rate, including temperature. A marked reduction in stored charge can result in either data loss or data corruption. In order to prevent this from happening SDRAM must be periodically refreshed by topping off the charge contained in each individual memory cell. The frequency with which this refresh need occur depends on the silicon technology used to manufacture the core memory die and the design of the memory cell itself.

Reading or writing to a memory cell has the same effect as refreshing the selected cell by issuing a Refresh (REF) command. Unfortunately, not all cells are read from or written to during the normal course of operation and so each cell in the array must be accessed and written back (restored) before the expiration of the refresh interval. In most cases, refresh cycles involve restoring the charge along an entire page. Over the course of the entire interval, every page is accessed and subsequently restored. At the end of the interval, the process begins again. A typical Refresh Period (tREF) is hundreds to possibly a thousand or more clocks.

All banks must be precharged and idle for a minimum of the RAS Precharge (tRP) delay before the Refresh (REF) command can be applied. An address counter, internal to the device, supplies the bank address used during the course of the refresh cycle. When the refresh cycle has completed, all banks are left in the precharged (idle) state. A delay between the REF command and the next Activate (ACT) command or subsequent REF command must be greater than or equal to the Row Refresh Cycle Time (tRFC). In other words, a minimum wait of tRFC cycles is required following a refresh to an idle bank before it can be again activated for access.

Before the SDRAM is ready to respond to read and write commands, a bank must first be opened (activated). The memory controller accomplishes this by sending the appropriate command (ACT), specifying the rank, bank, and page (row) to be accessed. The time to activate a bank is called the Row-Column (or Command) Delay and is denoted by the symbol tRCD. This variable represents the minimum time needed to latch the command at the command interface, program the control logic, and read the data from the memory array into the Sense Amplifiers in preparation for column-level access.

Following activation, the open bank contains within the array of Sense Amps a complete page of memory only 8KB in length. At this time, multiple Read (READ) and Write (WRI) commands can be issued, specifying the starting column address to be accessed. The time to read a byte of data from the open page is called the Column Address Strobe (CAS) Latency and is denoted by the symbol CL or tCAS. This variable represents the minimum time needed to latch the command at the command interface, program the control logic, gate the requested data from the Sense Amps into the Input/Output (I/O) Buffers, through a process known as pre-fetching, and place the first word of data on the Memory Bus.

Only one page per bank may be open at a time. Access to other pages in the same bank demands the open page first be closed. As long as the page remains open the memory controller can issue any combination of READ or WRI commands, sometimes switching back and forth between the two, until such time as the open page is no longer needed or a pending request to read/write data from an alternate page in the same bank requires the current page be closed so that another may be accessed. This is done by either issuing a Precharge (PR) command to close the specified bank only or a Precharge All (PRA) command to close all open banks in the rank.

Alternatively, the Precharge command can be effectively combined with the last read or write operation to the open bank by sending a Read with Auto-Precharge (RDA) or Write with Auto-Precharge (WRA) command in place of the final READ or WRI command. This allows the SDRAM control logic to automatically close the open page as soon as certain conditions have been met: (1) A minimum of RAS Activation Time (tRAS) has elapsed since the ACT command was issued, and (2) a minimum of Read to Precharge Delay (tRTP) has elapse since the most recent READ command was issued.

Precharging prepares the data lines and sense circuitry to transmit the stored charge in the Sense Amps back into the open page of individual memory cells, undoing the previous destructive read, making the DRAM core ready to sample the next page of memory to be accessed. The time to Precharge an open bank is called the Row Access Strobe (RAS) Precharge Delay and is denoted by the symbol tRP. The minimum time interval between successive ACT commands to the same bank is determined by the Row Cycle Time of the device, tRC, found by simply summing tRAS and tRP (to be defined). The minimum time interval between ACT commands to different banks is the Read-to-Read Delay (tRRD).