(DRAM) A type of semiconductor
memory in which the information is stored in capacitors
on a MOS integrated circuit
. Typically each bit
is stored as an amount of electrical charge in a storage cell consisting of a capacitor and a transistor
. Due to leakage the capacitor discharges gradually and the memory cell loses the information. Therefore, to preserve the information, the memory has to be refreshed periodically. Despite this inconvenience, the DRAM is a very popular memory technology because of its high density and consequent low price.
The first commercially available DRAM chip was the Intel 1103, introduced in 1970.
Early DRAM chips, containing up to a 16k x 1 (16384 locations of one bit each), needed 3 supply voltages (+5V, -5V and +12V). Beginning with the 64 kilobit chips, charge pumps were included on-chip to create the necessary supply voltages out of a single +5V supply. This was necessary to fit the device into a 16-pin DIL
package, which was the preferred package at the time, and also made them easier to use.
To reduce the pin count, thereby helping miniaturisation, DRAMs generally had a single data line which meant that a computer with an N bit wide data bus
needed a "bank" of (at least) N DRAM chips. In a bank, the address and control signals of all chips were common and the data line of each chip was connected to one of the data bus lines.
Beginning with the 256 kilobit DRAM, a tendency toward surface mount packaging arose and DRAMs with more than one data line appeared (e.g. 64k x 4), reducing the number of chips per bank. This trend has continued and DRAM chips with up to 36 data lines are available today. Furthermore, together with surface mount packages, memory manufacturers began to offer memory modules, where a bank of memory chips was preassembled on a little printed circuit
board (SIP = Single Inline Pin Module, SIMM = Single Inline Memory Module, DIMM = Dual Inline Memory Module). Today, this is the preferred way to buy memory for workstations
and personal computers
DRAM bit cells are arranged on a chip in a grid of rows and columns where the number of rows and columns are usually a power of two. Often, but not always, the number of rows and columns is the same. A one megabit device would then have 1024 x 1024 memory cells. A single memory cell can be selected by a 10-bit row address and a 10-bit column address.
To access a memory cell, one entire row of cells is selected and its contents are transferred into an on-chip buffer. This discharges the storage capacitors in the bit cells. The desired bits are then read or written in the buffer. The (possibly altered) information is finally written back into the selected row, thereby refreshing all bits (recharging the capacitors) in the row.
To prevent data loss, all bit cells in the memory need to be refreshed periodically. This can be done by reading all rows in regular intervals. Most DRAMs since 1970 have been specified such that one of the rows needs to be refreshed at least every 15.625 microseconds. For a device with 1024 rows, a complete refresh of all rows would then take up to 16 ms; in other words, each cell is guaranteed to hold the data for 16 ms without refresh. Devices with more rows have accordingly longer retention times.
Many varieties of DRAM exist today. They differ in the way they are interfaced to the system - the structure of the memory cell itself is essentially the same.
"Traditional" DRAMs have multiplexed address lines and separate data inputs and outputs. There are three control signals: RAS\ (row address strobe), CAS\ (column address strobe), and WE\ (write enable) (the backslash indicates an active low signal). Memory access procedes as follows: 1. The control signals initially all being inactive (high), a memory cycle is started with the row address applied to the address inputs and a falling edge of RAS\ . This latches the row address and "opens" the row, transferring the data in the row to the buffer. The row address can then be removed from the address inputs since it is latched on-chip. 2. With RAS\ still active, the column address is applied to the address pins and CAS\ is made active as well. This selects the desired bit or bits in the row which subsequently appear at the data output(s). By additionally activating WE\ the data applied to the data inputs can be written into the selected location in the buffer. 3. Deactivating CAS\ disables the data input and output again. 4. Deactivating RAS\ causes the data in the buffer to be written back into the memory array.
Certain timing rules must be obeyed to guarantee reliable operation. 1. RAS\ must remain inactivate for a while before the next memory cycle is started to provide sufficient time for the storage capacitors to charge (Precharge Time). 2. It takes some time from the falling edge of the RAS\ or CAS\ signals until the data appears at the data output. This is specified as the Row Access Time and the Column Access Time. Current DRAM's have Row Access Times of 50-100 ns and Column Access Times of 15-40 ns. Speed grades usually refer to the former, more important figure.
Note that the Memory Cycle Time, which is the minimum time from the beginning of one access to the beginning of the next, is longer than the Row Access Time (because of the Precharge Time).
Multiplexing the address pins saves pins on the chip, but usually requires additional logic in the system to properly generate the address and control signals, not to mention further logic for refresh. Therefore, DRAM chips are usually preferred when (because of the required memory size) the additional cost for the control logic is outweighed by the lower price.
Based on these principles, chip designers have developed many varieties to improve performance or ease system integration of DRAMs:
PSRAMs (Pseudo Static Random Access Memory) are essentially DRAMs with a built-in address multiplexor
and refresh controller. This saves some system logic and makes the device look like a normal SRAM
. This has been popular as a lower cost alternative for SRAM in embedded systems. It is not a complete SRAM substitute because it is sometimes busy when doing self-refresh, which can be tedious. Nibble Mode DRAM
can supply four successive bits on one data line by clocking the CAS\ line. Page Mode DRAM
is a standard DRAM where any number of accesses to the currently open row can be made while the RAS signal is kept active.
Static Column DRAM is similar to Page Mode DRAM, but to access different bits in the open row, only the column address needs to be changed while the CAS\ signal stays active. The row buffer essentially behaves like SRAM.
Extended Data Out DRAM (EDO DRAM) can continue to output data from one address while setting up a new address, for use in pipelined
DRAM used for Video RAM (VRAM
) has an additional long shift register that can be loaded from the row buffer. The shift register can be regarded as a second interface to the memory that can be operated in parallel to the normal interface. This is especially useful in frame buffers for CRT
displays. These frame buffers generate a serial data stream that is sent to the CRT to modulate the electron beam. By using the shift register in the VRAM to generate this stream, the memory is available to the computer through the normal interface most of the time for updating the display data, thereby speeding up display data manipulations.
SDRAM (Synchronous DRAM) adds a separate clock signal to the control signals. It allows more complex state machines on the chip and high speed "burst" accesses that clock a series of successive bits out (similar to the nibble mode).
CDRAM (Cached DRAM) adds a separate static RAM array used for caching. It essentially combines main memory and cache
memory in a single chip. The cache memory controller needs to be added externally.
RDRAM (Rambus DRAM) changes the system interface of DRAM completely. A byte-wide bus is used for address, data and command transfers. The bus operates at very high speed: 500 million transfers per second. The chip operates synchronously with a 250MHz clock. Data is transferred at both rising and falling edges of the clock. A system with signals at such frequencies must be very carefully designed, and the signals on the Rambus Channel use nonstandard signal levels, making it incompatible with standard system logic. These disadvantages are compensated by a very fast data transfer, especially for burst accesses to a block of successive locations.
A number of different refresh modes can be included in some of the above device varieties:
RAS\ only refresh: a row is refreshed by an ordinary read access without asserting CAS\. The data output remains disabled.
CAS\ before RAS\ refresh: the device has a built-in counter for the refresh row address. By activating CAS\ before activating RAS\, this counter is selected to supply the row address instead of the address inputs.
Self-Refresh: The device is able to generate refresh cycles internally. No external control signal transitions other than those for bringing the device into self-refresh mode are needed to maintain data integrity.