DDR SDRAM Memory

- overview, tutorial about the basics of what is DDR SDRAM, its technology operation, advantages & disadvantages.

A further advance in SDRAM memory technology is known as DDR SDRAM, or double data rate SDRAM.

This form of SDRAM provides data transfer at twice the speed of traditional SDRAM memory. This is achieved by transferring data twice per cycle, i.e. on both the rising and then the falling edge of the clock signal.

As the name implies, DDR SDRAM gains its name from the fact that it achieves nearly twice the bandwidth of a single data rate, SDR SDRAM running at the same clock frequency.

As a result of its speed, DDR SDRAM was quickly adopted and single data rate, SDRAM soon became obsolete.


DDR SDRAM basics

DDR SDRAM utilises techniques including very tight timing controls to increase the data transfer rates by almost a fact of two.

The very tight timing requirements often require the use of phase locked loops and self-calibration techniques to ensure the timing is accurate.

The key to the operation is that the DDR SDRAM is able to transfer data on both the rising as well as the falling edges of the clock pulse. This has many advantages, not only increasing the data rate, but also reducing other problems such as the signal integrity requirements. At these speeds, signal integrity can become a significant issue, and maximising the data transfer rate for a given clock rate provides improvements in this area.


DDR SDRAM Type Data Rate
Mb/s/pin
Memory Clock Speed
(MHz)
DDR-266
266
133
DDR-333
333
166
DDR-400
400
200

DDR SDRAM data rates and clock speeds

DDR SDRAMs access multiple memory locations in a single read or write command.

A memory read operation entails sending an "Activate" command followed by a "Read" command.

The memory has a certain latency after which the data is available - the memory provides a burst of data from two, four, or eight memory locations at a rate of two memory locations per clock cycle. It is therefore possible to read four memory locations in two consecutive clock cycles.


DDR SDRAM banks and arrays

DDR SDRAM memory has multiple banks. This enables the memory to provide multiple interleaved memory access, and this enables the overall memory bandwidth to be increased. A bank of memory is equal to an array or memory.

These banks can be addressed separately, and to accommodate this memory addressing is required. As this is done in binary notation, four DDR SDRAM memory banks require two lines for addressing: BA0 & BA1.

To provide an example of how DDR SDRAM operates in banks, a four bank DDR SDRAM may operate as follows:

  1. An activate command opens a row in the first SDRAM bank.
  2. A second Activate command activates a row in the second bank.
  3. Read or Write commands can be sent to columns in the rows in banks one and two where the rows are open.
  4. A Precharge command is sent once the read or write operations are complete. This closes the row and bank areas open.
  5. The memory is ready for the next Activate command.

DDR SDRAM power

While DDR SDRAM provides an improvement in speed, this comes at a cost of the power dissipated.

The power required by a DDR SDRAM is related to the number of rows that are open at any one time. Thus to gain the fastest operation, it is necessary to open a number of rows together, but this consumes more power. For low power operation, only one row should be open at any one time in each bank, and there should also not be multiple banks each with open rows.


Although DDR SDRAM was a major step forwards, it was soon superseded by the next improvement which was referred to as DDR2 SDRAM.

By Ian Poole


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