Monday, December 17, 2001
VIA Technologies pushed for DDR memory while Intel insisted on Rambus. Was VIA''s stratey correct?
DDR (Double Data Rate) Memory, Characteristics and Benefits
As the CPU speed and function continue to increase, DRAM speed will continue to climb. Since the structure of DDR are very similar to the SDRAM both in function and in production cost, it is expected to gradually become the mainstream memory. In the past, the bottleneck of the PC lies on the limitation of the memory bandwidth. Following the increase of CPU frequency, DRAM products were also marching towards higher speed. Since DDR is a natural evolution of SDRAM, it uses the same existing production equipment to lower its production cost. Besides, it does not involve in intellectual property(IP) dispute.
In performance comparision, it is not far behind Rambus. These factors caused Intel to finally submit to DDR adoption even after it had spent much effort in promoting Rambus technology. As the number of DDR application product expand further, it will eventually succeed PC133 SDRAM as the next generation mainstream memory. This is the latest new DRAM revolution since SDRAM had taken over EDO memory in 1986. We will highlight the functional characteristics in the following introductions.
Characteristics of DDR and its advantages
Information or Data Transfer are now moved at twice the efficiency of SDRAM. DDR(Double Data Rate SDRAM), now has comparable performance with the DRDRAM (Direct Rambus DRAM).
DDR SDRAM memory was conceived on the fact that PC133 SDRAM was unable to satisfy Intelís leading CPU bandwidth requirement of 1.6GB/sec. DDR now offers the double data rate DRAM that delivers twice the PC133 bandwidth without any drastic change of the memory manufacturing structure. Since it transfers data during the rising and the falling edge of the clock, it can now reach the peak bandwidth of 2.1GB/sec. According to many industry benchmark test results, the performance of DDR memory is not far behind DRDRAM (Direct Rambus) that Intel had originally pushed. With this advantage, DDR is therefore, in a much better position to succeed as the mainstream product in the DRAM market.
DDR was evolved from the SDRAM structure, it does not involve royalties and problems like DRDRAM
SDRAM was first introduced by South Korean company Samsung in 1985. The specification was established and agreed upon then by Hyundai of South Korea; Mitsubishi, Fujitsu, Toshiba, Hitachi, and Oki of Japan; together with Texas Instruments. DDR is a new product simply evolved from the SDRAM structure. Due to its open standard, no intellectual license is required. Since it requires no additional equipment to enter into production, it virtually encourages manufacturers to jump in. On the contrary, DRDRAM requires a paid license controlled by Rambus Inc. Rambus structure is also totally different from SDRAM. Both DRAM manufacturer and controller manufacturer have to obtain Rambus license for manufacturing. Although many manufacturers had signed on to support Rambus standard under the tremendous pressure from Intel, many semiconductor companies were reluctant to enter into large-scale production.
Utilizing the SDRAM manufacturing infrastructure gives it the advantage in mass production
Out of the 20 to 22 photo masks required to produce SDRAM, manufacturers only have to change the top metal mask and connection pads to arrive at producing DDR. Manufacturers can enter into DDR production using their existing machine platforms. This has not only save the cost of investing in new machines, but also minimized the learning curve. The similarity in IC die size and test also meant no additional production equipment and cost. Even the packaging remains to be in the TSOP II resulting in minimal cost difference. Therefore, all existing SDRAM manufacturers can quickly adapted to the DDR SDRAM production.
Battle between DDR and Rambus to become the mainstream
From the cost point of view: Rambus requires major overhaul of infrastructure. Rambus die size is 10-15% larger than SDRAM. Its yield is, therefore, difficult to control. It results in higher production cost. Packaging also requires uBGA that cost twice as much as the TSOP package. In the area of testing, it requires the most advanced equipment to accommodate the much higher clock frequency. All these factors attributed to the cost of DRDRAM to double that of SDRAM. It also prevents the price to come down. On the other hand, Micron announced that their cost in manufacturing DDR has approached that of SDRAM. The Taiwan manufacturers, however, are skeptical about that. They have declared a manufacturing cost of U$4.5 per piece mainly due to the learning curve and the production change over. They considering initial price increase is inevitable. The higher frequency also prolonged the learning curve at the initial phase. But the bottom line is that Rambus memory requires royalty that adds to the cost. This has caused Intel to reconsider.
Comparing DDR performance with Rambus memory, neither type of memories would double the performance of SDRAM. In reality, they can only deliver 10-20% advantages over SDRAM. This is perhaps the reason why they have difficulty in driving to a rapid market change over. However, in parameter comparison (see table), DDR is indeed technically superior than Rambus.
According to the table, we can see that the structure of DDR is better than DRDRAM. It is likely that DDR would replace SDRAM to become the mainstream memory.
Click Here for Part-3
By: Johnson Wang
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