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The introduction of double-data-rate (DDR) memory has afforded a quantum leap in performance for commercially available desktop and portable computers. That's a step much warranted by the multigigahertz processor speeds found on today's motherboards.
With a 2.6-GHz CPU, typical 100-Mbit/s SDRAM would be a throughput bottleneck. Brute-force speed increases in standard SDRAM create power-dissipation, noise-margin, transmission-line, and cost problems. Adding DDR memory doubles the data-transfer rate without doubling the clock rate, avoiding pc-board design and layout complexity. However, it requires that the DDR regulators have tighter dc regulation, higher currents, and close tracking for both the VTT (termination supply voltage) and VDD (memory bus supply voltage) regulators.
For the PC2100 device discussed in the article, a new approach to data transmission and line termination was adopted so that the data lines can switch at twice their previous speeds over the same pc-board traces. The DDR266 device employed has a clock frequency of 133 MHz and a peak data transfer of 266 Mbits/s. That's 2.1 Gbytes/s (PC2100) for a 64-bit wide bus over the same pc-board traces of the PC133 (266 × 8 = 2100).
Power considerations when moving from single-data-rate (SDR) to DDR memory are discussed. Newer DDR2-type memories should bring considerable power savings over the current DDR1 version due to lower-voltage rails.
Other key considerations are dc-dc converter compliance with DDR power and bus accuracy, along with meeting timing and sequencing guidelines. The regulator comes into play here.
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