Bubble, Bubble, Toil and Trouble;
Terminate On Die and Let Bandwidth Double
Fortunately, DDRII brings more to the table than simply extra fetching of adjacent memory cells. JEDEC and Intel took a page from Rambus and incorporated On-Die Termination (ODT) into the DDRII. ODT involves moving the capacity to terminate the stub signal onto all of the DRAMs in the system, instead of simply having a single stub terminator on the motherboard. This helps to clean up the signal quite a bit, primarily by eliminating any reflections from other DRAMs. This nets a 400mV gain on voltage swings, and reduces ISI skew by a little over 100ps, quite an improvement over DDR.
The other noticeable changes from DDR are differential signaling and Off-Chip Driver calibration. In differential signaling schemes, the magnitude of the difference between two signals is used to indicate a logical 0 or 1, rather than the actual voltage level . This eliminates any clock skew from noise that affects both signal lines equally, since the difference between the two signals will remain unchanged. OCD improves upon this one step further, by allowing the memory controller to adjust the strength of the drivers for each line to better synchronize the pull-up and pull-down of the two signal lines. Documents presented by Intel and Elpida indicate that the data skew was decreased from 140 to 55ps, as shown in Figures 2 and 3.
Figure 2 – Data Eye at Memory Controller without OCD
Figure 3 – Data Eye at Memory Controller with OCD
Other key benefits of DDRII are the more physically tangible aspects: density, power and heat. Out of the 23 additional signals in DDRII, two of them are used to expand the addressing capacity of DRAMs, from 1Gb to 4Gb. Combined with a doubling in the number of banks to eight (which uses another one of the 23 signals for addressing) this means that 4GB DIMMs are easily within the reach of memory vendors. Since DDRII operates at 1.8V, rather than 2.5V, there are substantial power savings to be realized as well. Figure 4 demonstrates the power savings that can be realized by DDRII DRAMs versus DDR.
Figure 4 – Power Consumption of DDR and DDRII
While it is not an equal speed comparison, Figure 4 does give a rough idea of the advantages. A 70% drop in I/O voltage translates into about a 50% drop in power consumption. This will be quite welcome by motherboard manufacturers who no longer need to route so much power to the memory, as well as consumers, who will get better battery life or a lower electric bill. Less power consumption in turn means less heat production. Initial estimates comparing DDR-400 to DDRII-400, show junction temperatures roughly 29° C cooler, which will be a further boon to system integrators and consumers . This is particularly important for HPC and telecom blade servers and other areas where the thermal footprint must be small.
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