X-bit labs

DDR vs. DDRII: Fight! (page 5)

Category: Memory

by Victor Kartunov

[ 05/19/2004 | 11:30 AM ]

In real life, cells do not work at the same frequency. For example, PC133 was once widespread where DRAM cells work at 133MHz. Accordingly, DDR200 had a higher bandwidth than PC133, but was slower as concerned latencies. In fact, the number of latency clocks being the same, PC133 has a 33% higher clock rate (the clock is thrice shorter). As a result, only DDR266 that had the same latencies as PC133 showed the real advantages of DDR SDRAM.

We see a similar situation today. Yes, the latencies are the same for DDR200 and DDR2-400 and the second module will have a higher bandwidth. But in fact DDR2-400 will compete with DDR400 rather than DDR200! And that’s where everything changes: first, the bandwidth of the modules is the same, 3.2GB/s. Second, the frequency of the DRAM array is 200MHz with DDR400, while DDR2 works at 100MHz. As a result, the latencies are noticeably smaller with DDR400, even considering that the latencies of DDR400 are typically three, rather than two clocks.

Let’s again view it in numbers. For DDR400, we usually have 2 or 2.5 latency clocks, sometimes 3. That is, from 10 to 15 nanoseconds. For DDR2-400 we calculate the latency in the following way: let the core has 2 latency clocks at 100MHz. It means we have 20ns latency. It means 4 latency clocks at the interface frequency (as the interface works at a higher clock rate). Thus, the resulting latencies for a DDR2 module will be 4-4-4 clocks. Considering the relatively low core clock rate, we may hope to see DDR2-400 modules with 3-3-3 characteristics in the future. But even such DDR2-400 modules will lose to a DDR400 module as concerns the time of accessibility of the data.

Overall, the situation seems absurd. Yes, DDR2 is potentially faster, since it provides a much higher bandwidth. But the transition to DDR2 will first somewhat slow down the systems that use it compared to systems with DDR. As you understand, the industry needs to produce some other advantages to attract the end-buyer.

Besides the Speed

DDR memory modules typically use TSOP-packaged chips (known to you by the numerous snapshots of memory modules). This packaging is all right with frequencies up to 200MHz, but starts to fail on higher clock rates. The chips in a TSOP package have too high resistance and inductance capacity, which hinders further frequency growth.

That’s why another packaging – BGA or Ball Grid Array – is long employed for higher frequencies, for example in graphics cards. This packaging is good as it has lower resistance and capacitance, smaller geometrical dimensions, and permits to take heat off more efficiently. That’s why DDR2 modules will use the BGA packaging. There are no miracles in this world, though, and this package costs more to manufacture.

Note that the BGA packaging itself is no prerogative of DDR2 memory. For example, Kingmax uses BGA for DDR memory as the company is into producing overclocker memory modules and the packaging makes the memory more tolerant to high frequencies.