Page 74 - A Practical Guide from Design Planning to Manufacturing

50 Chapter Two

The double data rate SDRAM (DDR SDRAM) standard provides more
bandwidth by supplying two pieces of data per memory bus clock in
burst mode instead of just one. This concept has been extended by the
DDR2 standard that operates in the same fashion as DDR but uses dif-
ferential signaling to achieve higher frequencies. By transmitting data
as a voltage difference between two wires, the signals are less suscepti-
ble to noise and can be switched more rapidly. The downside is that two
package pins and two wires are used to transmit a single bit of data.
Rambus DRAM (RDRAM) achieves even higher frequencies by plac-
ing more constraints on the routing of the memory bus and by limiting
the number of bits in the bus. The more bits being driven in parallel,
the more difficult it is to make sure they all arrive at the same moment.
As a result, many bus standards are shifting toward smaller numbers
of bits driven at higher frequencies. Some typical memory bus stan-
dards are shown in Table 2-4.
To make different DRAM standards easier to identify, early SDRAM
standards were named “PC#” where the number stood for the bus fre-
quency, but the advantage of DDR is in increased bandwidth at the same
frequency, so the PC number was used to represent total data band-
width instead. Because of the confusion this causes, DDR and DDR2
memory are often also named by the number of data transfers per second.
Just as with processor buses, transfers per cycle and clocks per cycle
are often confused, and this leads to DDR266 being described as 266-MHz
memory even though its clock is really only half that speed. As if things
weren’t confusing enough, the early RDRAM standards used the PC
number to represent transfers per cycle, while later wider RDRAM bus
standards have changed to being labeled by total bandwidth like DDR
memory.

TABLE 2-4 Memory Bus Standards
Bus Memory Transfers Max data
width bus clock Transfers per second bandwidth
Memory bus (b) (MHz) per clock (MT/s) (MB/s)
PC100 SDRAM 64 100 1 100 800
PC133 SDRAM 64 133 1 133 1066
PC2100 DDR (DDR266) 64 133 2 266 2133
PC2700 DDR (DDR333) 64 167 2 333 2667
PC2-3200 DDR (DDR2-400) 64 200 2 400 3200
PC2-4300 DDR2 (DDR2-533) 64 267 2 533 4267
PC800 RDRAM 16 400 2 800 1600
PC1066 RDRAM 16 533 2 1066 2133
PC3200 RDRAM 32 400 2 800 3200
PC4200 RDRAM 32 533 2 1066 4267