FIELD OF THE INVENTION
The present invention relates to a memory system that includes a memory controller and one or more memory modules, each with a bank of memory chips.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the invention will be apparent from the following description of example embodiments, with reference to the accompanying drawings, wherein:
FIG. 1 is a schematic diagram of a conventional multiple slot memory system;
FIG. 2 is a similar diagram of the inventive multiple slot memory system;
FIG. 3 is a schematic diagram of a DDR3 memory module; and
FIG. 4 is an exemplary signal diagram of CA signals on a post-register CA bus.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
A memory system for a PC or server typically has a memory controller and a plurality of RAM modules. In FIG. 1, a memory controller 10 is associated with a number of registered DIMMs 12a, 12b, . . . plugged into corresponding sockets or “slots” 14a, 14b, . . . Each DIMM 12a, 12b . . . has an associated register 16a, 16b, The memory controller 10 is connected to each register 16a, 16b . . . through a pre-register command/address (CA) bus 18 and through a pair of chip select lines 20a1 and 20a2, 20b1, 20b2 . . . Each DIMM has a post-register CA bus 22a, 22b . . . to which all memory chips (not shown) are connected in parallel. The post-register bus 22a, 22b . . . is terminated with a termination resistor 24 on both ends. The termination resistor 24 is connected to a voltage level that is approximately half of the supply voltage for the memory modules. For simplicity, other connections such as the clock and data connections are not shown in the figures.
In FIG. 1, active chip select lines 20a1 and 20a2 are shown with full lines and inactive chip select lines 20b1 and 20b2 are shown in dotted lines. Only register 16a, which receives active chip select signals, switches the CA signals received from the pre-register bus 18 to the associated post-register CA bus 22a, as indicated by a hatched arrow, and register 16b, which receives inactive chip select signals, blocks the CA signals from being switched to the associated post-register CA bus 22b, as indicated by a hollow arrow.
In FIG. 2, the chip select lines 20a1 and 20a2 are active whereas chip select lines 20b1 and 20b2 are inactive. However, (different from FIG. 1) register 16b is also switching the CA signals from pre-register CA bus 18 onto the associated post-register bus 22b, as indicated by a hatched arrow. In fact, all registers of all memory modules in the memory system switch the CA signals (from the pre-register CA bus 18 onto the associated post-register bus) independent of the chip select signals 20a1, 20a2, 20b1, 20b2 . . . received from the memory controller 10.
FIG. 3 is a schematic illustration of a single registered memory module 12 (RDIMM). A number of memory chips 26 are aligned on a circuit board on both sides of register 16 and in parallel along post-register CA bus 22. As is seen, the post-register CA bus 22 is terminated at each of its outer ends with a termination resistor 24. Inputs to the register 16 are the pre-register CA bus 18 and a pre-register clock 28 received from the memory controller 10. The signal clock is also applied in parallel to all memory chips 26 by register 16.
With reference now to FIG. 4, typical differential CA signals are shown at 30 and 32 as would be switched onto the post-register CA bus 22 by register 16. At 34 and 36 complementary “static” signals are shown which would result from tying a bus line to either of the opposite supply terminals of the memory module. Bearing in mind that the post-register CA bus 22 is terminated by resistors connected to a voltage level that is approximately half of the supply voltage, it is seen that power dissipation is considerably less when the bus lines are switched than when they are kept at either of the supply levels. Accordingly, by keeping the register switching the CA signals independent of the chip select signal, power dissipation is reduced and the system speed can be increased.