Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1999-09-13
2001-07-24
Guffin, Jeffrey (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C710S108000, C710S108000, C710S107000, C710S120000
Reexamination Certificate
active
06266252
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to computer systems with enhanced memory access performance, and more specifically, to systems which so provide by electronically terminating the memory bus in its characteristic impedance.
DESCRIPTION RELATIVE TO THE PRIOR ART
Increasing computer performance and capacity has resulted in a constant demand for larger amounts of RAM (random-access memory), and faster RAM memory.
Factors which limit the amount and speed of RAM include the configuration of the memory chips which contain the RAM, and the interconnections on the printed circuit boards which carry the chips.
As it pertains to the memory packaging used today in the industry to assemble memory, and in order to meet a desired DATA BUS width which is standard in the industry, a cluster of memory chips are assembled together on a printed circuit board. These boards are of several types, known as SIMMs, DIMMs, SODIMMs, RIMMS, etc. However, for the sake of brevity, the term DIMM will be used hereinafter to refer to any or all of these different types.
The DIMMs have conductive pads at the edge of the boards, called edge connectors, which make the electronic connection required when inserting into connectors, which also act to support the DIMMs and the memory chips which are assembled on DIMM boards.
The connectors are generally soldered onto a motherboard to facilitate channeling the conductive lines from the computer processor (CPU), or from the controller chip to the DIMM memory chips, or DRAMs (or other type of memory chip as previously described). There are DATA LINES, ADDRESS LINES, AND CONTROL LINES, which together form the computer BUS SYSTEM.
The DATA LINES are bi-directional. They connect bi-directional points of the CPU or controller with the bi-directional points of the DRAM chips that are on the DIMMs. Any physical printed wire length between two points will be driven by an electronic component, the driver (D), and be received by another electronic component, the receiver (R). The speed by which the physical printed wire is charged up depends on several factors one of which is the ability of the electronic component that drives or activates this line to provide such required charge, and the other is the total capacitance of the line been charged and other parameters. The amount of charge accumulated on the line is determined by the Capacitance of the line. By the laws of physics, each printed wire forms a capacitor whose capacitance is the capacitance of the printed wire line length and the capacitance of all the electronic component pins and circuits connected to the line.
When connectors for D are clustered on the BUS to make up the desired memory density for the specified system, the cumulative capacitance on each DATA LINE is increased. In order to attain desired speed performance, it is specified for the system to have a limited number of DIMMs attached to the BUS before the DATA LINES are required to be re-powered, or redriven.
The present construction of the DIMMs is such that a number of DRAMS are connected together in order to increase the memory density. Each DRAM chip pin presents a specified capacitive load. The printed wire line that is used to connect all the DRAM chips together adds to the capacitive load. The total allowable capacitive load that is measured at the entry TAB of the DATA LINE at the DIMM is specified by the system board designer and becomes the limiting factor of how many DIMMs can be used on the BUS to make up the desired density.
During operation, only one DIMM is selected at a time. However the selected DIMM data lines see the entire capacitive load that is present on the entire bus by all other DIMMs, the connectors, and the motherboard printed wire length, plus all other factors.
The individual leads are not perfect conductors, however, especially at the high speeds at which modem computers run. The memory access speeds are today measured in nanoseconds (10
−9
seconds), with picosecond (10
−12
seconds) speeds on the horizon for personal computers.
At such speeds, resistance of the leads, and the capacitance between leads forms a resistance-capacitance circuit which causes the pulses traveling between the connectors and the memory chips to become degraded, sometimes to the point of becoming unreliable.
It is well known that a resistance in series with a capacitance will effect a time delay which is described by
&Dgr;t=1/(RC)
where
&Dgr;
=the time delay caused by the RC circuit
R=the resistance
C=the capacitance
The result of such an RC delay is to cause a series of sharply defined pulses to become undetectable at some point. Consider, as an example, the idealized pulses shown in
FIG. 2
a
. These have perfectly sharp comers
52
,
54
, and are easy to detect, but such a waveform is rarely seen at high speeds. Rather, the capacitance which is always present may cause each pulse to exhibit a rise time
56
, and a fall time
58
. As seen in
FIG. 2
c
, when the rise time
60
, and fall time
62
, become excessive compared to the pulse width
64
, the pulses become highly distorted, and difficult to detect, resulting in detection errors which may become unacceptable.
Thus, the speed at which the memory can be accessed is a direct function of the capacitance an resistance of the leads, as well as other factors. The capacitance is especially troubling, because the capacitance of each line adds to the capacitance of the others. This addition of the capacitances also limits the number of memory chips which can be used to populate a memory board, since the more memory chips, the more leads, and the more leads, the more capacitance is introduced.
To date, no provisions have been made by the industry to use any means to isolate a selected memory module from non-selected memory modules in order to reduce capacitive load and increase speed. No prior art has been found which has utilized FET switching to accomplish such isolation.
The current invention solves this problem by isolating the data leads from the edge connector by means of high-speed FET switches, located close to the connector, which effectively negates the additive effect of line capacitances, as will be described infra.
As used in many personal computer systems, the system is provided with less than the maximum number of memory cards possible, leaving connectors available for later memory expansion. In high speed systems the bus connecting the DIMMs is, in effect, a transmission line, with the attending, well-known problems of standing waves, reflections at the terminations, etc. A standard technique for reducing or eliminating reflections and similar transmission line problems is to terminate the line in its characteristic impedance. However, in a system in which only one of several DIMMS are selected in any given memory access, the characteristic impedance will change according to which DIMM is selected. As a result, reflections on the bus, resulting in standing waves, may degrade signal performance to the point where the signal is unreliable.
The current invention solves this problem by means of a high-speed FET switch, located close to the connector on the DIMM termination board, which selects one of several different terminations, as will be described infra.
SUMMARY OF THE INVENTION
It is the general object of this invention to provide an high-speed RAM memory for use in computer systems without sacrificing capacity, or alternatively, to provide a high-capacity memory without sacrificing speed. It is a specific object of this invention to provide such high speed, or high capacity memory by terminating the memory bus with a termination card containing an electronically switchable termination circuit.
According to one aspect of the invention, a method for enhancing the performance of an electronic system which comprises a multiplicity of printed circuit boards, a motherboard containing a bus, having a characteristic impedance, includes a number of elements. These include terminating the bus with a termination card, mounting onto
Guffin Jeffrey
Tran Thanh Y.
White Mark P.
LandOfFree
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