Memory system including a point-to-point linked memory...

Details Memory system including a point-to-point linked memory... Memory system including a point-to-point linked memory...

2000-01-05

2002-12-31

Verbrugge, Kevin (Department: 2187)

Electrical computers and digital processing systems: memory

Addressing combined with specific memory configuration or...

For multiple memory modules

Reexamination Certificate

active

06502161

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to memory systems, memory subsystems, memory modules or a system having memory devices. More specifically, this invention is directed toward memory system architectures which may include integrated circuit devices such as one or more controllers and a plurality of memory devices.
Some contemporary memory system architectures may demonstrate tradeoffs between cost, performance and the ability to upgrade, for example; the total memory capacity of the system. Memory capacity is commonly upgraded via memory modules or cards featuring a connector/socket interface. Often these memory modules are connected to a bus disposed on a backplane to utilize system resources efficiently. System resources include integrated circuit die area, package pins, signal line traces, connectors, backplane board area, just to name a few. In addition to upgradeability, many of these contemporary memory systems also require high throughput for bandwidth intensive applications, such as graphics.
With reference to
FIG. 1
, a representational block diagram of a conventional memory system employing memory modules is illustrated. Memory system
100
includes memory controller
110
and modules
120
a
-
120
c
. Memory controller
110
is coupled to modules
120
a
-
120
c
via control/address bus
130
, data bus
140
, and corresponding module control lines
150
a
-
150
c
. Control/address bus
130
typically comprises a plurality of address lines and control signals (e.g., RAS, CAS and WE).
The address lines and control signals of control/address bus
130
are bussed and “shared” between each of modules
120
a
-
120
c
to provide row/column addressing and read/write, precharge, refresh commands, etc., to memory devices on a selected one of modules
120
a
-
120
c
. Individual module control lines
150
a
-
150
c
are typically dedicated to a corresponding one of modules
120
a
-
120
c
to select which of modules
1120
a
-
120
c
may utilize the control/address bus
130
and data bus
140
in a memory operation.
Here and in the detailed description to follow, “bus” denotes a plurality of signal lines, each having more than two connection points for “transceiving” (i.e., transmitting or receiving). Each connection point electrically connects or couples to a transceiver (i.e., a transmitter-receiver) or one of a single transmitter or receiver circuit.
With further reference to
FIG. 1
, memory system
100
may provide an upgrade path through the usage of modules
120
a
-
120
c
. A socket and connector interface may be employed which allows each module to be removed and replaced by a memory module that is faster or includes a higher capacity. Memory system
100
may be configured with unpopulated sockets or less than a full capacity of modules (i.e., empty sockets/connectors) and provided for increased capacity at a later time with memory expansion modules. Since providing a separate group of signals (e.g., address lines and data lines) to each module is avoided using the bussed approach, system resources in memory system
100
are efficiently utilized.
U.S. Pat. No. 5,513,135 discloses a contemporary dual inline memory module (DIMM) having one or more discrete buffer devices. In this patent, the discrete buffer devices are employed to buffer or register signals between memory devices disposed on the module and external bussing (such as control/address bus
130
in memory system
100
). The discrete buffer devices buffer or register incoming control signals such as RAS, and CAS, etc., and address signals. Local control/address lines are disposed on the contemporary memory module to locally distribute the buffered or registered control and address signals to each memory device on the module. By way of note, the discrete buffer devices buffer a subset of all of the signals on the memory module since data path signals (e.g., data bus
140
in
FIG. 1
) of each memory device are connected directly to the external bus.
In addition to the discrete buffer device(s), a phase locked Loop (PLL) device may be disposed on the contemporary DIMM described above. The PLL device receives an external clock and generates a local phase adjusted clock for each memory device as well as the discrete buffer devices.
Modules such as the DIMM example disclosed in U.S. Pat. No. 5,513,135 feature routed connections between input/outputs (I/Os) of each memory device and connector pads disposed at the edge of the module substrate. These routed connections introduce long stub lines between the signal lines of the bus located off of the module (e.g., control address bus
130
and data bus
140
), and memory device I/Os. A stub line is commonly known as a routed connection that deviates from the primary path of a signal line. Stub lines commonly introduce impedance discontinuities to the signal line. Impedance discontinuities may produce undesirable voltage reflections manifested as signal noise that may ultimately limit system operating frequency.
Examples of contemporary memory systems employing buffered modules are illustrated in
FIGS. 2A and 2B
.
FIG. 2A
illustrates a memory system
200
based on a Rambus™ channel architecture and
FIG. 2B
illustrates a memory system
210
based on a Synchronous Link architecture. Both of these systems feature memory modules having buffer devices
250
disposed along multiple transmit/receive connection points of bus
260
. In both of these examples, the lengths of stubs are significantly shortened in an attempt to minimize signal reflections and enable higher bandwidth characteristics. Ultimately however, memory configurations such as the ones portrayed by memory systems
100
,
200
and
210
may be significantly bandwidth limited by the electrical characteristics inherent in the bussed approach as described below.
In the bussed approach exemplified in
FIGS. 1
,
2
A and
2
B, the signal lines of the bussed signals become loaded with a (load) capacitance associated with each bus connection point. These load capacitances are normally attributed to components of input/output (I/O) structures disposed on an integrated circuit (IC) device, such as a memory device or buffer device. For example, bond pads, electrostatic discharge devices, input buffer transistor capacitance, and output driver transistor parasitic and interconnect capacitances relative to the IC device substrate all contribute to the memory device load capacitance.
The load capacitances connected to multiple points along the length of the signal line may degrade signaling performance. As more load capacitances are introduced along the signal line of the bus, signal settling time correspondingly increases, reducing the bandwidth of the memory system. In addition, impedance along the signal line may become harder to control or match as more load capacitances are present along the signal line. Mismatched impedance may introduce voltage reflections that cause signal detection errors. Thus, for at least these reasons, increasing the number of loads along the bus imposes a compromise to the bandwidth of the memory system.
In an upgradeable memory system, such as conventional memory system
100
, different memory capacity configurations become possible. Each different memory capacity configuration may present different electrical characteristics to the control/address bus
130
. For example, load capacitance along each signal line of the control/address bus
130
may change with two different module capacity configurations.
As memory systems incorporate an increasing number of memory module configurations, the verification and validation of the number of permutations that these systems make possible may become increasingly more time consuming. Verification involves the confirmation of operation, logical and/or physical functionality of an IC by running tests on models of the memory, associated devices and/or bus prior to manufacturing the device. Validation involves testing the assembled system or components thereof (e.g., a memory module). Validation typically must account for a majority of the com

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