Electrical computers and digital processing systems: memory – Storage accessing and control – Access timing
Reexamination Certificate
1998-08-28
2002-01-08
Kim, Matthew (Department: 2186)
Electrical computers and digital processing systems: memory
Storage accessing and control
Access timing
C713S400000
Reexamination Certificate
active
06338127
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to integrated circuit devices, and more particularly to resynchronizing a plurality of internal clock signals derived from respective external clock signals to ensure the internal clock signals can be used in latching respective external digital signals at the optimum times.
BACKGROUND OF THE INVENTION
Conventional computer systems include a processor (not shown) coupled to a variety of memory devices, including read-only memories (“ROMs”) which traditionally store instructions for the processor, and a system memory to which the processor may write data and from which the processor may read data. The processor may also communicate with an external cache memory, which is generally a static random access memory (“SRAM”). The processor also communicates with input devices, output devices, and data storage devices.
Processors generally operate at a relatively high speed. Processors such as the Pentium® and Pentium II® microprocessors are currently available that operate at clock speeds of at least 400 MHz. However, the remaining components of existing computer systems, with the exception of SRAM cache, are not capable of operating at the speed of the processor. For this reason, the system memory devices, as well as the input devices, output devices, and data storage devices, are not coupled directly to the processor bus. Instead, the system memory devices are generally coupled to the processor bus through a memory controller, bus bridge or similar device, and the input devices, output devices, and data storage devices are coupled to the processor bus through a bus bridge. The memory controller allows the system memory devices to operate at a lower clock frequency that is substantially lower than the clock frequency of the processor. Similarly, the bus bridge allows the input devices, output devices, and data storage devices to operate at a substantially lower frequency. Currently, for example, a processor having a 300 MHz clock frequency may be mounted on a mother board having a 66 MHz clock frequency for controlling the system memory devices and other components.
Access to system memory is a frequent operation for the processor. The time required for the processor, operating, for example, at 300 MHz, to read data from or write data to a system memory device operating at, for example, 66 MHz, greatly slows the rate at which the processor is able to accomplish its operations. Thus, much effort has been devoted to increasing the operating speed of system memory devices.
System memory devices are generally dynamic random access memories (“DRAMs”). Initially, DRAMs were asynchronous and thus did not operate at even the clock speed of the motherboard. In fact, access to asynchronous DRAMs often required that wait states be generated to halt the processor until the DRAM had completed a memory transfer. However, the operating speed of asynchronous DRAMs was successfully increased through such innovations as burst and page mode DRAMs which did not require that an address be provided to the DRAM for each memory access. More recently, synchronous dynamic random access memories (“SDRAMs”) have been developed to allow the pipelined transfer of data at the clock speed of the motherboard. However, even SDRAMs are incapable of operating at the clock speed of currently available processors. Thus, SDRAMs cannot be connected directly to the processor bus, but instead must interface with the processor bus through a memory controller, bus bridge, or similar device. The disparity between the operating speed of the processor and the operating speed of SDRAMs continues to limit the speed at which processors may complete operations requiring access to system memory.
A solution to this operating speed disparity has been proposed in the form of a computer architecture known as a synchronous link architecture. In the synchronous link architecture, the system memory may be coupled to the processor either directly through the processor bus or through a memory controller. Rather than requiring that separate address and control signals be provided to the system memory, synchronous link memory devices receive command packets that include both control and address information. The synchronous link memory device then outputs or receives data on a data bus that may be coupled directly to the data bus portion of the processor bus.
An example of a computer system
10
using the synchronous link architecture is shown in FIG.
1
. The computer system
10
includes a processor
12
having a processor bus
14
coupled through a memory controller
18
and system memory bus
22
to three packetized or synchronous link dynamic random access memory (“SLDRAM”) devices
16
a-c
. The computer system
10
also includes one or more input devices
20
, such as a keypad or a mouse, coupled to the processor
12
through a bus bridge
22
and an expansion bus
24
, such as an industry standard architecture (“ISA”) bus or a peripheral component interconnect (“PCI”) bus. The input devices
20
allow an operator or an electronic device to input data to the computer system
10
. One or more output devices
30
are coupled to the processor
12
to display or otherwise output data generated by the processor
12
. The output devices
30
are coupled to the processor
12
through the expansion bus
24
, bus bridge
22
and processor bus
14
. Examples of output devices
24
include printers and a video display units. One or more data storage devices
38
are coupled to the processor
12
through the processor bus
14
, bus bridge
22
, and expansion bus
24
to store data in or retrieve data from storage media (not shown). Examples of storage devices
38
and storage media include fixed disk drives floppy disk drives, tape cassettes and compact-disk read-only memory drives.
In operation, the processor
12
sends a data transfer command via the processor bus
14
to the memory controller
18
, which, in turn, communicates with the memory devices
16
a-c
via the system memory bus
22
by sending the memory devices
16
a-c
command packets that contain both control and address information. Data is coupled between the memory controller
18
and the memory devices
16
a-c
through a data bus portion of the system memory bus
22
. During a read operation, data is transferred from the SLDRAMs
16
a-c
over the memory bus
22
to the memory controller
18
which, in turn, transfers the data over the processor
14
to the processor
12
. The processor
12
transfers write data over the processor bus
14
to the memory controller
18
which, in turn, transfers the write data over the system memory bus
22
to the SLDRAMs
16
a-c
. Although all the memory devices
16
a-c
are coupled to the same conductors of the system memory bus
22
, only one memory device
16
a-c
at a time reads or writes data, thus avoiding bus contention on the memory bus
22
. Bus contention is avoided by each of the memory devices
16
a-c
on the system memory
22
having a unique identifier, and the command packet contains an identifying code that selects only one of these components.
The computer system
10
also includes a number of other components and signal lines that have been omitted from
FIG. 1
in the interests of brevity. For example, as explained below, the memory devices
16
a-c
also receive a master clock signal to provide internal timing signals, a data clock signal clocking data into and out of the memory device
16
, and a FLAG signal signifying the start of a command packet.
A typical command packet CA<
0
:
39
> for an SLDRAM is shown in FIG.
2
and is formed by 4 packet words CA<
0
:
9
>, each of which contains 10 bits of data. As will be explained in more detail below, each packet word CA<
0
:
9
> is applied on a command address bus CA including 10 lines CA
0
-CA
9
. In
FIG. 2
, the four packet words CA<
0
:
9
> comprising a command packet CA<
0
:
39
> are designated PW
1
-PW
4
. The first packet word PW
1
contains 7 bits of data identifying the packetized DRAM
16
a
Anderson Matthew D.
Dorsey & Whitney LLP
Kim Matthew
LandOfFree
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