Apparatus for providing reference voltages to memory modules...

Static information storage and retrieval – Read/write circuit – Including reference or bias voltage generator

Reexamination Certificate

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C365S226000

Reexamination Certificate

active

06707724

ABSTRACT:

CROSS REFERENCE
This application claims the benefit of Korean Patent Application No. 2001-6188, filed on Feb. 8, 2001, under 35 U.S.C. §119, the entirety of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a memory system having memory devices for storing data, and more particularly, to an apparatus for providing reference voltages to memory modules on which semiconductor memory devices are mounted.
2. Description of Related Art
FIG. 1
is a schematic view illustrating a configuration of a typical Rambus memory system. The Rambus memory system includes a master element
11
having a Rambus ASIC cell (RAC) and a Rambus memory controller (see
21
FIG.
2
A), Rambus in-line module (RIMM) connectors
12
to
14
, first and second RIMM memory modules
15
and
16
mounted on the RIMM connectors
12
and
13
, a RIMM continuity module
17
, and a direct Rambus clock generator (DRCG)
18
. The RIMM memory modules
15
and
16
each include a plurality of Rambus DRAMs (RDRAMs), e.g., 256 MB direct RDRAMs.
A Rambus channel
19
provides an interface between the master element
11
and the RDRAMs of each of the first and second RIMM memory modules
15
and
16
. The Rambus channel
19
is a transmission line connected between the master element
11
and a terminal resistor Rterm to which a terminal voltage Vterm is applied. The Rambus channel
19
can support a predetermined number of RDRAMs, e.g., 32 RDRAMs, without additional drivers.
In the Rambus memory system of
FIG. 1
, reference voltages applied to the respective RIMM memory modules
15
and
16
mounted on the respective RIMM connectors
12
and
13
are all equal to each other in level. That is, the reference voltages with the same level are applied to the RDRAMs of the RIMM memory module
15
and the RDRAMs of the RIMM memory module
16
.
FIGS. 2A and 2B
are block diagrams illustrating a conventional system of providing a reference voltage Vref to RIMM memory modules in a Rambus memory system. As shown in
FIGS. 2A and 2B
, a memory controller
21
is connected to each of the first and second RIMM memory modules
22
and
23
mounted on the RIMM connectors
12
and
13
(referring to FIG.
1
), respectively, through a Rambus channel
24
. The reference voltage Vref is applied from a reference voltage source
25
to each of the RIMM memory modules
22
and
23
, including RDRAMs
22
-
1
and
22
-
2
of the first RIMM memory module
22
and RDRAMs
23
-
1
and
23
-
2
of the second RIMM memory module
23
.
The reference voltage source
25
includes serial resistors R
21
and R
22
that are serially connected between the terminal voltage Vterm and ground. The reference voltage source
25
supplies the reference voltage Vref that is voltage-divided by the serial resistors R
21
and R
22
, to the RDRAMs
22
-
1
and
22
-
2
of the RIMM memory module
22
and to the RDRAMs
23
-
1
and
23
-
2
of the RIMM memory module
23
. As a result, the same level reference voltage is applied to the respective RDRAMs
22
-
1
,
22
-
2
,
23
-
1
and
23
-
2
.
Each of the RDRAMs
22
-
1
to
23
-
2
includes a DRAM core region, a Rambus interface, and an analog region for a delay lock loop (DLL) and a power supply. The DRAM core region is almost the same as a typical DRAM core. The Rambus interface is to provide an interface between the DRAM core region and an external portion of a memory chip, and an open drain type driver is widely used as the Rambus interface.
FIG. 3
is a circuit diagram illustrating an example of the open drain type Rambus interface. When an NMOS transistor MN of a driver
31
is turned on, the Rambus interface of
FIG. 3
outputs data having a logic “low” level. When the NMOS transistor MN is turned off, the Rambus interface of
FIG. 3
outputs data having a logic “high” level which is obtained from the terminal voltage Vterm connected to the terminal resistor Rterm. In
FIG. 3
, the reference numeral “
32
” denotes an output terminal.
FIG. 4
shows an example of a Rambus signaling level (RSL) output level of the conventional RDRAM. When the RDRAM normally outputs data of the RSL output level through the Rambus interface of
FIG. 3
, the level of a high voltage Voh (i.e., the terminal voltage Vterm) that is set to logic “
0
” is about 1.8 volts, the level of the reference voltage Vref is about 1.4 volts, and the level of a low voltage Vol that is set to logic “
1
” is 1.0 volt. Therefore, the normal RSL output level has Voh−Vref=Vol−Vref, and has a voltage swing Vsw of about 0.8 volts. The RDRAM can have the RSL output level having an amplitude less than 1.0 volt by terminating one end of the Rambus channel and connecting the terminated end to the terminal voltage Vterm having a level of about 1.8 volts that is lower than a power voltage for a driving current to pass through the terminal resistor Rterm.
However, in the conventional Rambus memory system, when data is read from one of the RDRAMs of one RIMM memory module and data are continuously, without any gap, read from one of the RDRAMs of another RIMM memory module, a so called “back-to-back noise” may be generated from the data read from a RDRAM of one RIMM memory module, affecting data read from a RDRAM of another RIMM memory module.
FIG. 5
is a schematic block diagram illustrating a conventional Rambus memory system having three kinds of waves generated due to inherent characteristics of the Rambus channel whose one end is terminated. When data is read from one of the RDRAMs of a RIMM memory module, three types of waves are generally generated such as an incident read wave W
1
, a reflected wave W
2
, and an incident terminating wave W
3
.
The incident read wave W
1
proceeds from one of the RDRAMs
42
to
44
to the memory controller
41
. The reflected wave W
2
is generated due to the reflection of the incident wave W
1
from the memory controller
41
. The incident terminating wave W
3
proceeds from the RDRAMs to the terminal resistor Rterm. The three waves W
1
to W
3
have a level of Vsw/2 (here, Vsw is the voltage swing shown in FIG.
4
).
FIG. 6
is a waveform diagram illustrating the different states of the three waves W
1
to W
3
of
FIG. 5
when data are normally read from the RDRAMs. In
FIG. 6
, “T=0” to “T=4” each denotes a lapse of time. As shown in
FIG. 6
, data can be read normally when there is no back-to-back noise due to a previous read operation. However, when noise (e.g., back-to-back noise) due to a previous read operation occurs, the noise affects the next data read operation as shown in FIG.
7
.
FIG. 7
is a waveform diagram illustrating the waves affected by the back-to-back noise. In
FIG. 7
, references W
11
to W
13
denote the three types of waves generated when data are read from a RDRAM during a first read cycle, and references W
21
to W
23
denote the three types of waves generated when data is read from the RDRAM during a second read cycle. As shown in
FIG. 7
, the reflected wave W
12
generated during the first read cycle and the incident terminating wave W
23
generated during the second read cycle are overlapped at time T
4
, so that the RDRAM is driven at a relatively low voltage when data is read from the RDRAM
62
. Therefore, a problem exists that data is erroneously read from the RDRAM
62
, leading to an abnormal operation.
That is, since the same reference voltage is applied to all of the memory modules, the RSL output level of data outputted from the RDRAM varies. As a result, the relation of Vol−Vref=Vref−Vol (referring to
FIG. 4
) cannot be maintained and data cannot be read normally. Thus, abnormal operations occur due to the back-to-back noise in the conventional Rambus memory system.
In efforts to overcome the problem described above, there has been proposed a Rambus memory system employing an “over driving factor” (ODF) technique in which a driving capability of the system during a data read operation is controlled according to conditions of memory modules of the syste

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