U.S. patent application number 12/347199 was filed with the patent office on 2009-07-02 for reduction of signal skew.
This patent application is currently assigned to STMicroelectronics Pvt. Ltd.. Invention is credited to Paras Garg, Saiyid Mohammad Irshad Rizvi.
Application Number | 20090167363 12/347199 |
Document ID | / |
Family ID | 40797429 |
Filed Date | 2009-07-02 |
United States Patent
Application |
20090167363 |
Kind Code |
A1 |
Garg; Paras ; et
al. |
July 2, 2009 |
REDUCTION OF SIGNAL SKEW
Abstract
Skew is reduced by extracting the AC component of an input
signal and superimposing it on a common reference voltage to
produce a resulting voltage. The resulting voltage is provided as
an input to a comparator, which compares it to the reference
voltage to provide a final output. Thus, all signals fed to a
system, in accordance with an embodiment, are referenced at the
same DC level and hence, skew is reduced.
Inventors: |
Garg; Paras; (Noida, IN)
; Rizvi; Saiyid Mohammad Irshad; (New Delhi, IN) |
Correspondence
Address: |
STMicroelectronics Inc.;c/o WOLF, GREENFIELD & SACKS, P.C.
600 Atlantic Avenue
BOSTON
MA
02210-2206
US
|
Assignee: |
STMicroelectronics Pvt.
Ltd.
Greater Noida
IN
|
Family ID: |
40797429 |
Appl. No.: |
12/347199 |
Filed: |
December 31, 2008 |
Current U.S.
Class: |
327/72 |
Current CPC
Class: |
H03K 5/04 20130101 |
Class at
Publication: |
327/72 |
International
Class: |
H03K 5/22 20060101
H03K005/22 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 26, 2007 |
IN |
2727/DEL/2007 |
Claims
1. A system for reduction of signal skew comprising: an extraction
module for extracting the AC component of an input signal; a
superimposing module coupled to the extraction module for
superimposing the extracted AC component of the input signal on a
common reference voltage to produce a resulting signal; and a
comparator module coupled to the superimposing module for comparing
the resulting signal with the reference voltage to give a final
output.
2. A system as claimed in claim 1, wherein the extracting module
comprises a capacitor for blocking a DC component of the input
signal.
3. A system as claimed in claim 1 wherein the superimposing module
comprises a summing amplifier for superimposing the extracted AC
component of the signal
4. A skew reduction device comprising: an extraction module for
extracting an AC component of an input signal; a superimposing
module coupled to the extraction module for superimposing the
extracted AC component of the input signal on a common reference
voltage to produce a resulting signal; and a comparator module
coupled to the superimposing module for comparing the resulting
signal with the reference voltage to give a final output.
5. A skew reduction device as claimed in claim 4, wherein the
extracting module comprises a capacitor for blocking a DC component
of the input signal.
6. A skew reduction device as claimed in claim 4 wherein the
superimposing module comprises a summing amplifier for
superimposing the extracted AC component of the signal.
7. A DDR/SSTL memory interface comprising: a. a transmission unit;
b. an interconnect unit; c. a receiver unit coupled to the
transmission unit through said interconnect unit; d. a skew
reduction unit coupled to output of the receiver unit comprising:
i. an extraction module for extracting an AC component of an input
signal; ii. a superimposing module coupled to the extraction module
for superimposing the extracted AC component of the input signal on
a common reference voltage to produce a resulting signal; and iii.
a comparator module coupled to the superimposing module for
comparing the resulting signal with the reference voltage to give a
final output.
8. A DDR/SSTL memory interface as claimed in claim 7, wherein the
extracting module comprises a capacitor for blocking a DC component
of the input signal.
9. A DDR/SSTL memory interface as claimed in claim 7 wherein the
superimposing module comprises a summing amplifier for
superimposing the extracted AC component of the signal.
10. A method for reduction of skew in memory interfaces comprising:
a. extracting the AC component of an input signal; b. superimposing
the extracted AC component on a common reference voltage to produce
a resulting signal; and c. comparing the resulting signal with the
reference voltage to give a final output.
11. A method as claimed in claim 10 wherein extracting the AC
component of an input signal comprises blocking a DC component of
the input signal by a capacitor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present disclosure relates to reduction of signal skew
and more specifically to reduction of signal skew in memory
interfaces.
[0003] 2. Discussion of the Related Art
[0004] Receiver sections in memory interfaces comprise clock/strobe
pads and data receiver pads. For efficient functioning of the
receiver section, outputs of the clock/strobe pads and data
receiver pads should be closely matched. However, the clock/strobe
pads are fully differential in nature while the data receiver pads
are pseudo differential. This results in skew between the two pads
due to which performance is compromised.
[0005] Generally, the receiver in clock/strobe pads consists of a
differential comparator at the first stage receiving two
complementary signals. The receiver in the data receiver pads also
consists of a differential comparator receiving a data signal as
one input and a reference voltage as second input.
[0006] According to conventional art, to ensure effective matching
between the outputs of the clock/strobe pads and the data receiver
pads, the same differential comparator is used for both clock pads
and data receiver pads. However, the characteristics of
clock/strobe pads do not completely match the characteristics of
data receiver pads owing to the fact that clock/strobe pads are
fully differential while data receiver pads are pseudo
differential.
[0007] Further, the mean voltage levels of the fully differential
signal of clock/strobe pads and the pseudo differential signal of
data receiver pads are not equivalent to the reference voltage
which causes common mode errors.
BRIEF DESCRIPTION OF DRAWINGS
[0008] Features and aspects of various embodiments of the
disclosure will be better understood when the following detailed
description is read with reference to the accompanying drawings in
which like characters represent like parts throughout the
drawings:
[0009] FIG. 1 illustrates a block diagram of a system for reduction
of signal skew according to an embodiment of the present
disclosure.
[0010] FIG. 2 illustrates extraction of the AC component of an
input signal and superimposition on a common reference voltage
according to an embodiment of the present disclosure.
[0011] FIG. 3 illustrates a transistor level representation for
reduction of signal skew according to an embodiment of the present
disclosure.
[0012] FIG. 4 shows a DDR/SSTL memory interface in accordance with
an example of an embodiment of the present disclosure.
[0013] FIG. 5 illustrates a method for reduction of signal skew
according to an embodiment of the present disclosure.
DETAILED DESCRIPTION
[0014] The embodiments of the present disclosure will be described
in detail with reference to the accompanying drawings. However, the
present disclosure is not limited to the illustrated embodiments.
The present disclosure can be modified in various forms. The
embodiments of the present disclosure are only provided to explain
more clearly the present disclosure to the ordinarily skilled in
the art of the present disclosure. In the accompanying drawings,
like reference numerals are used to indicate like components.
[0015] The embodiments of the present disclosure describe reduction
of skew between clock/strobe pads and data receiver pads in memory
interfaces. A system in accordance with an embodiment of the
present disclosure comprises an extraction module coupled to a
superimposing module further coupled to a comparator. The
extraction module extracts the AC component of an input signal by
blocking the DC component and provides the extracted AC component
as an input to the superimposing module. The superimposing module
superimposes the AC component on a common reference voltage to
provide a resulting signal which is then compared to the reference
voltage by the comparator. This ensures that all signals are at the
same level, thereby reducing the signal skew.
[0016] FIG. 1 illustrates the block diagram of a system for
reduction of signal skew according to an embodiment of the present
disclosure. The system 100 includes an extraction module 101 to
extract the AC component of an input signal. The embodiment also
comprises a superimposing module 102 and a comparator 103. The
extracted AC component of the input signal is superimposed on a
common reference voltage VREF to provide a resulting signal. The
resulting signal is then input to the comparator 103 for comparing
it with the reference voltage to provide a final output which is
input to a differential operational amplifier. Thus, all signals
received from the system described above are at the same reference
levels, thus, there are no matching issues and skew is effectively
reduced.
[0017] FIGS. 2(a) and 2(b) illustrate extraction of the AC
component of an input signal and superimposition of extracted AC
component on a common reference voltage according to an embodiment
of the present disclosure. The present embodiment of the disclosure
is directed at clock pads and data receiver pads in the receiver
section of a memory interface, the input signal comprises a clock
signal and a data signal i.e. IP and IPD respectively.
[0018] Capacitors 202(a), 202(b) block the DC level of the incoming
signal (IP, IPD) but allow the transient signal to pass through.
Thus, the AC level from the incoming signal (IP, IPD) is extracted.
The extracted signal is then applied as one input to a summing
amplifier 201(a), 201(b), which receives reference voltage VREF as
the second input. The extracted signal is then superimposed on the
common reference voltage to produce a resulting signal (IP', IPD').
All resulting signals are referenced to the same DC level and are
applied as an input to a differential operational amplifier. This
has been further illustrated in FIG. 3. The resulting signal (IP',
IPD') is applied as an input to MP1 (301(a), 301(b)) of the
differential operational amplifier while maintaining gate of MP2
(302(a), 302(b)) at reference voltage VREF even for a clock signal.
Thus, both signals are identical and hence skew is dramatically
reduced.
[0019] The generation of the resulting signal by superimposition of
the AC level on the common reference voltage helps avoid any common
mode errors arising due to mismatch of signals with the reference
voltage.
Example of an Embodiment in Accordance with the Present
Disclosure:
[0020] FIG. 4 illustrates a DDR/SSTL memory interface in accordance
with an example of an embodiment of the present disclosure. SSTL is
an electrical interface used with the DDR memory standard. It
comprises a receiver section 403 coupled to a transmission section
401 through an interconnect unit 402. As is shown in the figure,
there is one clock driver receiver (analogous to clock/strobe pads)
with corresponding signals DQS_P_A and DQS_N_A and eight data
driver receivers (analogous to data receiver pads) with
corresponding signals DQ_A<1:8>. In a typical DDR based
architecture, the transmission section receives clock pattern
through 400(a) and data patterns through 400(i) to 400(viii).
DQ_P_A and DQS_N_A have different mean values compared to
DQ_A<1:8>. This results in different common mode voltages for
the two cases. Assuming that average of voltage levels of DQS_P_A
and DQS_N_A i.e. (Vip+Vin)/2 is less than V.sub.IPD. Thus, a bigger
gain and small delay is achieved owing to higher V.sub.GS in case
of differential inputs while the reverse is true for when the
average of DQS_P_A and DQS_N_A is greater than VIPD. The difference
arising in the two cases is removed in the present embodiment by
applying signal DQS_P_A, DQS_N_A and DQ_A<1:8> to a skew
reduction unit 404. The skew reduction unit produces signals which
are referenced at the same DC level and are then applied as an
input to the differential operational amplifier.
[0021] An embodiment of method of reduction of signal skew is
illustrated in FIG. 5. The method is illustrated as a collection of
blocks in a logical flow graph, which represents a sequence of
operations that can be implemented in hardware, software, or a
combination thereof. The order in which the process is described is
not intended to be construed as a limitation, and any number of the
described blocks can be combined in any order to implement the
process, or an alternate process.
[0022] FIG. 5 illustrates a method for reduction of skew between a
clock buffer and data buffers according to an embodiment of the
present disclosure. At step 501, the AC component of an input
signal is extracted and the extracted signal is superimposed on a
common reference voltage to produce a resulting signal 502. At step
503, the resulting signal is compared with the reference voltage to
give a final output. The output received from the method results in
all signals being referenced at the same voltage level thus,
efficient matching is achieved and skew is reduced.
[0023] According to yet another embodiment of the present
disclosure, a skew reduction device is provided. The device
comprises an extraction module, a superimposing module and a
comparator. The extraction module extracts the AC component of an
input signal and provides it as an input to the superimposing
module. The superimposing module superimposes the extracted AC
component on a common reference voltage to produce a resulting
signal which after comparison with the reference voltage is
provided as the final output. In an embodiment, the extraction
module is a capacitor which blocks DC component while the
superimposing module is a summing amplifier.
[0024] The reduced signal skew in the embodiments of the present
disclosure ensures efficient clock calibration when desired.
Further, the embodiments of the present disclosure help reduce
signal skew between clock/strobe pads and data receiver pads in
receiver sections of memory interfaces.
[0025] Although the disclosure of system and method has been
described in connection with the embodiment of the present
disclosure illustrated in the accompanying drawings, it is not
limited thereto. It will be apparent to those skilled in the art
that various substitutions, modifications and changes may be made
thereto without departing from the scope and spirit of the
disclosure.
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