U.S. patent application number 12/635785 was filed with the patent office on 2010-04-15 for circuit and method for sampling valid command using extended valid address window in double pumped address scheme memory device.
This patent application is currently assigned to SAMSUNG ELECTRONICS CO., LTD.. Invention is credited to Seong-Jin Jang, Hyun-Jin Kim, Woo-Jin Lee, Jeong-Don Lim, Kwang-Il Park, Ho-Young Song.
Application Number | 20100091600 12/635785 |
Document ID | / |
Family ID | 37815394 |
Filed Date | 2010-04-15 |
United States Patent
Application |
20100091600 |
Kind Code |
A1 |
Kim; Hyun-Jin ; et
al. |
April 15, 2010 |
CIRCUIT AND METHOD FOR SAMPLING VALID COMMAND USING EXTENDED VALID
ADDRESS WINDOW IN DOUBLE PUMPED ADDRESS SCHEME MEMORY DEVICE
Abstract
Provided are a circuit and method for sampling a valid command
using a valid address window extended for a high-speed operation in
a double pumped address scheme memory device. A method for
extending the valid address window includes: inputting a valid
command signal and a first address signal at the first cycle of a
clock signal; inputting a second address signal at the second cycle
of the clock signal; generating a decoded command signal and
extended first and second internal address signals respectively in
response to the command signal and the address signals; and
latching and decoding the extended first and second internal
address signals in response to the decoded command signal.
Inventors: |
Kim; Hyun-Jin; (Gyeonggi-do,
KR) ; Jang; Seong-Jin; (Gyeonggi-do, KR) ;
Lim; Jeong-Don; (Gyeonggi-do, KR) ; Park;
Kwang-Il; (Gyeonggi-do, KR) ; Song; Ho-Young;
(Gyeonggi-do, KR) ; Lee; Woo-Jin; (Gyeonggi-do,
KR) |
Correspondence
Address: |
VOLENTINE & WHITT PLLC
ONE FREEDOM SQUARE, 11951 FREEDOM DRIVE SUITE 1260
RESTON
VA
20190
US
|
Assignee: |
SAMSUNG ELECTRONICS CO.,
LTD.
Suwon-si
KR
|
Family ID: |
37815394 |
Appl. No.: |
12/635785 |
Filed: |
December 11, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12128464 |
May 28, 2008 |
7656742 |
|
|
12635785 |
|
|
|
|
11560746 |
Nov 16, 2006 |
7394720 |
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12128464 |
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Current U.S.
Class: |
365/230.06 ;
365/233.1 |
Current CPC
Class: |
G11C 7/109 20130101;
G11C 7/1078 20130101; G06F 12/02 20130101 |
Class at
Publication: |
365/230.06 ;
365/233.1 |
International
Class: |
G11C 8/00 20060101
G11C008/00; G11C 8/18 20060101 G11C008/18 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 2005 |
KR |
10-2005-0111418 |
Claims
1. A method for extending a valid address window comprising:
inputting a valid command signal and a first address signal at the
first cycle of a clock signal; inputting a second address signal at
the second cycle of the clock signal; generating a decoded command
signal and extended first and second internal address signals
respectively in response to the command signal and the address
signals; and latching and decoding the extended first and second
internal address signals in response to the decoded command signal,
wherein the valid window of the extended first and second internal
address signals corresponds to at least two clock cycles of the
clock signal.
2. The method of claim 1, wherein the valid window of the decoded
command signal corresponds to at least one clock cycle of the clock
signal.
3. The method of claim 1, wherein the valid address window
extending method is applied to a double pumped address scheme
memory device.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a Continuation of U.S. non-provisional application
Ser. No. 12/128,464, filed May 28, 2008, now pending, which is a
Divisional of U.S. non-provisional application Ser. No. 11/560,746,
now U.S. Pat. No. 7,394,720, issued Jul. 1, 2008, which claims
priority from Korean Patent Application No. 10-2005-0111418, filed
on Nov. 21, 2005, all of which are hereby incorporated by reference
in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a semiconductor memory
device, and more particularly, to a circuit and method for sampling
a valid command using an extended valid address window in a double
pumped address scheme memory device.
[0004] 2. Description of the Related Art
[0005] As the capacity of a memory device increases, the number of
address signals for addressing memory cells increases. The address
signals are provided to the memory device through address pins. In
a synchronous memory device such as SRAM, command signals CMD and
address signals ADDRs are input in synchronization with a clock
signal CLK to the memory device, as shown in FIG. 1. The command
signals CMD include signals /RAS, /CAS, /CS and /WE and the address
signals ADDRs are, for example, 12 address signals A0 through
A11.
[0006] To input the address signals A0 through A11 to the memory
device, the 12 address signals are connected to the memory device
through routing of a system board. The number of address pins of
the memory device can be reduced from 12 (A0 through A11) to 6 (A0
through A5) in a double pumped address scheme. This simplifies
system board design. Furthermore, internal power of a chip can be
stabilized by using a power pad instead of an address pad removed
due to the decrease in the number of address signal lines.
Accordingly, the double pumped address scheme was developed.
[0007] In the double pumped address scheme, a valid command CMD and
a first address ADDR1 are input at the first rising edge of a clock
signal CLK and a second address ADDR2 is input at the second rising
edge of the clock signal CLK, as shown in FIG. 2. In addition, a
valid command is input at the first rising edge of the clock signal
CLK and an invalid command is input at the second rising edge of
the clock signal CLK. The 12 address signals A0 through A11 are
divided into two groups and respectively set to the first address
ADDR1 and the second address ADDR2. For example, the 6 address
signals A0 through A5 are set to the first address ADDR1 and the 6
address signals A6 through A11 are set to the second address
ADDR2.
[0008] FIG. 3 is a block diagram of a conventional double pumped
address scheme memory device 300. Referring to FIG. 3, the memory
device 300 includes a clock buffer 310, an address buffer 320, a
command buffer 330, and an address latch circuit 340.
[0009] The clock buffer 310 receives an external clock signal CLK
and generates a first internal clock signal CLK1. The address
buffer 320 receives address signals ADDRs in response to the first
internal clock signal CLK1 and outputs first and second internal
address signals TAFi and TASi. The command buffer 330 receives
command signals CMD in response to the internal clock signal CLK1
and generates a second internal clock signal PCLKD and internal
command signals TCAS/TRAS/TCS/TWE. The address latch circuit 340
receives the internal command signals TCAS/TRAS/TCS/TWE and the
first and second internal address signals TAFi and TASi in response
to the second internal clock signal PCLKD and generates a row
address RAi and a column address CAi.
[0010] FIG. 4 is a block diagram of the address buffer 320.
Referring to FIG. 4, the address buffer 320 includes an input
buffer 410 receiving the address signals ADDRs, an input sampler
420 sampling the output signal of the input buffer 410 in response
to the first internal clock signal CLK1 and generating a clocked
address signal C_ADDR, a delay 430 delaying the first internal
clock signal CLK1, and a flip-flop 440 receiving the clocked
address signal C_ADDR in response to the delayed first internal
clock signal CLK1_D. The output signal of the input sampler 420
corresponds to the second internal address signal TAFi and the
output signal of the flip-flop 440 corresponds to the first
internal address signal TAFi.
[0011] FIG. 5 is a block diagram of the command buffer 330 and the
address latch circuit 340. Referring to FIG. 5, the command buffer
330 includes an input buffer 510 receiving the command signal CMD,
an input sampler 520 sampling the output signal of the input buffer
510 in response to the first internal clock signal CLK1, a first
delay 530 delaying the first internal clock signal CLK1, a second
delay delaying the first internal clock signal CLK1 to generate the
second internal clock signal PCLKD, and a flip-flop 550 receiving
the output signal of the input sampler 520 in response to the
delayed first internal clock signal CLK1_D and generating the
internal command signals TCAS/TRAS/TCS/TWE.
[0012] The address latch circuit 340 includes a command decoder 560
decoding the internal command signals TCAS/TRAS/TCS/TWE in response
to the second internal clock signal CLK2 and generating a decoded
command signal D_CMD, an address decoder 570 receiving the first
and second internal address signals TAFi and TASi and generating
decoded first and second internal address signals D_TAFi and D_TASi
in response to the second internal clock signal PCLKD, and a latch
580 receiving the decoded first and second internal address signals
D_TAFi and D_TASi and generating the row address RAi and column
address CAi in response to the decoded command signal D_CMD.
[0013] FIG. 6 is a timing diagram for explaining the operation of
the memory device 300 of FIG. 3. Referring to FIG. 3, the command
signal CMD and a first address signal AF1 are input at the first
rising edge C1 of the external clock signal CLK and a second
address signal AS1 is input at the second rising edge C2. The
internal command signal and the first and second internal address
signals TAFi and TASi are generated at the third rising edge C3 of
the external clock signal CLK. The second internal clock signal
PCLKD is generated being delayed from the external clock signal CLK
by a predetermined time. The decoded first and second internal
address signals D_TAFi and D_TASi are generated in response to the
internal clock signal PCLKD, and the decoded command signal D_CMD
is generated. The decoded first and second internal address signals
D_TAFi and D_TASi are latched in response to the decoded command
signal D_CMD and the row address RAi and the column address CAi are
generated.
[0014] The row address RAi and the column address CAi can be stably
generated only when the decoded command signal D_CMD is generated
within the window of the decoded first and second internal address
signals D_TAFi and D_TASi. As shown in FIG. 6, the valid address
window of the decoded first and second internal address signals
D_TAFi and D_TASi corresponds to one clock cycle of the external
clock signal CLK.
[0015] However, the first and second internal address signals TAFi
and TASi input to the address decoder 570 have a skew between them
in order to generate the decoded first and second internal address
signals D_TAFi and D_TASi. Furthermore, the valid address window of
the decoded first and second internal address signals D_TAFi and
D_TASi becomes narrower as the operating frequency of the memory
device 300 increases. Accordingly, a margin between the first and
second internal address signals D_TAFi and D_TASi and the decoded
command signal D_CMD is insufficient.
SUMMARY OF THE INVENTION
[0016] The present invention provides a double pumped address
scheme for generating the first and second internal address signals
D_TAFi and D_TASi having a valid window greater than two clock
cycles of the external clock signal CLK.
[0017] The present invention also provides a method for extending a
valid address window in the double pumped address scheme.
[0018] According to an aspect of the present invention, there is
provided a memory device comprising: a valid command signal
generator receiving command signals in response to a clock signal
and generating a valid command signal; an address buffer receiving
first and second address signals in response to the clock signal,
generating first and second internal address signals from the first
and second address signals, and generating extended first and
second internal address signals in response to the valid command
signal; a command buffer generating internal command signals from
the command signals in response to the clock signal and delaying
the clock signal to generate an internal clock signal; and an
address latch circuit decoding the internal command signals in
response to the internal clock signal to generate a decoded
internal command signal, and latching and decoding the extended
first and second internal address signals in response to the
decoded internal command signal.
[0019] The valid command signal generator may include a first input
sampler sampling a RAS signal in response to the clock signal, a
second input sampler sampling a CAS signal in response to the clock
signal, a third input sampler sampling a chip select signal in
response to the clock signal, a delay delaying the clock signal; a
valid command decoder receiving the output signals of the first,
second and third input samplers and judging whether the RAS, CAS
and chip select signals are valid, and a flip-flop receiving the
output signal of the valid command decoder in response to the
output signal of the delay and generating the valid command
signal.
[0020] The address buffer may include an input buffer receiving the
first and second address signals, an input sampler sampling the
output signal of the input buffer in response to the clock signal
and generating the second internal address signal, a delay delaying
the clock signal, a flip-flop receiving the output signal of the
input sampler in response to the output signal of the delay and
generating the first internal address signal from the received
signal, a first latch latching the first internal address signal in
response to the valid command signal and outputting the extended
first internal address signal, and a second latch latching the
second internal address signal in response to the valid command
signal and generating the extended second internal address
signal.
[0021] The command buffer may include an input buffer receiving the
command signals, an input sampler sampling the output signal of the
input buffer in response to the clock signal and generating the
internal command signal, and a delay delaying the clock signal to
generate the internal clock signals.
[0022] The address latch circuit may include a command decoder
decoding the internal command signals in response to the internal
clock signal to generate the decoded internal command signal, and
an address decoder latching and decoding the extended first and
second internal address signals in response to the decoded internal
command signal.
[0023] The address latch circuit may further include a delay
delaying the decoded internal command signal, and the address
decoder may latch and decode the extended first and second internal
address signals in response to the delayed decoded internal command
signal.
[0024] The memory device can be a double pumped address scheme
memory device. According to another aspect of the present
invention, there is provided method for extending a valid address
window comprising: inputting a valid command signal and a first
address signal at the first cycle of a clock signal; inputting a
second address signal at the second cycle of the clock signal;
generating a decoded command signal and extended first and second
internal address signals respectively in response to the command
signal and the address signals; and latching and decoding the
extended first and second internal address signals in response to
the decoded command signal.
[0025] The valid window of the decoded command signal may
correspond to at least one clock cycle of the clock signal, and the
valid window of the extended first and second internal address
signals may correspond to at least two clock cycles of the clock
signal.
[0026] The valid address window extending method may be applied to
a double pumped address scheme memory device.
[0027] According to another aspect of the present invention, there
is provided a method for extending a valid address window of a
double pumped address scheme comprising: receiving a clock signal
and generating a first internal clock signal; receiving command
signals in response to the first internal clock signal, judging
whether the command signals are valid and generating a valid
command signal; receiving first and second address signals in
response to the first internal clock signal and generating first
and second internal address signals; latching the first and second
internal address signals in response to the valid command signal to
generate extended first and second internal address signals;
generating internal command signals from the command signals in
response to the first internal clock signal; delaying the first
internal clock signal to generate a second internal clock signal;
decoding the internal command signals in response to the second
internal clock signal to generate a decoded internal command
signal; and decoding the extended first and second internal address
signals in response to the decoded internal command signal.
[0028] According to the double pumped address scheme memory device
according to the present invention, the valid window of the
extended first and second internal address signals E_TAFi and
E_TASi corresponds to at least two clock cycles of the clock signal
CLK and thus a margin between the decoded internal command signal
D_CMD and the extended first and second internal address signals
E_TAFi and E_TASi for latching the extended first and second
internal address signals E_TAFi and E_TASi can be easily
secured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The above and other features and advantages of the present
invention will become more apparent by describing in detail
exemplary embodiments thereof with reference to the attached
drawings in which:
[0030] FIG. 1 is a timing diagram of signals of a conventional
synchronous memory device;
[0031] FIG. 2 is a timing diagram of signals of a double pumped
address scheme;
[0032] FIG. 3 is a block diagram of a conventional memory device
for realizing the double pumped address scheme;
[0033] FIG. 4 is a block diagram of an address buffer of the memory
device of FIG. 3;
[0034] FIG. 5 is a block diagram of a command buffer and an address
latch circuit of the memory device of FIG. 3;
[0035] FIG. 6 is a timing diagram for explaining the operation of
the memory device of FIG. 3;
[0036] FIG. 7 is a timing diagram of signals of a double pumped
address scheme having an extended valid address window according to
the present invention;
[0037] FIG. 8 is a block diagram of a memory device for realizing
the double pumped address scheme having an extended valid address
window of FIG. 7 according to an embodiment of the present
invention;
[0038] FIG. 9 is a block diagram of a valid command signal
generator of FIG. 8;
[0039] FIG. 10 is a block diagram of an address buffer of FIG.
8;
[0040] FIG. 11 is a block diagram of a command buffer and an
address latch circuit of FIG. 8; and
[0041] FIG. 12 is a timing diagram for explaining the operation of
the memory device of FIG. 8.
DETAILED DESCRIPTION OF THE INVENTION
[0042] The present invention will now be described more fully with
reference to the accompanying drawings, in which exemplary
embodiments of the invention are shown. The invention may, however,
be embodied in many different forms and should not be construed as
being limited to the embodiments set forth herein; rather, these
embodiments are provided so that this disclosure will be thorough
and complete, and will fully convey the concept of the invention to
those skilled in the art. Throughout the drawings, like reference
numerals refer to like elements.
[0043] FIG. 7 is a timing diagram of signals of a double pumped
address scheme having an extended valid address window according to
the present invention. Referring to FIG. 7, similarly to the
conventional double pumped address scheme shown in FIG. 2, a valid
command signal CMD1 and a first address signal ADDR1 are input at
the first rising edge C1 of an external clock signal CLK and a
second address signal ADDR2 is input at the second rising edge C2
of the external clock signal CLK.
[0044] A decoded command signal D_CMD and extended first and second
internal address signals E_ADDR1 and E_ADDR2 are generated
respectively in response to the command signal CMD1 and the address
signals ADDRs. The valid window of the decoded command signal D_CMD
corresponds to one clock cycle of the external clock signal CLK and
the valid window of the extended first and second internal address
signals E_ADDR1 and E_ADDR2 corresponds to at least two clock
cycles of the external clock signal CLK. Accordingly, a margin
capable of latching the extended first and second internal address
signals E_ADDR1 and E_ADDR2 in response to the decoded command
signal D_CMD is increased.
[0045] FIG. 8 is a block diagram of a double pumped address scheme
memory device 800 having the extended valid address window shown in
FIG. 7 according to an embodiment of the present invention.
Referring to FIG. 8, the memory device 800 includes a clock buffer
810, a valid command signal generator 815, an address buffer 820, a
command buffer 830, and an address latch circuit 840.
[0046] The clock buffer 810 generates a first internal clock signal
in response to the external clock signal CLK. The valid command
signal generator 815 receives a command signal CMD in response to
the first internal clock signal CLK1 and generates a valid command
signal V_CMD. The address buffer 820 receives the first internal
clock signal CLK1 and the valid command signal V_CMD in response to
the first internal clock signal CLK1 and generates the first and
second internal address signals TAFi and TASi.
[0047] The command buffer 830 receives the command signal CMD in
response to the first internal clock signal CLK1 and generates a
second internal clock signal PCLKD and internal command signals
TCAS/TRAS/TCS/TWE. The address latch circuit 840 receives the first
and second address signals TAFi and TASi in response to the second
internal clock signal PCLKD and the internal command signals
TCAS/TRAS/TCS/TWE and generates a row address RAi and a column
address CAi.
[0048] FIG. 9 is a block diagram of the valid command signal
generator 815. Referring to FIG. 9, the valid command signal
generator 815 includes first, second and third input samplers 910,
920 and 930, a delay 940, a valid command decoder 950 and a
flip-flop 960.
[0049] The first input sampler 910 samples a row address select
(RAS) signal /RAS in response to the first internal clock signal
CLK1, the second input sampler 920 samples a column address select
(CAS) signal /CAS in response to the first internal clock signal
CLK1, and the third input sampler 930 samples a chip select signal
/CS in response to the first internal clock signal CLK1. The output
signals of the first, second and third input samplers 910, 920 and
930 are provided to the valid command decoder 950 which judges
whether the signals /RAS, /CAS and /CS are valid. The delay 940
delays the first internal clock signal CLK1. The flip-flop 940
receives the output signal of the valid command decoder 950 in
response to the output signal of the delay 940 and generates the
valid command signal V_CMD. The valid command signal V_CMD
represents that all the command signals /RAS, /CAS and /CS input to
the memory device 800 are valid.
[0050] FIG. 10 is a block diagram of the address buffer 820 of FIG.
8. Referring to FIG. 10, the address buffer 820 includes an input
buffer 1010 receiving the address signals ADDRs, an input sampler
1020 receiving the output signal of the input buffer 1010 in
response to the first internal clock signal CLK1 and generating the
first internal address signal TAFi, a delay 1030 delaying the first
internal clock signal CLK1, and a flip-flop 1040 receiving the
output signal of the input sampler 1020 in response to the output
signal of the delay 1030 and generating the second internal address
signal TASi. The address buffer 820 further includes a first latch
1050 latching the output signal of the flip-flop 1040 in response
to the valid command signal V_CMD and generating the extended first
internal address signal E_TAFi, and a second latch 1060 latching
the output signal of the input sampler 1020 in response to the
valid command signal V_CMD and generating the extended second
internal address signal E_TASi. The address buffer 820 generates
the first and second internal address signals TAFi and TAS1 from
the received address signals ADDRS in response to the first
internal clock signal CLK1 and generates the extended first and
second internal address signals E_TAFi and E_TASi from the first
and second internal address signals TAFi and TASi in response to
the valid command signal V_CMD.
[0051] FIG. 11 is a block diagram of the command buffer 830 and the
address latch circuit 840 of FIG. 8. Referring to FIG. 11, the
command buffer 830 includes an input buffer 1110 receiving the
command signal CMD, an input sampler 1120 sampling the output
signal of the input buffer 1010 in response to the first internal
clock signal CLK1 and generating the internal command signals
TCAS/TRAS/TCS/TWE, and a first delay 1130 delaying the first
internal clock signal CLK1 to generate a second internal clock
signal PCLKD.
[0052] The address latch circuit 840 includes a command decoder
1140 decoding the internal command signals TCAS/TRAS/TCS/TWE in
response to the second internal clock signal PCLKD to generate the
decoded command signal D_CMD, and an address decoder 1150 decoding
the extended first and second internal address signals E_TAFi and
E_TASi in response to the decoded command signal D_CMD and
generating the row address RAi and the column address CAi.
[0053] The decoded command signal D_CMD can be an active signal
ACT, a precharge signal PRE, a refresh signal REF and a mode
register signal MRS, which designate the internal operation of the
memory device 800.
[0054] The address latch circuit 840 can further include a second
delay 1145 for delaying the decoded command signal D_CMD. The
output signal of the second delay 1145 can be provided to the
address decoder 1150. The reason why the decoded command signal
D_CMD is delayed will be explained with reference to FIG. 12.
[0055] FIG. 12 is a timing diagram for explaining the operation of
the memory device of FIG. 8. Referring to FIG. 12, the command
signal CMD1 and the first address signal AF1 are input at the first
rising edge C1 of the external clock signal CLK and the second
address signal AS1 is input at the second rising edge C2 of the
external clock signal CLK.
[0056] The valid command signal V_CMD is generated from the input
command signal CMD1 in response to the first internal clock signal
(not shown in FIG. 12) similar to the external clock signal CLK,
and the first and second internal address signals TAFi and TASi are
generated from the input address signals AF1 and AS1. The extended
first and second internal address signals E_TAFi and E_TASi are
generated from the first and second internal address signals TAFi
and TASi in response to the valid command signal V_CMD. The valid
window of the extended first and second internal address signals
E_TAFi and E_TASi corresponds to at least two clock cycles of the
external clock signal.
[0057] The internal command signals TCAS/TRAS/TCS/TWE are generated
from the command signal CMD1 in response to the first internal
clock signal (not shown). The decoded internal command signal D_CMD
is generated from the internal command signals TCAS/TRAS/TCS/TWE in
response to the second internal clock signal PCLKD. The first and
second internal address signals TAFi and TASi are latched in
response to the decoded internal command signal D_CMD. The decoded
internal command signal D_CMD is within the valid window of the
extended first and second internal address signals E_TAFi and
E_TASi.
[0058] The decoded internal command signal D_CMD can be delayed by
a predetermined time by the second delay 1145 shown in FIG. 11. The
decoded internal command signal D_CMD is generated in response to
the second internal clock signal PCLKD generated at the first
rising edge C1 of the external clock signal CLK. The instant of
time when the decoded internal command signal D_CMD is generated
can precede the instant of time when the extended first and second
internal address signals E_TAFi and E_TAS1 are generated because
the signals input at the first rising edge C1 of the external clock
signal CLK are directly generated as the internal command signals
TCAS/TRAS/TCS/TWE in response to the first internal clock signal
and then the decoded internal command signal D_CMD is generated in
response to the second internal clock signal PCLKD corresponding to
the first rising edge C1 of the external clock signal CLK.
[0059] That is, the decoded internal command signal D_CMD is
delayed by a predetermined time using the second delay 1145 of FIG.
11 such that the instant of time when the decoded internal command
signal D_CMD is generated falls in the valid window of the extended
first and second internal address signals E_TAFi and E_TASi. Then,
the extended first and second internal address signals E_TAFi and
E_TASi are latched in response to the decoded command signal D_CMD
and the row address RAi and the column address CAi are
generated.
[0060] According to the double pumped address scheme memory device
according to the present invention, the valid window of the
extended first and second internal address signals E_TAFi and
E_TASi corresponds to at least two clock cycles of the clock signal
CLK and thus a margin between the decoded internal command signal
D_CMD and the extended first and second internal address signals
E_TAFi and E_TASi can be easily secured.
[0061] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
* * * * *