U.S. patent application number 09/832851 was filed with the patent office on 2002-05-23 for synchronous semiconductor device and method for latching input signals.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Ikeda, Shinichiro.
Application Number | 20020060945 09/832851 |
Document ID | / |
Family ID | 18825503 |
Filed Date | 2002-05-23 |
United States Patent
Application |
20020060945 |
Kind Code |
A1 |
Ikeda, Shinichiro |
May 23, 2002 |
SYNCHRONOUS SEMICONDUCTOR DEVICE AND METHOD FOR LATCHING INPUT
SIGNALS
Abstract
A compact synchronous semiconductor device having an improved
set-up/hold time is disclosed. A decoder receives input signals and
generates decoded signals. A delay-adjusting unit adjusts the delay
time of each of the decoded signals and provides adjusted decoded
signals. A latch circuit unit latches the adjusted decoded signals
in synchronism with a clock signal.
Inventors: |
Ikeda, Shinichiro;
(Kasugai-shi, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN & HATTORI, LLP
1725 K STREET, NW.
SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki
JP
|
Family ID: |
18825503 |
Appl. No.: |
09/832851 |
Filed: |
April 12, 2001 |
Current U.S.
Class: |
365/233.1 |
Current CPC
Class: |
G11C 8/06 20130101; G11C
7/1078 20130101; G11C 7/109 20130101; G11C 8/18 20130101; G11C
7/1087 20130101 |
Class at
Publication: |
365/233 |
International
Class: |
G11C 008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 2000 |
JP |
2000-352636 |
Claims
What is claimed is:
1. A synchronous semiconductor device comprising: a decoder for
receiving a plurality of input signals to generate a plurality of
decoded signals; a delay adjusting unit connected to the decoder to
adjust a delay time of each of the decoded signals and to provide a
plurality of adjusted decoded signals; and a latch circuit unit
connected to the delay adjusting unit for latching the adjusted
decoded signals in synchronism with a clock signal.
2. A synchronous semiconductor device comprising: a plurality of
decode circuits for receiving a plurality of input signals and for
producing a plurality of decoded signals; a plurality of delay
adjusting circuits connected to the plurality of decode circuits,
respectively, to adjust the respective delay times of the plurality
of decode signals so that a variation in the set-up/hold time of
the plurality of decode signals is reduced, thus providing a
plurality of adjusted decoded signals; and a plurality of latch
circuits connected to the plurality of delay adjusting circuits for
latching the plurality of adjusted decoded signals, respectively,
in synchronism with a clock signal.
3. A synchronous semiconductor device according to claim 2, wherein
each of the delay adjusting circuits adjusts the delay time of an
associated one of the decoded signals depending on the operational
condition of the synchronous semiconductor device.
4. A synchronous semiconductor device according to claim 2, wherein
each of the delay adjusting circuits includes a capacitor that
delays an associated one of the decoded signals.
5. A synchronous semiconductor device according to claim 2, wherein
each of the plurality of delay adjusting circuits includes: an
input terminal to which the decoded signal is provided; an output
terminal from which the decoded signal is output; at least one
switching element connected to at least one node between the input
terminal and the output terminal; and at least one capacitor
connected to the at least one switching element, the at least one
switching element selectively connecting the at least one capacitor
and the at least one node.
6. A synchronous semiconductor device according to claim 5, wherein
the delay time is changed as the at least one switching element is
switched.
7. A synchronous semiconductor device according to claim 5, wherein
the capacitor includes MOS capacitor.
8. A synchronous semiconductor device according to claim 2, wherein
when the input signal is to be decoded, one of the plurality of
decode circuits operates.
9. A synchronous semiconductor device according to claim 2, wherein
the input signal is at least one of an address signal and a command
signal that are used to operate a synchronous semiconductor
memory.
10. A method of latching a plurality of input signals in a
synchronous semiconductor device, comprising the steps of: decoding
the plurality of input signals to provide a plurality of decoded
signals; adjusting a delay time of each of the decoded signals so
that a variation in the set-up/hold time for the decoded signals is
reduced, thus providing a plurality of adjusted decoded signals;
and latching the adjusted decoded signals in synchronism with a
clock signal.
11. A method of latching the plurality of input signals according
to claim 10, wherein the delay time is adjusted in the manner
depending on the operational condition of the synchronous
semiconductor device.
12. A synchronous semiconductor device comprising: a plurality of
command input terminals for receiving a plurality of command
signals; a plurality of decode circuits connected to the command
input terminals, respectively, to decode the command signals,
thereby providing a plurality of decoded signals; a plurality of
delay adjusting circuits connected to the decode circuits to adjust
a delay time of each of the decoded signals in a manner dependent
on the variety of the command signals, thereby providing a
plurality of adjusted decoded signals; and a plurality of latch
circuits connected to the delay adjusting circuits for latching the
adjusted decoded signals in synchronism with a clock signal.
13. A synchronous semiconductor device according to claim 12,
wherein each of the delay adjusting circuits includes a capacitor
for delaying an associated one of the decoded signals.
14. A synchronous semiconductor device according to claim 12,
wherein each of the delay adjusting circuits includes: an input
terminal for receiving an associated one of the decoded signals; an
output terminal connected to an associated one of the latch
circuits; a plurality of MOS transistors connected to the ground;
and a plurality of switching elements connected between a plurality
of nodes, which are defined between the input terminals and the
output terminals, and the MOS transistors, for selectively
connecting the nodes and the MOS transistors.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a synchronous semiconductor device,
and in particular, to a method of latching an input signal to a
synchronous semiconductor device.
[0002] In a synchronous semiconductor device such as synchronous
dynamic random access memory (SDRAM), there are demands for
reducing the set-up time and the set-up/hold time for input signals
(command signal and address signal) which follow a clock signal in
order to accommodate for the acceleration of the synchronous clock
cycle.
[0003] Japanese Unexamined Patent Publication No. Hei 8-17182
discloses a synchronous semiconductor device according to a first
prior art. A decoder which precedes a latch circuit is provided in
the semiconductor circuit to enable a rapid internal operation. As
a consequence, the decoding rate of the decoder has a direct
influence upon the set-up/hold time.
[0004] The first prior art semiconductor device includes a logic
circuit disposed between an external command input terminal and the
latch circuit. The logic circuit decodes an external command signal
fed to each external command input terminal, and the decoded signal
is held by the latch circuit in synchronism with the clock signal.
This technique is commonly referred to as "command prefetch
approach".
[0005] FIG. 1 is a block diagram of a command decoder circuit 60 of
a synchronous semiconductor device according to a second prior art.
The command decoder circuit 60 operates according to the command
prefetch approach. Specifically, the command decoder circuit 60
includes a decode circuit 51 connected between four external
command input terminals T1-T4 and a latch circuit 50. The decode
circuit 51 includes a decode unit 52 having six AND circuits
52a-52f, and four input buffers 53a-53d. Clock signal CLK is fed to
an external clock signal input terminal TO and passed through a
clock buffer 54 to the latch circuit 50. External command signals
applied to the input terminal T1-T4 are passed through
corresponding input buffers 53a-53d, respectively, to the decode
unit 52, which then decodes the external command signals to provide
decoded signals, which are in turn received and held by the latch
circuit 50 in synchronism with the clock signal CLK.
[0006] FIG. 2 is a block diagram of one of the input buffers
53a-53d. Thus any one of the input buffers 53a-53d includes a level
conversion circuit 56, a delay circuit 57 connected to the level
conversion circuit 56, an inverter circuit 58 connected to the
delay circuit 57 and an inverter circuit 59 connected to the
inverter circuit 58. In each of the input buffers 53a-53d, the
delay circuit 57 controls the signal input to the decode unit 52,
whereby the set-up/hold time for each external command input
terminal T1-T4 is controlled.
[0007] Japanese Unexamined Patent Publication No. Hei. 9-153279
discloses a semiconductor device according to a third prior art.
This semiconductor device also includes a decoder which precedes a
latch circuit. Specifically, the semiconductor device includes a
plurality of external command input terminals, D-type flip-flop
circuits each for temporarily storing one of a plurality of
external command signals applied to the plurality of external
command input terminals, and a plurality of command decode
circuits. The D-type flip-flop circuits deliver the plurality of
external command signals which they temporarily store to the
respective command decode circuits. The plurality of command decode
circuits operate to decode the external command signal delivered
from the D-type flip flop circuit to provide decoded signals, which
are then supplied to a plurality of latch circuits to be held
therein. At this time, a clock signal which depends on the delay
time of an associated command decode circuit is fed to each latch
circuit, thus enabling it to latch the decoded signal in accordance
with the corresponding clock signal.
[0008] The first prior art semiconductor device has an access time
which is more rapid than the access time of a semiconductor device
in which a latch circuit precedes a logic circuit. However, a
decoder having multiple stages of circuits and elements is
connected between the external command input terminal and the latch
circuit, and this results in a relatively long time interval or
delay time from a point in time when an external command signal is
applied to an external command input signal until the logic circuit
delivers a decoded signal by decoding the external command signal.
As a consequence, the set-up/hold dead zone of the decoded signal
relative to the clock signal will be offset toward the set-up side.
In other words, the set-up time of the decoded signal will be
shortened.
[0009] With the semiconductor device according to the first prior
art, the delay time within the logic circuit varies from decoded
signal to decoded signal as a result of the potential transition
situation of the external command input terminals and operational
noises of the semiconductor device.
[0010] Consequently, the total dead zone in the set-up/hold of the
decoded signal relative to the clock signal further increases.
[0011] By contrast, in the command decoder circuit 60 shown in FIG.
1, the delay circuit 57 controls the set-up/hold time for each
external command terminal. However, a delay time from the
transition of the potential of the signal applied to each external
command input terminal T1-T4 to the transition of a potential
occurring in the decoded signal from each of AND circuits 52a-52f
varies from decoded signal to decoded signal. In addition, the
delay time of the decoded signal D1-D6 changes depending on the
direction of transition of the potential on each external command
input terminal T1-T4 (i.e, from H level to L level or from L level
to H level).
[0012] In addition, the delay time of the decoded signal D1-D6
changes due to operational noises of the semiconductor device such
as a variation in the supply voltage, for example. By way of
example, if the command decoder circuit 60 is used in a synchronous
DRAM, the following difficulties are experienced:
[0013] A mode register set command or self-refresh command is
applied to a semiconductor device during its idle condition where
the operational noises of the semiconductor device remain
relatively low. By contrast, an active command, a read/write
command or a precharge command is applied to the semiconductor
device during its active condition where the operational noises are
relatively high. Accordingly, with the active command, the
read/write command and the precharge command, a variation in the
set-up/hold time attributable to noises in the AND circuits 52a-52f
of the decode unit 52 is greater than a corresponding variation
experienced by the mode register set command or the self-refresh
command. In other words, there is a large variation in the
set-up/hold time between different processing commands.
[0014] It is difficult to adjust such variation by using a
plurality of delay circuits 57 in each of the input buffers 53a-53d
which precedes the decode unit 52. Specifically, to accommodate for
such variation, it would be necessary to choose an individual delay
time for each of the plurality of delay circuits 57, but in
practice, such control would be difficult, and there remains a
certain variation, which causes the dead zone breadth of the
set-up/hold to increase when viewed from the whole assembly of
external command input terminals T1-T4. If the command decoder
circuit 60 is used in an address decode circuit or a variety of
test mode decision circuits, a similar problem occurs.
[0015] For a semiconductor device including the input buffers
53a-53d as shown in FIG. 2, there is a difference between a
positive logic output circuit and a negative logic output circuit
in the number of stages of constituting circuit elements.
Obviously, there results an offset in the output timing between the
positive logic output circuit and the negative logic output
circuit, and this leads to an offset between the operations of AND
circuits 52a-52f which receive complimentary logic signals.
Specifically, with the AND circuits 52a-52f, when producing a
decoded output signal having an L level (or when selecting two
input signals each having an L level), the transition of the
decoded output signal is fastest while degrading the hold time. On
the contrary, when producing a decoded output signal having an H
level (or when selecting two input signals each having an H level),
the transition of the decoded output signal is slowest while
degrading the set-up time.
[0016] The latch circuit 50 includes latches 50a-50f each
associated with AND circuits 52a-52f.
[0017] On the other hand, in the semiconductor device according to
the third prior art, while the delay time can be adjusted for each
external command terminal, the setup/hold time for each external
clock signal is determined by the D-type flip-flop circuit. In
other words, it is impossible to adjust the set-up/hold time for
each command with the semiconductor device according to the third
prior art.
[0018] Furthermore, in the semiconductor device according to the
third prior art, each command decode circuit follows the D-type
flip-flop circuit. Each latch circuit latches an output signal from
the corresponding command decode circuit, and at this end, each
latch circuit is supplied with a clock signal which takes the delay
time into consideration. However, such clock signal is required to
have the same delay time as the set-up/hold window width that is
determined by the D-type flip-flop circuit. In addition, it is
necessary to provide a number of clock signal generator circuits
which is equal to the number of the latch circuits. Because the
clock signal generator circuit requires a relatively increased
number of elements, it follows that the circuit area for a
synchronous DRAM, which is provided with a number of commands and
address, decodes increases.
SUMMARY OF THE INVENTION
[0019] It is an object of the invention to provide a synchronous
semiconductor device that is compact and has an improved
set-up/hold time.
[0020] To achieve the above object, the present invention provides
a synchronous semiconductor device including a decoder, a delay
adjusting unit, and a latch circuit unit. The decoder receives a
plurality of input signals and produces a plurality of decoded
signals. The delay adjusting unit is connected to the decoder and
adjusts a delay time of each of the decoded signals and provides a
plurality of adjusted decoded signals. The latch circuit unit is
connected to the delay adjusting unit and latches the adjusted
decoded signals in synchronism with a clock signal. The decoder
typically includes a plurality of decode circuits, and the delay
adjusting unit includes a plurality of delay adjusting circuits,
each of which is connected with each of the decode circuits. The
latch circuit unit includes a plurality of latch circuits, each of
which is connected with each of the delay adjusting circuits. The
input signal includes one of an address signal and a command signal
used to operate a synchronous semiconductor memory. One of the
decoded circuits will operate when the input signal is decoded. The
synchronous semiconductor device further includes a plurality of
command input terminals for receiving the command input
signals.
[0021] The delay adjusting circuit includes an input terminal to
which the decoded signal is provided, an output signal from which
the decoded signal is output, at least one switching element
connected to at least one node between the input terminal and the
output terminal, and at least one capacitor, which can be a MOS
capacitor, connected to the switching element and the ground. The
delay adjusting circuit adjusts the delay time of the associated
decoded signals depending on the operational condition of the
synchronous semiconductor device, and the delay time is changed
when the switching element is switched.
[0022] The synchronous semiconductor device described above is used
to latching a plurality of input signals. The input signals are
first decoded to provide decoded signals. The delay time of each of
the decoded signals is adjusted so that a variation in the
set-up/hold time for the decoded signals is reduced, thus providing
adjusted decoded signals. The adjusted decoded signals are then
latched in synchronism with a clock signal. The delay time of each
decoded signal is adjusted depending on the operational condition
of the synchronous semiconductor device.
[0023] Other aspects and advantages of the present invention will
become apparent from the following description, taken in
conjunction with the accompanying drawings, illustrating by way of
example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The features of the present invention that are believed to
be novel are set forth with particularity in the appended claims.
The invention, together with objects and advantages thereof, may
best be understood by reference to the following description of the
presently preferred embodiments together with the accompanying
drawings in which:
[0025] FIG. 1 is a schematic block diagram of a decode circuit and
a latch circuit according to a prior art;
[0026] FIG. 2 is a schematic block diagram of an input buffer of
the prior art;
[0027] FIG. 3 is a schematic block diagram of a command decode
circuit for a synchronous DRAM according to a first embodiment of
the present invention;
[0028] FIG. 4 is a schematic block diagram of an input buffer shown
in FIG. 3;
[0029] FIG. 5 is a schematic block diagram of a variable delay
circuit shown in FIG. 3;
[0030] FIG. 6 is a schematic block diagram of an SFF circuit shown
in FIG. 3; and
[0031] FIG. 7 is a schematic block diagram of an address decode
circuit for a synchronous DRAM according to a second embodiment of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] A synchronous DRAM according to a first embodiment of the
invention will now be described. FIG. 3 is a block diagram of a
command decode circuit 100 for a synchronous DRAM.
[0033] The command decode circuit 100 has an external clock signal
input terminal T0, and four or first to fourth external command
input terminals T1-T4. The external clock terminal T0 receives an
external clock signal CLK from an external unit, not shown. The
first to the fourth external command input terminal T1-T4 receive
external command signals from an external unit. In the first
embodiment, the external command signals include a chip select
signal CSB, a row address strobe signal RASB, a column address
strobe signal CASB and a write enable signal WEB. The command
decode circuit 100 decodes external command signals to provide
decoded signals.
[0034] The external commands also include an active command, a read
command, a write command, a precharge command, and a mode register
set command. An active command activates a synchronous DRAM in
response to an access demand from an external unit, downloads a row
address during the same cycle through a separate route, not shown,
to select any desired word line and allows a memory cell data to be
amplified by a sense amp.
[0035] A read command follows the active command to download a
column address to deliver the memory cell data. A write command
follows the active command to download a column address and data to
be written to cause the data to be written into a selected memory
cell. A precharge command follows either one of the active command,
the read command and the write command to deactivate a word line or
a bit line, thereby initializing internal nodes within the
synchronous DRAM. The mode register set command occurs during one
of the active command, the read command and the write command to
set up parameters to control the internal circuit of the
synchronous DRAM closely.
[0036] The command decoder circuit 100 includes a decode unit 10
connected to the first to the fourth external command input
terminal T1-T4, a set-up/hold latch circuit 15 connected to the
external clock signal input terminal TO and a delay adjusting unit
14 connected between the decode unit 10 and the set-up/hold latch
circuit 15.
[0037] The decode unit 10 includes a first to a fourth input buffer
11a-11d, and a decode circuit 12 including a first to a fifth AND
circuit 12a-12e. Each of the first to the fourth input buffer
11a-11d has a single input terminal and a pair of output terminals.
The input terminal of the first input buffer 11a is connected to
the first external command input terminal T1 to receive a chip
select signal CSB. The input terminal of the second input buffer
11b is connected to the second external command input terminal T2
to receive a row address strobe signal RASB. The input terminal of
the third input buffer 11c is connected to the third external
command input terminal T3 to receive a column address strobe signal
CASB. The input terminal of the fourth input buffer lid is
connected to the fourth external command input terminal T4 to
receive a write enable signal WEB.
[0038] FIG. 4 is a block diagram of the first input buffer 11a. It
is to be understood that the second to the fourth input buffer
11b-11d are similarly constructed as the first input buffer 11a.
Specifically, the first input buffer 11a includes a pair of
inverter circuits 13a, 13b, thereby producing two complimentary
signals outz, outx from a single input signal in. More
specifically, the first input buffer 11a produces an inversion of
the chip select signal CSB (or inverted chip select signal csz) and
a signal which is in phase with the chip select signal CSB (or a
chip select signal csx). The second input buffer 11b produces an
inversion of the row address strobe signal RASB (or an inverted row
address strobe signal rasz) and a signal in phase with the row
address strobe signal RASB (or a row address strobe signal rasx).
The third input buffer 11c produces an inversion of the column
address strobe signal CASB (or an inverted column address strobe
signal casz) and a signal in phase with the column address strobe
signal CASB (or a column address strobe signal casx). The fourth
input buffer lid produces an inversion of the write address enable
signal WEB (or an inverted write enable signal wez) and a signal in
phase with the write enable signal WEB (or a write enable signal
wex).
[0039] Output signals csz, rasz, rasx, casz, casx, wez, and wex
from the first to the fourth input buffers 11a-11d are supplied to
the decode circuit 12. Accordingly, the external commands
(specifically the chip select signal CSB, the row address strobe
signal RASB, the column address strobe signal CASB and the write
enable signal WEB) are decoded in the decode circuit 12.
[0040] The first AND circuit 12a receives the inverted chip select
signal csz, the inverted row address strobe signal rasz, the column
address strobe signal casx and the write enable signal wex to
produce a decoded signal D1 which assumes an H level when all of
the signals csz, rasz, casx and wex are at H levels.
[0041] The second AND circuit 12b receives the inverted chip select
signal csz, the inverted row address strobe signal rasz, the column
address strobe signal casx and the inverted write enable signal wez
to produce a decoded signal D2 which assumes an H level when all of
the signals csz, rasz, casx and wez are at H levels.
[0042] The third AND circuit 12c receives the inverted chip select
signal csz, the row address strobe signal rasx, the inverted column
address strobe signal casz and the write enable signal wex to
produce a decoded signal D3 which assumes an H level when all of
the signals csz, rasx, casz and wex are at H levels.
[0043] The fourth AND circuit 12d receives the inverted chip select
signal csz, the row address strobe signal rasx, the inverted column
address strobe signal casz and the inverted write enable signal wez
to produce a decoded signal D4 which assumes an H level when all of
the signals csz, rasx, casz and wez are at H levels.
[0044] The fifth AND circuit 12e receives the inverted chip select
signal csz, the inverted row address strobe signal rasz, the
inverted column address strobe signal casz and the inverted write
enable signal wez to produce a decoded signal D5 which assumes an H
level when all of the signals csz, rasz, casz and wez are at H
levels.
[0045] The decode circuit 12 decodes various external commands in a
manner such that one of the decoded signals D1-D5 rises to its H
level while the remaining decoded signals D1-D5 fall to their L
level. By way of example, when the external command is an active
command, the decode circuit 12 produces the decoded signal D1 of an
H level and decoded signals D2-D5 of an L level. When the external
command is a precharge command, the decode circuit 12 produces the
decoded signal D2 of an H level, and decoded signals D1, D3-D5 of
an L level. When the external command is a read command, the decode
circuit 12 produces the decoded signal D3 of an H level and decoded
signals D1, D2-D4 and D5 of an L level. When the external command
is a write command, the decode circuit 12 produces the decoded
signal D4 of an H level and decoded signals D1-D3 and D5 of an L
level. When the external command is a mode register set command,
the decode circuit 12 produces the decoded signal D5 of an H level
and decoded signals D1-D4 of an L level.
[0046] The decode circuit 12 delivers the decoded signals D1-D5 to
the set-up/hold latch circuit 15 through a delay adjusting unit 14.
More specifically, decoded signals D1-D5 from the AND circuits
12a-12e are supplied to the set-up/hold latch circuit 15 through
the corresponding ones of a first to a fifth variable delay circuit
14a-14e.
[0047] The set-up/hold latch circuit 15 will now be described.
[0048] The set-up/hold latch circuit 15 includes five SFF circuits
15a-15e, each of which is latched with one of the decoded signals
D1-D5 in synchronism with the external clock signal CLK.
Specifically, a clock buffer 20 receives an external clock signal
CLK and produces an internal clock signal clk which is in phase
with the external clock signal CLK. The clock buffer 20 delivers
the internal clock signal clk to each of SFF circuits 15a-15e.
[0049] FIG. 6 is a schematic diagram of SFF circuit 15a. It is to
be understood that remaining SFF circuits 15b-15e are constructed
in the same manner as the SFF circuit 15a. The SFF circuit 15a
includes a pair of latches 21a, 21b, a pair of transfer gates 22a,
22b and a pair of inversion circuits 23, 24. Each SFF circuit
15a-15e has an input terminal Pin which is connected to the
corresponding variable delay circuit 14a.
[0050] The first and the second latch 21a, 21b each include a pair
of inverter circuits. The first latch 21a is connected between the
first transfer gate 22a and the second transfer gate 22b. In other
words, the first latch 21a is connected to the input terminal Pin
through the first transfer gate 22a and the invention circuit 23.
The second latch 21b is connected between the second transfer gate
22b and an output terminal Pout of the SFF circuit 15a. The output
terminal Pout is connected to an internal circuit, not shown.
[0051] The first transfer gate 22a includes an P-channel MOS (PMOS)
transistor Q1 and an N-channel MOS (NMOS) transistor Q2. The PMOS
transistor Q1 has a gate, to which the internal clock signal clk is
applied. On the other hand, the inverted internal clock signal clk
is applied to the gate of NMOS transistor Q2 through the invertion
circuit 24.
[0052] The second transfer gate 22b includes an PMOS transistor Q3
and an NMOS transistor Q4. The inverted internal clock signal clk
is applied to the gate of PMOS transistor Q3 through the invertion
circuit 24 while the internal clock signal clk is applied to the
gate of NMOS transistor Q4.
[0053] When the internal clock signal clk assumes an L level, the
first transfer gate 22a is opened while the second transfer gate
22b is closed. In this instance, the first latch 21a holds the
inverted decoded signal. When the internal clock signal clk changes
from its L level to its H level, the second transfer gate 22b is
opened while the first transfer gate 22a is closed. At this time,
the second latch 21b receives the decoded signal which has been
held by the first latch 21a and continues to hold such decoded
signal. In this manner, respective decoded signals D1-D5 are
latched into a corresponding second latch 21b in response to the
rising edge of the internal clock signal clk.
[0054] The set-up/hold time is adjusted by the adjusting unit 14
(or the first to the fifth variable delay circuit 14a14e).
Accordingly, a result of detection (decoded signals D1-D5) from the
decode circuit 12 is held by the SFF circuits 15a-15e in a positive
manner in response to the rising edge of the internal clock signal
clk.
[0055] The delay adjusting unit 14 will now be described. The delay
adjusting unit 14 includes a first to a fifth variable delay
circuit 14a-14e.
[0056] The first variable delay circuit 14a receives the decoded
signal D1 from the first AND circuit 12a and feeds it to the first
SFF circuit 15a without or with a delay. Similarly, the second to
the fifth variable delay circuit 14b-14e receive the decoded signal
D2-D5, respectively, from the second to the fifth AND circuit
12b-12e, respectively, and feed the decoded signal D2-D5 to the
second to the fifth SFF circuit 15b-15e, respectively, without or
with a delay.
[0057] FIG. 5 is a schematic diagram of the first variable delay
circuit 14a. It is to be understood that the second to the fifth
variable delay circuit 14b-14e are constructed in the similar
manner as the first variable delay circuit 14a. The first variable
delay circuit 14a has an input terminal in which is connected to
the AND circuit 12a and an output terminal out which is connected
to the SFF circuit 15a. The first variable delay circuit 14a
includes a pair of switch elements 16a, 16b and a pair of MOS
capacitors 17a, 17b. Specifically, the switch elements 16a, 16b are
connected to nodes 18a, 18b on a signal line 18 which joins the
input terminal in and the output terminal out, and the pair of MOS
capacitors 17a, 17b are connected between the respective switch
elements 16a, 16b and the ground. Preferably, each switch element
16a, 16b includes an MOS transistor.
[0058] An adjustment of the set-up/hold time will now be
described.
[0059] The delay time or the set-up/hold time of the decoded
signals D1-D5 is adjusted by each variable delay circuit 14a-14e.
In the variable delay circuit 14a-14e, the delay time is adjusted
by selectively turning the switch elements 16a, 16b on and/or off.
More specifically, when the both switch elements 16a, 16b are
turned off, a shortest delay time is set up. When the both switch
elements 16a, 16b are turned on, a longest delay time is set up.
When one of the switch elements 16a, 16b is turned off while the
other is turned on, the resulting delay time is intermediate the
longest and the shortest delay time.
[0060] In the first embodiment, for each external command, the
first to the fifth AND circuit 12a-12e produces one decoded signal
D1-D5 of an H level and four decoded signals of an L level. Paying
attention to this fact, it is examined in a test mode if one of the
SFF circuits 15a-15e is capable of holding one of the decoded
signals, which has an H level in accordance with the external
command, in accordance with the internal clock signal clk. The
delay times of the variable delay circuits 14a-14e are determined
so that the SFF circuit is capable of holding the decoded signal
having an H level in accordance with the internal clock signal
clk.
[0061] Specifically, when the external command is an active
command, only the decoded signal D1 assumes an H level. The
external clock signal CLK and the active command are supplied to
the synchronous DRAM. The delay time of the first variable delay
circuit 14a is adjusted so that the first SFF circuit 15a is
capable of holding the decoded signal D1 in accordance with the
internal clock signal clk.
[0062] When the external command is a precharge command, only the
decoded signal D2 assumes an H level. The external clock signal CLK
and the precharge command are supplied to the synchronous DRAM. The
delay time in the second variable delay circuit 14b is adjusted so
that the second SFF circuit 15b holds the decoded signal D2 in
accordance with the internal clock signal clk.
[0063] When the external command is a read command, only the
decoded signal D3 assumes an H level. The external clock signal CLK
and the read command are supplied to the synchronous DRAM. The
delay time of the third variable delay circuit 14c is adjusted so
that the third SFF circuit 15c holds the decoded signal D3 in
accordance with the internal clock signal clk.
[0064] When the external command is a read command, only the
decoded signal D3 assumes an H level. The external clock signal CLK
and the read command are supplied to the synchronous DRAM. The
delay time of the third variable delay circuit 14c is adjusted so
that the third SFF circuit 15c holds the decoded signal D3 in
accordance with the internal clock signal clk.
[0065] When the external command is a write command, only the
decoded signal D4 assumes an H level. The external clock signal CLK
and the write command are supplied to the synchronous DRAM. The
delay time of the fourth variable delay circuit 14d is adjusted so
that the fourth SFF circuit 15d holds the decoded signal D4 in
accordance with the internal clock signal clk.
[0066] When the external command is a mode register set command,
only the decoded signal D5 assumes an H level. The external clock
signal CLK and the mode register set command are supplied to the
synchronous DRAM. The delay time of the fifth variable delay
circuit 14e is adjusted so that the fifth SFF circuit 15c holds the
decoded signal D5 in accordance with the internal clock signal
clk.
[0067] The delay time for each of the decoded signals D1-D5 is
defined as a length of time passed from the transition of the
potential of the command signal applied to one of the external
command input terminal T1-T4 to the transition of the potential of
the decoded signal D1-D5 delivered from the AND circuits 12a-12e.
It should be noted that the delay time is different between decoded
signals D1-D5. In addition, the delay time also varies depending on
the direction of transition (from H level or L level or L level to
H level) on the external command input terminals T1-T4.
[0068] The delay time also varies due to operational noises of the
synchronous DRAM such as a variation in the supply voltage, for
example. In other words, depending on the operational condition of
the semiconductor device as the potential of the decoded signal
D1-D5 transitions, the influence of the noises upon the delay time
varies from decoded signal to decoded signal.
[0069] Specifically, when the mode register set command is applied
to the semiconductor device, the semiconductor device is in an idle
condition where the operational noises within the semiconductor
device are relatively low. By contrast, when the active command,
the read/write command and precharge command are applied to the
semiconductor device, the semiconductor device is in an active
condition where the operational noises are relatively high. As a
consequence, when the mode register set command is applied, the
set-up/hold time is less susceptible to the influence of noises.
However, conversely, when the active command, the read/write
command or precharge command is applied, the set-up/hold time of
each AND circuit 12a-12e is greatly influenced by the noises. This
means that when the active command, the read/write command and
precharge command are asserted, the set-up/hold time is susceptible
to variation.
[0070] A variation in the set-up/hold time is reduced by using the
variable delay circuits 14a-14e to adjust the delay time
(set-up/hold time) for each command. In this manner, the SFF
circuits 15a-15e are enabled to hold each decoded signal having an
H level in conformity to the internal clock signal clk. As a
consequence, the command decode circuit 100 exhibits a reduced dead
zone width for the set-up/hold when viewed from the entire external
command input terminals T1-T4.
[0071] The synchronous DRAM according to the first embodiment has
the following advantages:
[0072] (1) The first to the fifth variable circuit 14a-14e are
connected between the decode unit 10 and the setup/hold latch
circuit 15. The command decode circuit 100 has a number of circuit
elements which is less than the number of circuit elements used in
the third prior art, and thus there are provided a relatively
compact command decode circuit 100 and hence SDRAM. Since the delay
time is adjusted for each decoded signal in the command decode
circuit 100, the dead zone width of the set-up/hold time as viewed
from the entire input terminals T1-T4 is relatively narrow.
[0073] (2) Using the first to the fifth variable delay circuit
14a-14e, the delay time (set-up/hold time) is adjusted for each
command. Accordingly, if the operational noises such as a variation
in the supply voltage of the synchronous DRAM differ from command
to command, there is provided an optimum adjustment of the delay
time. As a consequence, a variation in the set-up/hold time from
command to command is reduced, and the decoded signals D1-D5 can be
latched in the set-up/hold latch circuit 15.
[0074] (3) Each of the delay circuits 14a-14e includes the switch
elements 16a, 16b and MOS capacitors 17a, 17b, and is relatively
simple in construction. The incorporation of MOS capacitors 17a,
17b is facilitated.
[0075] (4) In the first embodiment, only one of the first to the
fifth AND circuit 12a-12e operates in response to a variety of
external commands. Accordingly, the decode circuit 12 has a
relatively low power dissipation. It will be noted that only one of
the delay circuits which is associated with the operating AND
circuit operates. Accordingly, the power dissipation of the delay
circuits 14a-14e is also relatively low.
[0076] The first embodiment mentioned above can be modified in the
manner mentioned below.
[0077] The input buffers 11a-11d may deliver a single phase signal.
The input buffer may include a delay circuit. Where a level
conversion of an external signal is unnecessary, the input buffers
11a-11d can be omitted.
[0078] AND circuit 12a-12e may be replaced by a decode circuit 12
which employ other logic circuits.
[0079] The variable delay circuits 14a-14e are not limited to those
shown in FIG. 5. For example, the switch elements 16a, 16b may be
replaced by fuses. In this instance, the fuse which is connected to
MOS capacitors 17a, 17b which is turned off is cut by a laser. MOS
capacitors 17a, 17b may be replaced by capacitors other than MOS
capacitors. In addition, a delay circuit or set/reset delay circuit
which employs an inverter circuit may be used.
[0080] SFF circuits 15a-15e latch the decoded signals D1-D5 in
response to the rising edge of the internal clock signal clk.
However, as an alternative, SFF circuits 15a-15e may latch the
decoded signals DL-D5 in response to the falling edge of the
internal clock signal clk, for example.
[0081] The invention is not limited to its application to a command
decode circuit 100 for the synchronous DRAM, but may also be
applied to an address decode circuit of such DRAM or to a variety
of test mode decision circuits.
[0082] By way of example, FIG. 7 is a block diagram of an address
decode circuit 200 according to a second embodiment of the
invention. The address decode circuit 200 includes a decode unit
30, a delay adjusting unit 33 and a set-up/hold latch circuit 34.
An external address signal BA0 and BA1 are supplied to a first and
a second external address terminal T11 and T12, respectively.
[0083] The decode unit 30 includes a first and a second input
buffer 31a, 31b, and a first to a fourth AND circuit 32a-32d. The
first input buffer 31a produces complimentary address signals ba0x,
ba0z from an external address signal BA0. The second input buffer
31b produces complimentary address signals ba1x, ba1z from the
external address signal BA1. Each of AND circuits 32a-32d
represents a two input AND circuit and receives address signals
ba0x, ba0z, ba1x and ba1z in different combinations. Each of AND
circuits 32a-32d produces one of decoded signals D11-D14 of an H
level when two address signals assume an H level.
[0084] The delay adjusting unit 33 includes a first to a fourth
variable delay circuit 33a-33d. The set-up/hold latch circuit 34
includes a first to a fourth SFF circuit 34a-34d. The SFF circuits
34a-34d latch respective decoded signals D11-D14 supplied from
corresponding variable delay circuits 33a-33d in synchronism with
the external clock signal CLK.
[0085] It will be understood that in the address decode circuit
200, the difference in the number of inverter circuits within the
input buffers 31a and 31b causes a variation in the set-up/hold
time of the external address signal. In order to assure that a
result of detection (decoded signal D11-D14) of the decode unit 30
be positively held in response to the rising edge of the internal
clock signal clk, the set-up/hold time is adjusted by using the
variable delay circuits 33a to 33d in the similar manner as in the
first embodiment. Accordingly, the second embodiment provides the
same advantage as achieved by the first embodiment.
[0086] The command decode circuit 100 of the synchronous DRAM may
be formed on a single semiconductor substrate, or may be formed on
the semiconductor substrate in mixture with the logic circuit.
[0087] It should be apparent to those skilled in the art that the
present invention may be embodied in many other specific forms
without departing from the spirit or scope of the invention.
Therefore, the present examples and embodiments are to be
considered as illustrative and not restrictive and the invention is
not to be limited to the details given herein, but may be modified
within the scope and equivalence of the appended claims.
* * * * *