U.S. patent application number 09/761274 was filed with the patent office on 2001-05-24 for delayed locked loop implementation in a synchronous dynamic random access memory.
This patent application is currently assigned to MOSAID Technologies Incorporated. Invention is credited to Allan, Graham, Foss, Richard C., Gillingham, Peter B..
Application Number | 20010001601 09/761274 |
Document ID | / |
Family ID | 23240630 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001601 |
Kind Code |
A1 |
Foss, Richard C. ; et
al. |
May 24, 2001 |
Delayed locked loop implementation in a synchronous dynamic random
access memory
Abstract
A clock applying circuit for a synchronous memory is comprised
of a clock input for receiving a clock input signal, apparatus
connected to the synchronous memory for receiving a driving clock
signal, and a tapped delay line for receiving the clock input
signal and for delivering the clock driving signal to the
synchronous memory in synchronism with but delayed from the clock
input signal, the delay being a small fraction of the clock period
of the clock input signal.
Inventors: |
Foss, Richard C.; (Calabogie
Lake, CA) ; Gillingham, Peter B.; (Kanata, CA)
; Allan, Graham; (Stittsville, CA) |
Correspondence
Address: |
James M. Smith, Esq.
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
Two Militia Drive
Lexington
MA
02421-4799
US
|
Assignee: |
MOSAID Technologies
Incorporated
Kanata
ON
|
Family ID: |
23240630 |
Appl. No.: |
09/761274 |
Filed: |
January 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09761274 |
Jan 16, 2001 |
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09392088 |
Sep 8, 1999 |
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09392088 |
Sep 8, 1999 |
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08996095 |
Dec 22, 1997 |
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08996095 |
Dec 22, 1997 |
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08319042 |
Oct 6, 1994 |
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5796673 |
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Current U.S.
Class: |
365/233.1 |
Current CPC
Class: |
G11C 7/1072 20130101;
G11C 7/1051 20130101; G11C 7/22 20130101; G11C 7/222 20130101; H03L
7/0814 20130101; H03K 5/133 20130101; H03L 7/0816 20130101; G11C
11/4076 20130101 |
Class at
Publication: |
365/233 |
International
Class: |
G11C 008/00 |
Claims
What is claimed is:
1. A clock applying circuit for a synchronous memory comprising:
(a) a clock input for receiving a clock input signal; (b) means
connected to the synchronous memory for receiving a driving clock
signal; and (c) a tapped delay line for receiving the clock input
signal and for delivering said clock driving signal to the
synchronous memory in synchronism with but delayed from the clock
input signal, the delay line being comprised of a series of delay
elements for carrying said clock input signal; (d) means for
providing the driving clock signal from an output of one of the
delay elements; (e) means for selecting an output from said one of
the delay elements comprising a comparator for comparing the clock
input signal with said driving clock signal and for selecting said
output from said one of the delay elements based on a closest
predetermined one of a rising or falling edge of a clock input
signal following an enable time required at a particular enable
terminal of the synchronous memory; and (f) means for providing the
driving clock signal to said particular enable terminal.
2. A clock applying circuit as defined in claim 1 including delay
model means having a signal time delay simulating clock skew delay
between a clock input terminal of the synchronous memory for
receiving the clock input signal and said particular enable
terminal, an input port of the delay model means for receiving the
driving clock signal and for providing a delayed driving signal to
the comparator.
3. A clock applying circuit as defined in claim 2 in which the
selecting means is comprised of a multiplexer for receiving output
signals of plural ones of the delay elements at respective inputs
thereof, means for receiving an input select control signal from
the comparator resulting from said comparing for selecting one of
said output signals for passing through the multiplexer as the
driving clock signal.
4. A clock applying circuit as defined in claim 1 in which the
selecting means is comprised of a multiplexer for receiving output
signals of plural ones of the delay elements at respective inputs
thereof and for outputting one of the output signals as the driving
clock signal.
5. A clock applying circuit as defined in claim 1 in which the
selecting means is comprised of a multiplexer for receiving output
signals of plural ones of the delay elements at respective inputs
thereof, means for receiving an input select control signal from
the comparator resulting from said comparing for selecting one of
said output signals for passing through the multiplexer as the
driving clock signal.
6. A clock applying circuit for a synchronous memory comprising: a
clock input receiving a clock input signal; a tapped delay line
receiving the clock input signal and delivering a clock driving
signal to the synchronous memory in synchronism with but delayed
from the clock input signal, the delay line being comprised of a
series of delay elements for carrying said clock input signal; and
a selector selecting an output from said one of the delay elements
to provide the driving clock signal, the selector comprising a
comparator comparing the clock input signal with said driving clock
signal and the selector selecting said output from said one of the
delay elements based on a closest predetermined one of a rising or
falling edge of a clock input signal following an enable time
required at a particular enable terminal of the synchronous
memory.
7. A clock applying circuit as defined in claim 1 including a delay
model having a signal time delay simulating clock skew delay
between a clock input terminal of the synchronous memory for
receiving the clock input signal and said particular enable
terminal, an input port of the delay model receiving the driving
clock signal and providing a delayed driving signal to the
comparator.
8. A clock applying circuit as defined in claim 7 in which the
selector is comprised of a multiplexer receiving output signals of
plural ones of the delay elements at respective inputs thereof, and
receiving an input select control signal from the comparator
resulting from said comparing for selecting one of said output
signals for passing through the multiplexer as the driving clock
signal.
9. A clock applying circuit as defined in claim 6 in which the
selector is comprised of a multiplexer receiving output signals of
plural ones of the delay elements at respective inputs thereof and
for outputting one of the output signals as the driving clock
signal.
10. A clock applying circuit as defined in claim 6 in which the
selector is comprised of a multiplexer receiving output signals of
plural ones of the delay elements at respective inputs thereof, and
receiving an input select control signal from the comparator
resulting from said comparing for selecting one of said output
signals for passing through the multiplexer as the driving clock
signal.
Description
FIELD OF THE INVENTION
[0001] This invention relates to the field of semiconductor
memories, and in particular to a circuit for applying a clock to a
synchronous memory such as a synchronous dynamic random access
memory (SDRAM).
BACKGROUND TO THE INVENTION
[0002] An SDRAM, shown in block diagram in FIG. 1 typically
operates as follows, with reference to the signal timing diagram
shown in FIG. 2. A clock input terminal 1 receives a clock input
signal CLK. The remainder of the SDRAM is represented by the memory
array and support circuitry block 3. The clock signal arriving at
the clock input terminal 1 is buffered inside the SDRAM,
represented by the receiver 5 and buffer 6, and is distributed to
internal circuitry of the SDRAM.
[0003] A signal at the output of the memory array and support
circuitry 3 is applied to output buffers, represented by output
buffer 8, which is enabled by the clock signal to drive data onto
data terminals 10 of the SDRAM. However, due to the delays caused
by the internal buffering and the interconnect wire on the
integrated circuit chip that distributes the clock signal, the
clock signal arrives at the enable terminal of the buffers delayed
from the clock input signal. This delayed clock signal is
illustrated in FIG. 2 as signal ICLK.
[0004] Assuming that the system is responsive to the rising edge of
the clock signal, the delay between the rising edges is shown in
FIG. 2 as internal clock skew 12. This clock skew can be a
significant fraction of the clock period if the part is driven with
a high frequency clock. The clock skew typically determines the
maximum speed of the part. As the operating frequency of the part
increases, as determined by the clock frequency, the clock skew
delay causes enabling of the output buffer 8 too late relative to
the next rising clock edge and the valid data at the output data
terminals 10 will appear too late for the receiving chip.
[0005] Prior to the present invention, there were either of two
solutions used to deal with this problem: (a) making the clock
buffer circuitry between the clock input terminal 1 and the output
buffer circuit enable terminal as fast as possible, and (b) using a
phase locked loop (PLL) to drive the enable terminal of the output
buffer.
[0006] Implementing the first solution results in a limit to the
operating frequency of the part. There will always be a limit to
the operating frequency of the part, because there will always be
significant delay associated with the clock buffer and distribution
circuitry and delays introduced by parasitic resistance and
capacitance of the interconnection conductors used to distribute
the buffered clock signal to the output buffers, which is evident
from FIG. 1. Thus as shown in FIG. 2, after the read command to the
memory array circuitry 3 from the address and control input of the
memory array, to output data to the output buffers 8, there must be
a delay 12 until valid data is output to the data terminals 10, as
indicated by the timing diagram DQ. This time is the sum of the
internal clock skew from the rising edge of the clock input signal
CLK to the rising edge of the delayed clock signal ICLK, and the
time from the rising edge of the clock signal ICLK to the time that
valid data is output on the output terminals 10 caused by the
output buffer delay after it has been clocked by the ICLK
signal.
[0007] The second solution provides considerable improvement over
the first. An on chip oscillator is used in a phase locked loop
(PLL) which is synchronized with the input clock signal. The
internal clock signal can be either multiplied in frequency or
adjusted to remove internal clock skew as much as possible.
[0008] A system implementing the second solution is shown in FIG.
3, and a corresponding timing diagram is shown in FIG. 4. A PLL 15
is fed by the input clock signal from receiver 5, as well as by a
feedback signal on conductor 17 derived from the interconnection
conductor which distributes the output buffer enable clock signal.
The latter signal is received from the output of the PLL via the
internal buffering circuitry represented by buffer 6.
[0009] Thus the already buffered (and delayed) clock signal is
applied to the PLL and is compared with the input clock signal.
Since the operation of the PLL is to synchronize the two signals,
the clock signal to be distributed to the enable inputs of the
output buffers, represented by the timing diagram ICLK in FIG. 4,
is made as close as possible in timing to the input clock signal.
The internal clock skew is thus minimized, as illustrated by skew
time 19 shown in FIG. 4. Thus the output buffer is enabled much
closer to the clock edge that is received by the part and valid
data appears sooner relative to the clock edge, and thus allowing
higher frequency operation of the part. This is shown by access
time 21, which it may be seen is much shorter than access time 12
resulting from the first solution.
[0010] However it has been found that the PLL solution also suffers
from problems. It is complex, requiring an on-chip oscillator with
feedback control of the frequency depending on the monitored status
of the on-chip oscillator relative to the input clock. It requires
significant stand-by power due to its extra circuitry, and it
requires considerable start-up time for the on-chip oscillator to
synchronize and lock to the input clock frequency. It also requires
use of an analog oscillator in a digital circuit, which requires
significantly different and complex fabrication techniques.
SUMMARY OF THE INVENTION
[0011] The present invention minimizes the elapsed time between a
clock edge that is input to a synchronous memory such as an SDRAM
and the time at which the same clock edge eventually triggers the
output buffer of the SDRAM to drive valid data onto the output
terminals of the SDRAM. The present invention utilizes a delay
locked loop (DLL) instead of the phase locked loop used in the
second solution described above. The DLL allows higher clock
frequency operation while requiring less standby current and
start-up time than the system that uses the PLL. No oscillator is
required as is required using the PLL, and the entire system can be
fabricated using digital integrated circuit technology, rather than
a mixture of analog and digital technology.
[0012] In accordance with an embodiment of the invention, a clock
applying circuit for a synchronous memory is comprised of a clock
input for receiving a clock input signal, apparatus connected to
the synchronous memory for receiving a driving clock signal, and a
tapped delay line for receiving the clock input signal and for
delivering the clock driving signal to the synchronous memory in
synchronism with but delayed fro the clock input signal, the delay
being a small fraction of the clock period of the clock input
signal. The fraction can be negligibly small.
[0013] In accordance with another embodiment, a clock applying
circuit is comprised of a synchronous dynamic random access memory
(SDRAM) comprised of a memory array and an output buffer connected
to the memory array, the memory array having a clock input signal
terminal and the output buffer having an enable terminal for
receiving a driving clock signal, a clock input for receiving a
clock input signal, a tapped delay line comprised of a series of
delay elements and having an input, apparatus for applying the
clock input signal to the clock input signal terminal and to the
input of the tapped delay line, apparatus for receiving output
signals of plural ones of the delay elements and for providing one
of the output signals of the delay elements as the driving clock
signal, apparatus for applying the driving clock signal to the
enable terminal of the output buffer, and apparatus for selecting
said one of the output signals having a predetermined one of the
rising and falling edge time which follows a corresponding rising
or falling edge of the clock input signal by a clock skew delay
time of the SDRAM between said clock input signal terminal of the
memory array and the output buffer.
BRIEF INTRODUCTION TO THE DRAWINGS
[0014] A better understanding of the invention will be obtained by
reading the description of the invention below, with reference to
the following drawings, in which:
[0015] FIGS. 1 and 3 are block diagrams illustrating prior art
systems,
[0016] FIGS. 2 and 4 are timing diagrams corresponding to and used
in understanding operation of the systems of FIGS. 1 and 3
respectively, and
[0017] FIG. 5 is a block diagram illustrating an embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Turning to FIG. 5, an input clock signal is applied to a
tapped delay line formed of a series of delay elements 25 such as
inverters. The outputs of predetermined ones of the delay elements,
which can be each one of the delay elements, are provided to the
inputs of a selection apparatus such as a multiplexer 27. The
output of the multiplexer 29 provides a signal, referred to herein
as a driving clock signal, which in this embodiment is applied to
the enable terminal of the output buffer in a manner as described
above with respect to the prior art systems.
[0019] A delay comparator 31 has one input that receives the input
clock signal, and another input that receives the driving clock
signal. The comparator 31 outputs a control signal which has a
value that depends on the differential between the input clock
signal and the driving clock signal. That control signal is applied
to the control inputs of multiplexer 27, and determines which of
the inputs to it are passed through it to output 29 and forms the
driving clock signal. The value of the control signal is such that
the delay between the input clock signal and the driving clock
signal is minimized in the positive sense (i.e. the leading edge of
the driving clock signal will always be at the same time or later
than the leading edge of the input clock signal).
[0020] In this manner the output buffer of the memory will be
enabled either no or a minimum time following the input clock.
[0021] In another embodiment, the feedback signal (i.e. the driving
clock signal) is delayed by a delay circuit 33, referred to herein
as a delay model, which use similar elements as the real circuit
path taken by the input clock signal, including buffers, logic
gates, interconnect conductors, etc. The result is a signal for
comparison by the delay comparator 31 which is delayed by a value
which tracks the real circuit's performance as operating conditions
vary. It's use in a memory can allow the memory to operate at high
speeds and maintains its capability as operating conditions such as
temperature vary.
[0022] While the system requires some time on start-up to adapt
itself to a stable operating condition, the start-up modes on most
synchronous memories should be sufficient for the output buffer to
receive a properly adjusted clock signal. Due to the nature of the
delay locked loop, there will be a minimum frequency below which
the internal function of the clock will be uncertain. If such
frequencies are contemplated, external control circuitry can be
used to disable the delay locked loop, such as by using a register
bit which when set enables the delay locked loop and when reset
disables the delay locked loop. Then the chip operates with the
digital locked loop disabled, the start-up time and minimum
frequency requirements will be ignored.
[0023] If the delay locked loop derived clock is used only for the
output buffer, any chip mode registers can be set and data can be
written to memory before the delay locked loop has adapted. If the
chip enters a power down mode while retaining supply voltage
levels, the last tap position can be preserved so that normal
operation can be quickly re-enabled.
[0024] During a standby state of the memory, the delay locked loop
can be disabled, and the delay chain settings can be maintained, as
long as the power is applied, allowing the part to enter a low
power mode. Upon exit from the standby state into an active state,
the system will enter a faster lock since the delay chain settings
are maintained.
[0025] The delay locked loop can be disabled and the regular
buffered version of the system can be used as in the prior art,
enabling the output buffer with the prior art form of delayed clock
signal, which can allow the system to be tested or operated using a
low frequency clock.
[0026] The driving clock signal can be used as the clock for the
entire memory system, it can be used for only parts of the memory
system and the input clock signal used for others, or can be used
only to enable the output buffer with the input clock signal used
for the remainder of the memory system.
[0027] The present invention is not limited for use in conjunction
with an SDRAM which was used as an example, but can be used in
conjunction with other synchronous memories such as synchronous
static random access memories, video random access memories,
synchronous graphics random access memories, synchronous read only
memories. In addition, other designs of the delay locked loop may
be used than the one described herein.
[0028] A person understanding this invention may now conceive of
alternative structures and embodiments or variations of the above.
All of those which fall within the scope of the claims appended
hereto are considered to be part of the present invention.
* * * * *