U.S. patent application number 10/109974 was filed with the patent office on 2002-08-08 for process, voltage and temperature independent clock tree deskew circuitry -temporary driver method.
Invention is credited to Schultz, Richard T..
Application Number | 20020105366 10/109974 |
Document ID | / |
Family ID | 27082597 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020105366 |
Kind Code |
A1 |
Schultz, Richard T. |
August 8, 2002 |
PROCESS, VOLTAGE AND TEMPERATURE INDEPENDENT CLOCK TREE DESKEW
CIRCUITRY -TEMPORARY DRIVER METHOD
Abstract
A multilevel clock tree uses a temporary clock buffer or
reference signal in a clock tree deskew circuit to dynamically
minimize skew in a variable delay clock signal that synchronizes
operation of synchronized circuit components of an integrated
circuit. There are multiple temporary clock buffer signals at each
level of the multilevel clock tree. Skew between the temporary
clock buffer signals are minimized by providing identical path
lengths and path geometries at each level of the temporary clock
buffer. The clock tree deskew circuit reduces the clock tree skew,
on a level by level basis, in repeated intervals over a period of
time. When each level of the tree deskew circuit is deskewed, that
level of the clock tree deskew circuit is then turned off to
prevent unnecessary further adjustments to the clock signals, but
can be turned back on when conditions change that alter the clock
tree skew. The clock tree deskew circuit adjusts the variable delay
clock buffer signal of each pair toward the temporary clock buffer
signal of the pair to reduce the skew between the two clock buffer
signals. After a predetermined number of adjustment cycles, the
overall clock skew of the variable delay clock buffer signal is
minimized by repeated adjustments. The variable delay clock buffer
signals of each level may be optionally set as conditions
warrant.
Inventors: |
Schultz, Richard T.; (Fort
Collins, CO) |
Correspondence
Address: |
LSI Logic Corporation
1551 McCarthy Blvd.
M/S: D-106 Patent Department
Milpitas
CA
95035
US
|
Family ID: |
27082597 |
Appl. No.: |
10/109974 |
Filed: |
March 29, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10109974 |
Mar 29, 2002 |
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09915237 |
Jul 25, 2001 |
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09915237 |
Jul 25, 2001 |
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09596677 |
Jun 19, 2000 |
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6340905 |
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Current U.S.
Class: |
327/161 |
Current CPC
Class: |
G06F 1/10 20130101 |
Class at
Publication: |
327/161 |
International
Class: |
H03L 007/00 |
Claims
What is claimed is:
1. A method for deskewing clock signals generated from a multilevel
clock tree to synchronize operation of synchronized clock circuit
components in an integrated circuit, comprising: receiving a first
clock signal; generating a plurality of groups of second clock
signals from the first clock signal; designating one of each group
of the plurality of groups of second clock signals as a second
clock reference signal; detecting and adjusting the skew, in each
group of the plurality of groups of second clock signals, by
comparing the second clock reference signal of that group with each
of the other second clock signals of that group; and generating a
plurality of groups of third clock signals in which the third clock
signals are generated only from those second clock signals of that
group which are not second clock reference signals.
2. The method of claim 1, further comprising the steps of
designating one of each group of the plurality of groups of third
clock signals as a third clock reference signal; and detecting and
adjusting the skew of the plurality of groups of third clock
signals, by comparing the third clock reference signal of that
group with each of the other third clock signals, the means for
adjusting generating a plurality of groups of fourth clock signals
only from the third clock signals of that group which are not
reference signals in a multilevel clock tree structure.
3. The method of claim 2, wherein each level of the multilevel
clock tree has its own enable signal which permits the clock tree
to be calibrated for that level and that level only.
4. The method of claim 3, wherein each group of a selected level of
the multilevel clock tree does not receive any signals from and
does not transmit any signals to any other group of that selected
level of the multilevel clock tree.
5. An integrated circuit with clock deskewing circuitry,
comprising: means for receiving a first clock signal; means for
generating a plurality of groups of second clock signals from the
first clock signal; means for designating one of each group of the
plurality of groups of second clock signals as a second clock
reference signal; and means for detecting and adjusting the skew,
in each group of the plurality of groups of second clock signals,
by comparing the second clock reference signal of that group with
each of the other second clock signals, the means for adjusting
generating a plurality of groups of third clock signals only from
the second clock signals of that group which are not reference
signals.
6. The integrated circuit of claim 5, further comprising means for
designating one of each group of the plurality of groups of third
clock signals as a third clock reference signal; and means for
detecting and adjusting the skew of the plurality of groups of
third clock signals, by comparing the third clock reference signal
of that group with each of the other third clock signals, the means
for adjusting generating a plurality of groups of fourth clock
signals only from the third clock signals of that group which are
not reference signals in a multilevel clock tree structure.
7. The integrated circuit of claim 5, the plurality of groups of
third clock signals being used to time the active circuit
components of the integrated circuit.
8. The integrated circuit of claim 6, further comprising means for
generating a plurality of n th clock signals from (n-1) th, where n
is a natural number greater than 4.
9. The integrated circuit of claim 6, further comprising means for
ensuring synchronization between all the clock signals of a given
level in the multilevel clock tree structure in addition to the
means for detecting and adjusting the skew.
10. The integrated circuit of claim 9, wherein the means for
ensuring synchronization include means for providing identical
clock signal path lengths and symmetric layouts of the clock signal
paths for a given level.
11. The integrated circuit of claim 5, wherein each group of the
plurality of groups of second clock signals is isolated from other
groups of the plurality of second clock signals such that no group
of the plurality of groups of second clock signals receives any
signal from or transmits any signal to the other groups of the
plurality of groups of second clock signals.
12. The integrated circuit of claim 5, wherein the integrated
circuit is incorporated into a portable device.
13. The integrated circuit of claim 12, wherein the integrated
circuit is incorporated into one of the group consisting of a lap
top, a cell phone, a personal device assistant, and a pager.
14. The integrated circuit of claim 5, wherein the means for
detecting and adjusting are selectively enabled.
15. An integrated circuit for deskewing clock signals in a
multilevel clock tree structure, comprising: a buffer for receiving
a first clock signal and fanning out a plurality of second clock
signals derived from the first clock signal, the plurality of
second clock signals being divided into distinct groups; first
level skew detect and adjust circuitry, of a first level of the
multilevel clock tree, dedicated to each of the distinct groups of
the plurality of second clock signals so that each of the distinct
groups of the plurality of second clock signals each generates a
plurality of third clock signals, wherein in each of the distinct
groups of the plurality of second clock signals, one of the second
clock signals is a reference clock signal and the others are
variable delay clock buffer signals such that the reference clock
signal is used to reduce the skew of the variable delay clock
buffers, only the variable delay clock buffers from each distinct
group of the plurality of second clock signals being used to
generate the plurality of clock signals corresponding to each
distinct group of the plurality of second clock signals.
16. The integrated circuit of claim 15, wherein each of the
plurality of second clock signals travels along a conductive path
from the buffer to the first level skew detect and adjust circuitry
that is matched with the other conductive paths of the remainder of
the plurality of second clock signals in that the signal path
lengths are identical and the signal path layout has geometric
symmetry.
17. The integrated circuit of claim 16, wherein the geometric
symmetry includes an H-shaped layout.
18. The integrated circuit of claim 15, wherein there are n levels
of the multilevel clock tree, each level having distinct groups of
n th clock signals as input to and (n+1) th clock signals as output
from n th level skew detect and adjust circuitry which is uniquely
assigned to a distinct group of the n th clock signals and which
outputs a distinct group of (n+1) th clock signals, wherein one of
each distinct group of n th clock signals is a reference clock
signal by which the remainder of that distinct group of n th clock
signals is adjusted, wherein n is a natural number greater than or
equal to 2.
19. The integrated circuit of claim 18, wherein all the levels of
the skew detect and adjust circuitry is physically laid out on the
integrated chip in a concentric manner.
20. The integrated circuit of claim 15, wherein the skew adjust
circuitry is implemented using tristatable buffers to set an
adjustment time for clock signal adjustment.
21. The integrated circuit of claim 15, wherein the skew adjust
circuitry is implemented using switched capacitors.
22. The integrated circuit of claim 15, wherein the skew detect and
adjust circuitry is implemented using an internal microcontroller
and an external memory which stores adjustment data from the
integrated circuit and loads adjustment data to the integrated
circuit.
23. The integrated circuit of claim 15, wherein the skew detect and
adjust circuitry is implemented using an external microcontroller
and an external memory which stores adjustment data from the
integrated circuit and loads adjustment data to the integrated
circuit.
24. The integrated circuit of claim 23, wherein the external memory
is an electrically erasable programmable read only memory.
25. The integrated circuit of claim 18, wherein each level of the
multilevel clock tree is independently settable through its own
enable signal.
26. The integrated circuit of claim 25, wherein each level is
automatically set.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/915,237, filed Jul. 25, 2001, which is a
continuation-in-part of U.S. patent application Ser. No.
09/596,677, filed Jun. 19, 2000, now U.S. Pat. No. 6,340,905 B1.
U.S. patent application Ser. Nos. 09/915,237 and 09/596,677 are
herein incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to clock signal fan out within
integrated circuit (IC) chips and, specifically, to a method of
deskewing clock signals at various levels of a multilevel clock
tree.
BACKGROUND OF THE INVENTION
[0003] Conventional integrated circuits (ICs) use a clock signal
and branch it out through a series of buffers to form a plurality
of clock signals. The structure of the branching of the clock
signal is called a "clock tree." One or more clock trees can be
present in a single IC. For example, a clock signal at a given
branch, or level, of the clock tree may feed into three buffers to
produce three clock signals at the next level, which may each feed
into three more buffers to produce nine clock signals at the third
level. The clock signals at any level of the clock tree are sent to
various synchronous components of the IC to coordinate the
functions of these components. For various reasons, however, any
two clock signals, even at the same level of the same clock tree,
may be slightly different or offset from each other. This
difference in clock signals is called "clock skew," and differences
throughout several clock signals of any level of the clock tree is
called "clock tree skew".
[0004] Clock tree skew has several causes. For example, the buffers
between levels in the clock tree typically introduce a delay
between their input and output clock signals, so clock signals at
different levels of the clock tree are usually naturally skewed
from each other. Additionally, the load experienced by one clock
signal may introduce a delay into the clock signal different from
that of another load on another clock signal. Furthermore, changes
in temperature, different applied voltages and differing
semiconductor fabrication processes can affect the clock skew. Such
other causes of clock skew are typically due to temperature
variations, circuit load variations, different applied voltages,
different semiconductor fabrication processes and inadequate
tolerances in the semiconductor fabrication process.
[0005] One prior art device employs a power PC chip which has
buffers with multiple trees to drive a large bus. Performance is
less than optimal because changes in the rise and fall times of
these many drivers slows system operation.
[0006] Another prior art device relied on a neighboring clock
signal for deskewing in which each finally derived clock signal
drove the active components of the integrated circuit. This device
consumed a notable area of semiconductor substrate and was somewhat
susceptible to process, voltage, and temperature variations.
[0007] There is a need to provide a method to reliably generate
multiple levels of well calibrated clock signals which requires
minimal semiconductor substrate area and has reduced susceptibility
to process, voltage, and temperature variations.
SUMMARY OF THE INVENTION
[0008] The present invention enables dynamic self-detection and
correction of clock tree skew in an integrated circuit (IC) using a
multilevel clock tree. Each level has a temporary clock buffer or
reference signal which is used to deskew the variable delay clock
buffer signals. Several temporary clock buffer signals are
generated at each level. By designing their signal paths to be the
same length and geometry, the temporary clock buffer signals of a
given level are synchronized. Only the variable delay clock buffer
signals proceed to the next level of the multilevel clock tree.
[0009] Clock skew variations due to temperature changes, different
applied voltages and different semiconductor fabrication processes
are corrected at each level of the multilevel clock tree. Thus, as
clock skew increases or decreases during operation of the IC, the
present invention may dynamically detect and correct the changing
clock skew on-the-fly. In this manner, the adjustment of each clock
signal in a clock tree does not rely on a single determination and
adjustment of the anticipated clock skew during the design of the
IC, but is altered and re-altered as is dynamically determined to
be appropriate by a skew detection and adjustment circuitry,
particularly in response to differences in applied voltage,
temperature and fabrication process. The design combines signal
path length balancing with temporary clock buffers for calibration.
The clock skew of every variable delay clock buffer signal of each
level may be independently set as circumstances warrant.
[0010] In the skew detection and adjustment circuitry of the
present invention, each variable delay clock buffer signal in a
clock tree is paired with a temporary clock buffer signal at each
level in the clock tree. The absolute skew between the two clock
signals in each pair is detected, and the variable delay clock
buffer signal of each pair is adjusted forward or backward as
appropriate. Such adjustment of the clock signals is performed by
adding or subtracting a certain amount of delay. The detection and
adjustment is repeated in increments as necessary to reach an
acceptable minimum skew. Thus, the invention has the advantage of
automatically correcting for almost any amount of clock skew.
Additionally, since the skew detection and adjustment circuitry may
dynamically detect and correct absolute skew between a pair
consisting of a temporary clock buffer signal and a variable clock
delay buffer signal, the invention has the further advantage of
automatically correcting for clock skew variations due to different
applied voltages and/or different semiconductor fabrication
processes that could not be anticipated during the design of the
IC. The detection and adjustment may be performed during the
operation of the IC to account for changing clock skew patterns or
it may be performed as needed.
[0011] The clock tree deskew circuit preferably includes a
plurality of skew adjust circuits and a corresponding plurality of
skew detect circuits. The clock tree deskew circuit of a level also
preferably sends output clock signals to either the synchronized
circuit components or to the next level of the clock tree. Each
skew adjust circuit corresponds to and produces one of the variable
delay clock buffer signals. The skew detect circuits connect to
their corresponding skew adjust circuits to receive the
corresponding variable delay clock buffer signal. Each skew detect
circuit also receives a temporary clock buffer signal. Each skew
detect circuit produces an adjustment signal to its corresponding
skew adjust circuit indicative of whether the variable delay clock
signal preceded the temporary buffer signal. Each skew adjust
circuit preferably receives the adjustment signal and shifts the
corresponding variable delay clock buffer signal accordingly.
[0012] The multilevel clock tree of the present invention may be
arranged such that each level is concentric to the others. This
arrangement helps provide symmetry for the temporary clock buffer
signal paths to ensure synchronization of the temporary clock
buffer signals through the matching of signal path geometry and
path length. The present invention provides an improvement over
current methods in being able to adjust clock skew over
temperature, process, and voltage variations and on the fly within
just a few clock cycles.
[0013] In a portable device which incorporates an integrated
circuit which has a multilevel clock tree of the present invention,
a user may be able to selectively synchronize the timing of an
integrated circuit having the multilevel clock tree. This would
allow the user to synchronize the timing when he suddenly goes out
doors or experiences a change of environment or climate.
[0014] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive of the invention
claimed. The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate an embodiment of
the invention and together with the general description, serve to
explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The numerous objects and advantages of the present invention
may be better understood by those skilled in the art by reference
to the accompanying figures in which:
[0016] FIG. 1 is a physical layout of a two level clock tree of the
present invention;
[0017] FIG. 2 is an embodiment of a skew adjust circuit of the
clock tree deskew circuit;
[0018] FIG. 3 is another embodiment of the skew adjust circuit of
the clock tree deskew circuit;
[0019] FIG. 4 is a block diagram of a skew detect circuit of the
clock tree deskew circuit;
[0020] FIG. 5 is a waveform timing diagram illustrating the
deskewing of clock signals by the clock tree deskew circuit;
[0021] FIG. 6A illustrates a timing diagram of a prior art
device;
[0022] FIG. 6B illustrates a timing diagram of the present
invention; and
[0023] FIG. 7 illustrates an embodiment of the invention using a
microcontroller.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Reference will now be made in detail to an embodiment of the
invention, examples of which are illustrated in the accompanying
drawings.
[0025] In FIG. 1, in an exemplary embodiment, there are two levels
to the clock tree. FIG. 1 is merely illustrative as the present
invention is not limited to two levels, but may be practiced with
three, four, or more levels. It may also be practiced with but a
single level. The circuit operates based on two temporary clock
nets each turned on or off independently. The first level temporary
clock net 45 is turned on (by the level 1 enable net 60) and driven
by four first level temporary clock buffers 20. There may be a
greater or lesser number of first level temporary clock buffers 20.
There may be two, three, eight, etc. temporary clock buffers in the
first level. A natural limit on the maximum number of temporary
clock buffers is the fan out capability of the immediate source of
the input clock signal. The fanout may be limited to four, six,
eight, or another number. The second temporary clock tree net 48 is
turned off or grounded by the level 2 enable net 70. The first
level temporary clock buffers 20 are driven by one starting buffer
5 shown connected to the "clock" in the center of the chip. This
source clock which provides an input to buffer 5 may be provided by
an off chip voltage controlled oscillator, a crystal oscillator, or
the like, proximately disposed to the integrated circuit.
[0026] Since the delay along the four nets connected to the initial
"clock" buffer may not be identical due to intra die interconnect
process variations and due to different neighboring routes, the
first level temporary clock buffers 20 may not all turn on at the
same time. Additional differences in turn on times may be caused by
intra die transistor variation, variations in signal line lengths,
and differing capacitive effects. However, in the present
invention, the input signal paths of each temporary clock buffer of
a given level are designed to be the same length in an H style
layout of the signal paths. Thus, the clock signal into each
temporary clock buffer is presumed to be identical to the clock
signal of the other temporary clock buffers of the same level. In
other words, the temporary clock buffer signal paths are balanced
which results in fewer and minimal adjustments.
[0027] In FIG. 1, additional delay programmable delay buffers 10
are provided right next to each of the temporary clock buffers 20.
The temporary clock buffer signal is a reference signal used to
deskew at least one variable delay clock buffer signal but fewer
than all the variable delay clock buffer signals of a given level
of the clock tree. The variable delay clock buffer signal is
adjusted with respect to this reference signal to provide a portion
of the variable delay clock buffer signals and temporary clock
buffer signals of the next level or to provide a portion of the
clock signals which time the synchronized active components of the
integrated circuit.
[0028] The temporary clock buffers 20 are connected to the original
"clock" buffer 5 in the center of the chip. These programmable
clock tree buffers 10 are used to drive the next level of the clock
tree. The programmable clock tree buffers 10 receive a variable
delay clock buffer signal of a first level and provide either the
actual clocks which run the active circuitry in the IC or the
variable delay clock buffer signals and temporary buffer signals of
the next level. The programmable buffers turn on times may not be
identical and may have some skew between them. For this reason, the
programmable buffers outputs are also connected to a skew detect or
phase detect circuit 40. The skew detect circuit 40 produces a 1 or
0 depending on whether the temporary clock buffers 20 or the first
level programmable delay buffer output 10 arrives first. If the
programmable delay buffer 10 arrives last, its speed is increased
to match the temporary clock net 20. All first level programmable
delay buffers 10 are adjusted at the same time. The number of
cycles needed and deskew capability depends on the resolution of
the programmable delay buffer increments. With the present
invention, the final deskew may be kept considerably under 20 to 30
picoseconds and may even be as low as 5 picoseconds.
[0029] The first level programmable delays are first established.
Testing equipment may be used on a representative sample of IC
units to make a firm determination as to the variance of the clock
signals generated from the different tree levels. After the first
level programmable delays are set, the first level temporary clock
buffers are turned off or grounded by the first level enable signal
60. The programmable delay clock buffers 10 are also now held
constant in terms of delay and no further adjustments are
necessary. They still remain active and are not controlled by the
first level enable signal.
[0030] The first level programmable delay drivers 10 next to the
temporary buffers 20 are now deskewed and drive the second level
temporary clock buffers 30 and second level programmable delay
buffers 15 next to the temporary clock buffers 30. Now the second
level temporary clock buffers 30 are enabled via the level 2 enable
net 70. Once again, the net delays, programmable clock buffer
delays and temporary clock buffer delays will not be identical. The
second level clock net 48 may not have smooth rise and fall times.
The temporary 30 and programmable delay 15 buffer outputs are
connected to the skew detect or phase detect circuits.
[0031] The programmable clock buffers 15 are adjusted to match the
temporary clock net 48. The skew detect circuit 40 produces a 1 or
0 depending on whether the temporary net 48 or the second level
programmable delay buffer 15 arrives out first. If the programmable
delay buffer 15 arrives first, it is then slowed down to match the
temporary clock net 48. If the programmable delay buffer 15 arrives
last, its speed is increased to match the temporary clock net 48.
All second level programmable delay buffers 15 are adjusted at the
same time. The number of cycles needed and deskew capability
depends on the resolution of the programmable delay buffer
increments. The final deskew of the second level programmable clock
tree buffers 15 may be set below 20 to 30 picoseconds, even to 5
picosecond or less.
[0032] After the second level programmable delays are set, the
second level temporary clock buffers 30 are turned off or grounded
by the second level enable signal 70. The programmable delay clock
buffers are also now held constant in terms of delay and no further
adjustments are necessary. They still remain active and are not
controlled by the second level enable signal.
[0033] This process may be repeated many times depending on how
many levels of programmable clock buffers are needed. The resulting
skew will not increase with the number of levels added to the
design since each level is deskewed prior to the next level being
deskewed.
[0034] Referring now to FIG. 4, an exemplary skew detect circuit is
shown. The skew detect circuit may be conventional or specially
designed. It may include two sets of delay buffers, or inverters,
226a-226f and 228a-228f, a pass gate 230, a keeper cell 232 and an
output buffer 234. The variable delay clock buffer signal 161 is
fed through the delay buffers 226a-226f, and the temporary clock
buffer signal 162 is fed through the delay buffers 228a-228f. The
skew adjust signal 167 is produced by the output buffer 234. The
delay buffers 226a-226f and 228a-228f cause the skew adjust signal
167 to be produced with appropriate timing to be latched by the
set/reset flip-flops 185 and 186 (FIG. 2) of the skew adjust
circuit. From previous (or, N level) variable delay clock buffer
signal 160, N+1 level variable delay clock buffer signal 161 is
generated.
[0035] The delay buffers 226a-226f are connected to each other in
series, with the variable delay clock buffer signal 161 connected
to the input of the first delay buffer 226a. The output of the last
delay buffer 226f is connected to the pass gate 230. The pass gate
230 also connects to the keeper cell 232 to pass the output signal
of the delay buffer 226f to the keeper cell 232 when the pass gate
230 is closed, or turned "on." The pass gate 230 includes a
positive-active transistor 236 and a negative-active transistor 238
connected in parallel, such that they are closed when a logic 1
signal activates transistor 236 and a logic 0 signal activates
transistor 238. The keeper cell 232 connects to the output buffer
234 to provide the desired value of the skew adjust signal 167
(inverted) to the output buffer 234.
[0036] The keeper cell 232 includes two inverters 246 and 247
connected in a loop, such that the input of inverter 246 connects
to the output of inverter 247, and the output of inverter 246
connects to the input of inverter 247. The input of the inverter
246 also forms the input of the keeper cell 232, and the output of
the inverter 246 also forms the output of the keeper cell 232. In
this manner, since the inverter 247 feeds back into the inverter
246, the inverters 246 and 247 maintain the previously received
signal as long as no new signal is applied to the keeper cell 232
as described below. The output buffer 234 inverts the output signal
of the keeper cell 232 and provides the inverted signal as the skew
adjust signal 167 during the time that no new signal is applied to
the keeper cell 232. The timing of the skew detect circuit and the
skew adjust circuit 148 (FIG. 2) are such that the skew adjust
signal 167 is latched by the set/reset flip-flops 185 and 186 (FIG.
2) during this time.
[0037] The delay buffers 228a-228f are connected to each other in
series, with the temporary clock buffer signal 162 connected to the
input of the first delay buffer 228a. Outputs of the second-to-last
delay buffer 228e and the last delay buffer 228f are connected to
transistors 236 and 238, respectively, of the pass gate 230.
Control signals from the delay buffers 228e and 228f control the
transistors 236 and 238, respectively, to turn "on" and "off" the
pass gate 230 to permit the output signal from the delay buffer
226f to pass through, or not to pass through, the pass gate 230 to
the keeper cell 232.
[0038] A pull-down transistor 258 is connected between the input of
the keeper cell 232 and ground 260. The gate of the pull-down
transistor 255 is connected to the output of an OR gate 266. The OR
gate 266 receives its inputs from the outputs of the first two
delay buffers 226a and 228a. Thus, as long as at least one of the
variable delay clock buffer signal 161 and temporary clock buffer
signal 162 is a logic 0, then at least one of the input signals to
the OR gate 266 will be a logic 1, and the output signal from the
OR gate 266 will also be a logic 1. In this case, the pull-down
transistor 258 will be turned "on," and the input of the keeper
cell 232 will be pulled down to ground, so the output signal from
the pass gate 230 cannot be applied to the keeper cell 232 at this
time. It is only when both of the variable delay clock buffer
signal 161 and temporary clock buffer signal 162 go "low" (i.e.
logic 0) that the pull-down transistor 258 is turned "off," and the
output signal from the pass gate 230 is applied to the keeper cell
232.
[0039] The output signal from the delay buffer 226f is passed
through the pass gate 230 to the keeper cell 232 only when the pass
gate 230 is closed, or "on." The pass gate 230 is "on" only when
the output signals of the delay buffers 228e and 228f are logic 1
and logic 0, respectively. The output signals of the delay buffers
228e and 228f are logic 1 and logic 0, respectively, only when the
temporary clock buffer signal 162 is logic 0, and the logic 0 has
propagated through the delay buffers 228a-228f. In other words, the
output signal of the delay buffer 226f can pass through the pass
gate 230 to the keeper cell 232 only when the temporary clock
buffer signal 162 is logic 0. However, the pull-down transistor 258
permits the keeper cell 232 to receive the output signal from the
pass gate 230 only when both of the clock signals 161 and 162 are
logic 0. Due to the delay of the temporary clock buffer signal 162
through the delay buffers 228a-228f, however, there is a "window of
opportunity" for a short time interval after the temporary clock
buffer signal 162 transitions from logic 0 to logic 1 for the
output signal of the delay buffer 226f to pass through the pass
gate 230 before the pass gate 230 is turned "off." The pull-down
transistor 258 is turned "off" at the beginning of the "window of
opportunity," so that the output signal from the pass gate 230 can
be latched by the keeper cell 232. The pull-down transistor 258
remains "off" for an additional period of time during which the
keeper cell 232 "holds" the value of the most recent output signal
from the pass gate 230. During this period of time, the output
buffer 234 supplies the skew adjust signal 167 to the skew adjust
circuit 148 (FIG. 2).
[0040] If the clock skew between the variable delay clock buffer
signal 161 and temporary clock buffer signal 162 is such that the
variable delay clock buffer signal 161 precedes the temporary clock
buffer signal 162, then the variable delay clock buffer signal 161
will transition from logic 0 to logic 1 before the temporary clock
buffer signal 162 does the same. Therefore, since the number of
delay buffers 226a-226f is the same as the number of delay buffers
228a-228f, the output signal of delay buffer 226f will become logic
1 for a time approximately equal to the time of the clock skew
prior to the turning "off" of the pass gate 230. In this case, the
logic 1 value will be latched by the keeper cell 232 and held at
this logic value until the pull-down transistor 258 turns on.
During this time, the inverter 246 inverts the logic 1 to logic 0,
the output buffer 234 inverts the logic 0 to logic 1 for the skew
adjust signal 167, and the set/reset flip-flops 183 and 186 (FIG.
2) latch the skew adjust signal 167.
[0041] If the clock skew between the variable delay clock buffer
signal 161 and temporary clock buffer signal 162 is such that the
variable delay clock buffer signal 161 follows the temporary clock
buffer signal 162, then the variable delay clock buffer signal 161
will transition from logic 0 to logic 1 after the temporary clock
buffer signal 162 does the same. Therefore, the output signal of
the delay buffer 226f will still be logic 0 at the time of the
turning "off" of the pass gate 230. In this case, the logic 0 value
will be latched by the keeper cell 232 and held at this logic value
until the pull-down transistor 258 turns "on," between the time
that the keeper cell 232 latches the logic 0 and the time that the
pull-down transistor 258 turns "on," the inverter 246 inverts the
logic 0 to logic 1, the output buffer 234 inverts the logic 1 to
logic 0 for the skew adjust signal 167, and the set/reset
flip-flops 183 and 186 (FIG. 2) latch the skew adjust signal
167.
[0042] An exemplary clock tree deskew situation with different
examples of clock skew between the variable delay clock buffer
signal 161 and the temporary clock buffer signal is shown in FIG.
5. In this example, the rising edge of the variable delay clock
buffer signal 161 leads the rising edge of the reference signal 162
by a certain amount of time 272. The skew detect circuitry sends an
adjustment bit to the skew adjust circuitry. On the next clock
cycle, the rising edge of the variable delay clock buffer signal
161 again leads the rising edge of the reference signal 162 by an
amount of time 274. The skew detect circuitry sends an adjustment
bit to the skew adjust circuitry. The next rising edge of the
variable clock delay signal 161 coincides with the next rising edge
of the reference or temporary clock buffer signal 162. This skew
detect circuitry will continue to output adjustment bits, but now
they will toggle from one clock cycle to the next because the two
signals 161 and 162 are essentially synchronized to the circuitry
limits. The skew is no more than 30 picoseconds and may be less
than 5 picoseconds.
[0043] In FIG. 3, another embodiment of the skew adjust circuit is
shown. The buffer structure 389 generally includes inverters 396
and 397, a set of transistor switches 398, 399, 400, 401, 402, and
403 and a set of capacitors 404, 405, 406, 407, 408, and 409. The
output of inverter 396 connects to the input of inverter 397 across
signal line 410. The input of inverter 396 inverts the input clock
signal 360 and sends it across signal line 410 to the inverter 397,
which inverts the clock signal again and produces it as output
clock signal 361.
[0044] The transistor switches 398, 399, 400, 401, 402 and 403
connect between signal line 410 and the capacitors 404, 405, 406,
407, 408 and 409, respectively, which in turn connect to ground
411. When any one of the transistor switches 398, 399, 400, 401,
402, or 403 is closed, its respective capacitor 404, 405, 406, 407,
408, or 409 applies a capacitive load to the inverted signal on
signal line 410, due to the time required to charge and discharge
the capacitors 404, 405, 406, 407, 408 and 409.
[0045] When few or none of the transistor switches 398 to 303 are
closed, then the capacitive load, and thus the delay, applied to
the inverted signal is small or minimized. When most or all of the
transistor switches 398 to 403 are closed, then the capacitive
load, and thus the delay, applied to the inverted signal is large
or maximized. By selectively closing or opening the transistor
switches 398 to 403, the delay in the inverted signal is increased
or decreased in predetermined increments, or steps, as determined
by the capacitance of the capacitors 404 to 409.
[0046] Other operational details of FIG. 3 may be found in the
description of FIG. 3 of U.S. Pat. No. 6,340,905, which is herein
incorporated by reference.
[0047] The number of set of set/reset flip-flops in the skew adjust
circuit need not be six, but may be a number such as four, eight,
ten, or any other odd or even number that provides sufficient
calibration of the variable delay clock buffer. Other circuits may
be used to adjust the skew.
[0048] FIG. 6B illustrates a timing diagram of the present
invention. The transition of the clock of the present invention is
cleaner than the transition of the clock of the prior art (FIG. 6A)
since, in the present invention, there is no contention between
multiple clock buffers that drive the clock tree since the
temporary driver net is turned off after the variable delay clock
buffers are deskewed with respect to the temporary driver net. In
contrast, in the prior art, the temporary driver net is used to
drive the clock tree.
[0049] Each level of the clock tree may optionally be set rather
than successively set. For instance, a user may set the first and
third levels, but not the second. Or, the user may set the second
and third levels, but not the first. Or, the user may set the first
and fourth level, but not the second and third. Other combinations
of setting levels may be performed.
[0050] The programmable clock buffers may be controlled by external
firmware and/or an internal micro controller. All that is needed is
a register to keep track of what adjustments are needed for each
programmable buffer. The adjustment information may also be written
to an EPROM, flash memory, SRAM, DRAM, or hard drive. The advantage
of this method is that all adjustments are self calibrating and do
not need external software or hardware.
[0051] FIG. 7 illustrates an embodiment of the present invention
using a micro controller and EEPROM wherein the microcontroller and
EEPROM may be disposed external to the chip. Components 510, 520,
530, 540, 550, 560, 570, 580, 590, 600 form another embodiment of
skew detect and skew adjust circuits for one of the levels of the
multilevel clock tree. The microcontroller 500 provides the set,
reset, and enable signals for the deskew logic. As shown, the clock
may be controlled by the enable signal through a logic gate 510,
560. An enabled clock is used by a 10 bit register for timing the
adjustment for the particular variable delay clock buffer. The MUX
530, 580 switches on and off the data from the registers 520, 570
or allows the previous adjustments which were stored in the EEPROM
to load or control the programmable delay buffers. A programmable
delay clock buffer 540, 590 controls the output of the resulting
clock signal. The set and reset signals allow for initialization of
the adjustment circuitry. Other variations are within the purview
of the invention.
[0052] The exemplary embodiment of FIG. 7 also permits testing of
the integrated circuit's multilevel clock tree to finely control
for process variations during manufacture.
[0053] The present invention may be incorporated into a portable
device such as a lap top, cell phone, PDA, pager, and the like. An
advantage of the present invention is that the user of the device
which incorporates the integrated circuit may, as needed or
desired, recalibrate the clock tree. This would especially be
useful when the portable device experiences temperature and other
environmental changes since a user may elect to reset the skew
adjustments of the clock tree. This would be especially useful for
out of doors applications.
[0054] Further, it is believed that the present invention and many
of its attendant advantages will be understood by the foregoing
description, and it will be apparent that various changes may be
made in the form, construction, and arrangement of the components
thereof without departing from the scope and spirit of the
invention or without sacrificing all of its material advantages.
The form herein before described being merely an explanatory
embodiment thereof, it is the intention of the following claims to
encompass and include such changes.
* * * * *