U.S. patent application number 10/603722 was filed with the patent office on 2004-12-30 for cascaded phase-locked loops.
Invention is credited to Lim, Chee How, Wong, Keng L., Zhao, Cangsang.
Application Number | 20040263223 10/603722 |
Document ID | / |
Family ID | 33539794 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040263223 |
Kind Code |
A1 |
Wong, Keng L. ; et
al. |
December 30, 2004 |
CASCADED PHASE-LOCKED LOOPS
Abstract
A method and apparatus for generating clock frequencies using
cascaded phase-locked loop (PLL) circuits includes a first PLL
circuit coupled to a second PLL circuit to produce a microprocessor
I/O data clock signal and a microprocessor core clock signal,
respectively. In one embodiment, the first PLL produces the data
clock signal based upon a first reference signal and a first
feedback signal, where the first feedback signal is derived from
the data clock signal. Furthermore, the second PLL circuit produces
the core clock signal based at least in part upon a second
reference signal and a second feedback signal, where the second
reference signal is derived from the data clock signal and the
second feedback signal is derived from the core clock signal.
Inventors: |
Wong, Keng L.; (Portland,
OR) ; Zhao, Cangsang; (Portland, OR) ; Lim,
Chee How; (Hillsboro, OR) |
Correspondence
Address: |
SCHWABE, WILLIAMSON & WYATT
PACWEST CENTER, SUITES 1600-1900
1211 S.W. FIFTH AVE.
PORTLAND
OR
97204
US
|
Family ID: |
33539794 |
Appl. No.: |
10/603722 |
Filed: |
June 24, 2003 |
Current U.S.
Class: |
327/156 |
Current CPC
Class: |
H03L 7/0891 20130101;
H03L 7/23 20130101; G06F 1/04 20130101 |
Class at
Publication: |
327/156 |
International
Class: |
H03L 007/00 |
Claims
1. A circuit comprising: a first phase-locked loop to receive a
first reference signal and a first feedback signal, and to produce
a data clock signal having a first frequency based at least in part
upon the first reference signal and a first feedback signal; and a
second phase-locked loop coupled to the first phase-locked loop by
a first signal path, the second phase-locked loop to receive a
second reference signal on the first signal path and a second
feedback signal on a second signal path-, and to produce a second
clock signal having a second frequency based at least in part upon
the second reference signal and the second feedback signal, the
first feedback signal and the second reference signal being derived
from the data clock signal and the second feedback signal being
derived from the second clock signal wherein the second reference
signal and the second feedback signal have equivalent signal
distribution delays.
2. The circuit of claim 1, further comprising a first divide-by-N
circuit coupled to the first phase-locked loop circuit to divide
the first frequency of the data clock signal by a first integer
value N to produce the first feedback signal having a third
frequency.
3. The circuit of claim 2, further comprising a divide-by-M circuit
coupled between the first phase-locked loop circuit and the second
phase-locked loop circuit to divide the first frequency of the data
clock signal by an integer value M to produce the second reference
signal.
4. The circuit of claim 3, wherein the divide-by-M circuit is
equipped to divide the first frequency of the data clock signal by
a selected one of a factor of 4, 2 and 1.
5. The circuit of claim 4, wherein the first divide-by-N circuit
comprises a divide-by-4 circuit to divide the first frequency of
the data clock signal by a factor of 4.
6. The circuit of claim 3, further comprising a second divide-by-N
circuit coupled to the second phase-locked loop to divide the
second frequency of the second clock signal by a second integer
value N to produce the second feedback signal, wherein the second
integer value N is selected from a group of integer values
including a low integer value, and a high integer value that is
less than two times the low integer value.
7. (canceled)
8. The circuit of claim 1, wherein: the first signal path
corresponding to the second reference signal and the second signal
path corresponding to the second feedback signal, are equivalent in
length.
9. A processor comprising: a process core; input-output circuitry;
a first phase-locked loop coupled to the input-output circuitry by
a first signal trace, the first phase-locked loop producing a first
output signal having a first frequency based at least in part upon
a first reference signal and a first feedback signal derived from
the first output signal, and providing the first output signal as a
data clock signal to the input-output circuitry on the first signal
trace; and a second phase-locked loop coupled to the first
phase-locked loop by a second signal trace and to the processor
core circuitry by a third signal trace, the second phase-locked
loop providing a second output clock signal to the processor core
circuitry on the third signal trace, the second output signal
having a second frequency based at least in part upon a second
reference signal provided by the second signal trace and derived
from the first output signal and a second feedback signal derived
from the second output signal.
10. The processor of claim 9, further comprising a first
divide-by-N circuit coupled to the first phase-locked loop to
divide the first frequency of the data clock signal by a first
integer value N to produce the first feedback signal having a third
frequency.
11. The processor of claim 10, further comprising a divide-by-M
circuit coupled between the first Phase-locked loop and the second
Phase-locked loop to divide the first frequency of the data clock
signal by an integer value M to produce the second reference
signal.
12. The processor of claim 11, wherein the divide-by-M circuit is
equipped to divide the first frequency of the data clock signal by
a selected one of a factor of 4, 2 and 1.
13. The processor of claim 12, wherein the first divide-by-N
circuit comprises a divide-by-4 circuit to divide the first
frequency of the data clock signal by a factor of 4.
14. The processor of claim 11, further comprising a second
divide-by-N circuit coupled to the second phase-locked loop to
divide the second frequency of the core clock signal by a second
integer value N to produce the second feedback signal, wherein the
second integer value N is selected from a group of integer values
including a low integer value and a high integer value that is less
than two times the low integer value.
15. (canceled)
16. The processor of claim 9, further comprising: a fourth signal
trace to provide the second feedback signal, wherein the first and
second and fourth signal traces have equivalent signal distribution
delays.
17-19. (canceled)
20. The processor of claim 16, wherein the second and fourth signal
traces are equivalent in length.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] Embodiments of the present invention relate to the field of
circuit design. More specifically, embodiments of the present
invention relates to a cascaded PLL arrangement.
[0003] 2. Background Information
[0004] Over the years, ongoing advancements in the field of
microprocessor design and fabrication have facilitated the
continued increase in microprocessor clock frequencies. High
frequency microprocessor core clocks are typically synthesized from
existing, lower frequency external reference clocks such as a bus
clock. In the past, it was fairly simple to achieve a desired core
frequency using PLLs as microprocessor core-to-bus frequency ratios
were relatively low. Nowadays however, in order to generate the
multi-Gigahertz microprocessor core frequencies that are typical, a
large PLL synthesis factor is required. Unfortunately, as PLL
synthesis factors increase, the circuit area required to implement
the PLLs also increases.
[0005] In the past, microprocessor core clock signals and
microprocessor I/O clock signals have been generated by two PLLs
disposed in a parallel arrangement with respect to one another.
FIG. 1, for example, illustrates a prior art dual-PLL arrangement
where the PLLs are arranged in a parallel configuration. As shown,
the external reference clock, xclk, is routed to both I/O PLL 104
and Core PLL 114 to generate Data clock 109 and Core clock 119
respectively. In order to generate a Core clock signal having a
frequency that is N times greater than the frequency of xclk, the
damping factor (which is a measure of PLL stability) of Core PLL
114 is reduced by a factor equivalent to the square root of N. One
problem with this arrangement, however, is that a lower damping
factor can also cause jitter amplification in certain
phase-modulation frequencies. Additionally, any phase noise
associated with xclk will be propagated through to Core clock 119
with only one level of filtering. Moreover, since the bandwidth of
Core PLL 114 is very high, the effectiveness of the phase noise
filtering is reduced.
[0006] Another problem associated with the parallel PLL
configuration illustrated in FIG. 1 involves the synchronous
transfers between timing domains. Because the parallel PLLs do not
track each other very closely, a timing margin is often required to
facilitate synchronous data transfer from one timing domain to
another in a deterministic manner. Unfortunately, however, such
timing margins typically shrink as microprocessor core operating
frequencies increase potentially resulting in clock
instability.
BRIEF DESCRIPTION OF DRAWINGS
[0007] The present invention will be described by way of exemplary
embodiments, but not limitations, illustrated in the accompanying
drawings in which like references denote similar elements, and in
which:
[0008] FIG. 1 illustrates a first and second PLLs disposed in a
parallel relationship according to the prior art;
[0009] FIG. 2 illustrates clock generation circuitry utilizing
cascaded PLLs in accordance with one embodiment of the
invention;
[0010] FIG. 3 illustrates an example PLL suitable for cascading
with one or more additional PLLs in accordance with one embodiment
of the invention; and
[0011] FIG. 4 illustrates one embodiment of a microprocessor
including the cascaded PLL clock generation circuitry of FIG.
2.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0012] A method and apparatus for generating clock frequencies
using cascaded phase-locked loops (PLL) is described. In the
following detailed description, numerous specific details are set
forth in order to provide a thorough understanding of the present
invention. However, those skilled in the art will understand that
the present invention may be practiced without these specific
details, that the present invention is not limited to the depicted
embodiments, and that the present invention may be practiced in a
variety of alternative embodiments. In other instances, well known
methods, procedures, components, and circuits have not been
described in detail.
[0013] Various operations will be described as multiple discrete
steps performed in turn in a manner that is helpful for
understanding the present invention. However, the order of
description should not be construed as to imply that these
operations are necessarily performed in the order they are
presented, nor even order dependent.
[0014] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the invention. Thus, the
appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment or invention.
Furthermore, the particular features, structures, or
characteristics may be combined in any suitable manner in one or
more embodiments. Lastly, the terms "comprising", "including",
"having", and the like, as used in the present application, are
intended to be synonymous.
[0015] FIG. 2 illustrates clock generation circuitry utilizing
cascaded PLLs in accordance with one embodiment of the invention.
As shown, clock generation circuitry 200 includes I/O PLL 202
coupled to core PLL 212 in a cascaded manner, as shown. In
particular, I/O PLL 202 receives reference signal 201, and in turn
generates data clock signal 203. Data clock signal 203, or a
divided down version of data clock signal 203, is then used as a
reference clock signal for core PLL 212 which in turn generates
core clock 213. In one embodiment, data clock signal 203 is used
for source synchronous communication between a microprocessor and
its memory subsystem, whereas core clock 213 is utilized by the
microprocessor core.
[0016] In addition to I/O PLL 202 and core PLL 212, clock
generation circuitry 200 further includes feedback frequency
dividers 204, 206 and 214. Each of feedback frequency dividers 204,
206 and 214 represent circuitry to divide the frequency of an input
signal by an integer value to produce an output signal having a
reduced or "divided-down" frequency as compared to an input signal.
More specifically, in the illustrated embodiment, feedback
frequency divider 204 generates feedback signal 205 based upon data
clock signal 203, feedback frequency divider 206 generates
reference signal 209 based upon data clock signal 203, and feedback
frequency divider 214 generates reference signal 215 based upon
core clock signal 213. In one embodiment, one or more of feedback
frequency dividers 204, 206 and 214 are selectively programmable so
as to divide the frequency of an input signal by a selected factor.
In one embodiment, feedback frequency divider 206 may be
selectively programmed to divide the frequency of data clock signal
203 by a factor of 4, 2 or 1. Similarly, in one embodiment,
feedback frequency divider 214 may be programmed to divide the
frequency of core clock signal 213 by an integer value selected
from a group of integer values including a low integer value, and a
high integer value that is less than two times the low integer
value. In one embodiment, feedback frequency divider 214 may be
programmed to divide the frequency of core clock signal 213 by an
integer value that is greater than or equal to 12 and less than or
equal to 23. For example, if feedback frequency divider 206 is set
to divide data clock signal 203 by 1, 2 or 4, the effective
synthesis factor range of clock generation circuitry 200 would be
12-23, 24-46 and 48-92, respectively.
[0017] By selecting a higher multiplied input reference frequency
for the Core PLL, its output frequency can be ratcheted up to reach
a higher synthesis multiplication factor. Because the Core PLL's
ratio range is small, its damping factor fits in a tight design
range, and the filter capacitor required to stabilize the Core PLL
can be minimized. Since the implementation area of a PLL is largely
dependent upon the total capacitance of the PLL, a cascaded PLL
arrangement facilitates leveraging a wide synthesis range while
reducing the overall PLL implementation size. Moreover, the
cascaded arrangement of I/O PLL 202 and core PLL 212 provides an
additional level of filtering of external reference clock phase
noise not available in single PLL implementations.
[0018] FIG. 3 illustrates an example PLL suitable for cascading
with one or more additional PLLs in accordance with one embodiment
of the invention. As shown, PLL 300, includes phase-frequency
detector (PFD) 322 for monitoring a phase difference between a
reference signal 324 and a feedback signal 326 (in the form of a
frequency divided output signal of a voltage-controlled oscillator
(VCO 328). The PFD 322 generates an UP control signal 330 and a
DOWN control signal 332 to cause a charge pump 334 to respectively
charge and discharge a loop filter 336. The loop control voltage
338 developed across the loop filter 336 determines the output
frequency of the VCO 328. Furthermore, the UP and DOWN control
signals 330, 332 that drive the charge pump 334 set the proper loop
filter control voltage 338 at the input of the VCO to maintain a
minimal phase error between the input signals applied to the PFD
322.
[0019] During lock acquisition, the PFD attempts to correct for
frequency differences and/or phase misalignments between the
reference and feedback clocks 324, 326. The correction comes in the
form of the UP/DOWN control signals 330, 332 whose pulse-widths are
proportional to the frequency and/or phase error between the two
input signals. The pulse width of the UP/DOWN control signals 330,
332 informs the charge-pump as to how much current to source or
sink from loop filter capacitors.
[0020] FIG. 4 illustrates one embodiment of a microprocessor
including the cascaded PLL clock generation circuitry of FIG. 2. In
the illustrated embodiment of FIG. 4, microprocessor 475 includes
I/O PLL 202 and core PLL 212 coupled together in a cascaded manner
as shown. In one embodiment, the data clock signal generated from
I/O PLL 202 is used by I/O block 450 of microprocessor 475 to
facilitate data and address latching within a first time domain,
while the core clock signal generated from core PLL 212 is used by
core block 460 of microprocessor 475 to facilitate latching in a
second time domain. However, because the cascaded PLL arrangement
of FIGS. 2 and 4 facilitates tight tracking between the two clock
domains, a reduced timing margin between the domains may be
achieved. In one embodiment, the loop bandwidth of core PLL 212 is
at least three times greater than the loop bandwidth of I/O PLL
202.
[0021] Although it may not be readily apparent from FIG. 4, signal
path 411 which provides reference signal 409 to PLL 212, and signal
path 415 which provides feedback signal 415 to PLL 212 may be
associated with long distributions. In one embodiment, to reduce
the impact of global supply noise that may occur due to the long
signal distributions, signal path 411 and signal path 415 are
equalized in length so as to provide first-order common mode
tracking.
Epilog
[0022] While the present invention has been described in terms of
the above-illustrated embodiments, those skilled in the art will
recognize that the invention is not limited to the embodiments
described. For example, the PLL of FIG. 2 may be used in one or
more chipsets in addition to the microprocessor of FIG. 4. The
present invention can be practiced with modification and alteration
within the spirit and scope of the appended claims. Thus, the
description is to be regarded as illustrative instead of
restrictive on the present invention.
* * * * *