U.S. patent application number 12/685681 was filed with the patent office on 2011-07-14 for clock buffer circuit.
This patent application is currently assigned to FREESCALE SEMICONDUCTOR, INC.. Invention is credited to Chetan VERMA, Nitin VERMA.
Application Number | 20110169528 12/685681 |
Document ID | / |
Family ID | 44258079 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110169528 |
Kind Code |
A1 |
VERMA; Chetan ; et
al. |
July 14, 2011 |
CLOCK BUFFER CIRCUIT
Abstract
A clock buffer for a clock network that reduces leakage current
and lowers power consumption. The clock buffer includes a first
CMOS transistor, a second CMOS transistor, and a leakage current
prevention circuit connected to the first and second CMOS
transistors. The leakage current prevention circuit includes a
first PMOS transistor, which is connected between the source of a
PMOS transistor of the first CMOS transistor and a power supply
line, and a second PMOS transistor, which is connected between the
source of a PMOS transistor of the second CMOS transistor and the
power supply line. The first and second PMOS transistors are
deactivated in response to an enable signal generated when a
circuit block does not require the clock signal. The first and the
second PMOS transistors have predefined widths and lengths such
that the addition of these transistors in series with the CMOS
transistors does not increase the propagation delay of the clock
buffer circuit. Deactivating the first and second PMOS transistors
disconnects the CMOS transistors from the power supply line, which
prevents current leakage.
Inventors: |
VERMA; Chetan; (Noida,
IN) ; VERMA; Nitin; (Noida, IN) |
Assignee: |
FREESCALE SEMICONDUCTOR,
INC.
Austin
TX
|
Family ID: |
44258079 |
Appl. No.: |
12/685681 |
Filed: |
January 12, 2010 |
Current U.S.
Class: |
326/95 |
Current CPC
Class: |
G06F 1/10 20130101; H03K
5/1506 20130101 |
Class at
Publication: |
326/95 |
International
Class: |
H03K 19/096 20060101
H03K019/096 |
Claims
1-10. (canceled)
11. A clock buffer circuit for use in a clock network, the clock
buffer circuit consisting essentially of: a clock driver circuit
that receives an input signal and outputs a buffered signal, the
clock driver circuit consisting of: a first CMOS transistor,
connected between a power supply line and a ground line, that
receives the input signal; and a second CMOS transistor, connected
between the power supply line and the ground line, and to the first
CMOS transistor, wherein the second CMOS transistor receives the
input signal from the first CMOS transistor and generates the
buffered signal; a leakage current prevention circuit for
disconnecting the first and second CMOS transistors from the power
supply line, the leakage current prevention circuit consisting of:
a first PMOS transistor connected between a source of a PMOS
transistor in the first CMOS transistor and the power supply line,
wherein a gate of the first PMOS transistor receives an enable
signal; and a second PMOS transistor connected between a source of
a PMOS transistor in the second CMOS transistor and the power
supply line, wherein a gate of the second PMOS transistor receives
the enable signal; and wherein, the widths and lengths of the first
and the second PMOS transistors are predefined such that the
addition of these transistors in series to the first and second
CMOS transistors does not increase the activation time of the first
and second CMOS transistors, and wherein the first and second PMOS
transistors are deactivated in response to the enable signal
thereby disconnecting the first and second CMOS transistors from
the power supply line; and an output stabilization holding
transistor, connected between an output terminal of the second CMOS
transistor and the ground line, and receiving the enable signal,
wherein when the output stabilization holding transistor receives
the enable signal, the output of the second CMOS transistor is held
at ground level.
12. The clock buffer circuit of claim 11, wherein disconnecting the
first and second CMOS transistors from the power supply line when
the enable signal is active prevents the first and second CMOS
transistors from leaking current, thereby saving power.
13. (canceled)
14. The clock buffer circuit of claim 11, wherein the output
stabilization holding transistor comprises an NMOS transistor
having a drain connected to the output terminal of the second CMOS
transistor, a source connected to the ground line GND, and a gate
that receives the enable signal EN.
15. The clock buffer circuit of claim 14, wherein the NMOS
transistor of the output stabilization holding transistor has a
small driving capacity.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a clock network of an
integrated circuit, and more particularly, to a clock buffer
circuit for a clock network.
[0002] An integrated circuit (IC) includes a plurality of internal
circuits that receive a clock signal and operate in synchronism
with the clock signal. Each internal circuit receives the clock
signal via one or more clock buffers. However, there is a limit to
the number of internal circuits that can be operated by a clock
signal originating from a single clock buffer. Additionally, to
meet the timing of the circuit, it is essential to keep the clock
latency and skew (between different internal circuits receiving the
clock inputs), within prescribed limits. Thus, to provide the clock
signal to a plurality of internal circuits, the IC includes a
plurality of clock networks, each including a plurality of clock
buffers.
[0003] The recent tendency for the use of more internal circuits
has increased the number of clock buffers in a clock network.
However, as is well known, clock buffers slow down the clock signal
and contribute to the problem of clock skew. Thus, there is a
demand for faster clock buffers. To increase the speed of a clock
buffer circuit, faster transistors can be used to form the circuit,
such as low voltage threshold (LVT) transistors. However, LVT
transistors increase power consumption and their use is thus in
conflict with today's demands for low power circuits.
[0004] Thus, it would be desirable to have a low power but fast
clock buffer circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The present invention is illustrated by way of example and
is not limited by the accompanying figures, in which like
references indicate similar elements. Elements in the figures are
illustrated for simplicity and clarity and have not necessarily
been drawn to scale.
[0006] FIG. 1 is a schematic circuit diagram of a clock buffer
circuit according to an embodiment of the present invention;
and
[0007] FIG. 2 is a schematic circuit diagram showing a clock
network incorporating the clock buffer circuits of FIG. 1 in a
semiconductor integrated circuit device.
DETAILED DESCRIPTION OF THE INVENTION
[0008] The terms "assert" or "set" and "negate" (or "de-assert" or
"clear") are used herein when referring to the rendering of a
signal into its logically true or logically false state,
respectively. If the logically true state is a logic level one, the
logically false state is a logic level zero. And if the logically
true state is a logic level zero, the logically false state is a
logic level one.
[0009] Each signal described herein may be designed as positive or
negative logic, where negative logic can be indicated by a bar over
the signal name or a forward slash (/) preceding the name. In the
case of a negative logic signal, the signal is active low where the
logically true state corresponds to a logic level zero. In the case
of a positive logic signal, the signal is active high where the
logically true state corresponds to a logic level one. Note that
any of the signals described herein can be designed as either
negative or positive logic signals. Therefore, in alternate
embodiments, those signals described as positive logic signals may
be implemented as negative logic signals, and those signals
described as negative logic signals may be implemented as positive
logic signals.
[0010] The present invention provides a low power and low leakage
clock buffer circuit for use in a clock network of a semiconductor
integrated circuit. In one embodiment, the present invention is a
clock buffer circuit including a first CMOS transistor connected
between a power supply line and a ground line. A second CMOS
transistor is connected to the first CMOS transistor, and also
connected between the power supply line and the ground line. A
first PMOS transistor is connected between a source of a PMOS
transistor in the first CMOS transistor and the power supply line.
A gate of the first PMOS transistor receives an enable signal. A
second PMOS transistor is connected between a source of a PMOS
transistor in the second CMOS transistor and the power supply line,
and a gate of the second PMOS transistor receives the enable
signal. The first and second PMOS transistors are deactivated in
response to the enable signal, which disconnects the first and
second CMOS transistors from the power supply line.
[0011] In another embodiment, in an integrated circuit having a
plurality of internal circuit blocks, the present invention
provides a clock network for providing a clock signal to the
plurality of internal circuit blocks. The clock network includes a
plurality of clock buffer circuits that sequentially receive and
output a clock signal to the plurality of internal circuits. Each
of the plurality of clock buffer circuits includes a first CMOS
transistor connected between a power supply line and a ground line,
and a second CMOS transistor connected between the power supply
line and the ground line, and also connected to the first CMOS
transistor. A first switch is connected between a source of a PMOS
transistor of the first CMOS transistor and the power supply line.
A second switch is connected between a source of a PMOS transistor
of the second CMOS transistor and the power supply line. The first
and second switches are deactivated in response to an enable signal
that is generated when the circuit block to which the clock buffer
circuit is connected does not require the clock signal.
Deactivating the first and second switches disconnects the first
and second CMOS transistors from the power supply line.
[0012] In yet another embodiment, the present invention provides a
clock buffer circuit for use in a clock network. The clock buffer
circuit includes a clock driver circuit and a leakage current
prevention circuit. The clock driver circuits includes a first CMOS
transistor connected between a power supply line and a ground line,
and a second CMOS transistor connected to the first CMOS transistor
and also connected between the power supply line and the ground
line. The leakage current prevention circuit disconnects the first
and second CMOS transistors from the power supply line. In one
embodiment, the leakage current prevention circuit includes a first
PMOS transistor connected between a source of a PMOS transistor in
the first CMOS transistor and the power supply line. A gate of the
first PMOS transistor receives an enable signal. The leakage
current prevention circuit also includes a second PMOS transistor
connected between a source of a PMOS transistor in the second CMOS
transistor and the power supply line. A gate of the second PMOS
transistor receives the enable signal. The first and second PMOS
transistors are deactivated in response to the enable signal, which
disconnects the first and second CMOS transistors from the power
supply line, which prevents the first and second CMOS transistors
from leaking.
[0013] 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. A preferred embodiment of
the present invention will now be discussed with reference to FIG.
1.
[0014] FIG. 1 is a schematic circuit diagram of a clock buffer
circuit 10 in accordance with an embodiment of the present
invention. The clock buffer circuit 10 includes a clock driver
circuit 12 and a leakage current prevention circuit 14. As shown in
FIG. 1, each of the clock driver circuits includes a first CMOS
transistor 16 and a second CMOS transistor 18. The first CMOS
transistor 16 and the second CMOS transistor 18 are connected
between a power supply line (VDD) and a ground line (GND), and in
series with each other.
[0015] The first CMOS transistor 16 includes a PMOS transistor 20
connected in series with an NMOS transistor 22, as is well known to
those skilled in the art. The PMOS transistor 20 has a source
connected to a power supply line (VDD), a drain connected to the
drain of the NMOS transistor 22, and a gate connected to an input
terminal of the clock buffer circuit 10, which is an input clock
line CLK. The source of the NMOS transistor 22 is connected to a
ground line (GND) and the gate of the NMOS transistor 22 is
connected the input clock line CLK.
[0016] The second CMOS transistor 18 includes a PMOS transistor 24
connected in series with an NMOS transistor 26. The PMOS transistor
24 has a source connected to the power supply line (VDD), a drain
connected to the drain of the NMOS transistor 26, and a gate
connected to a node N1 between the drains of the PMOS transistor 20
and the NMOS transistor 22 of the first CMOS transistor 16. The
source of the NMOS transistor 26 is connected to the ground line
(GND) and the gate of the NMOS transistor 26 is connected the node
N1. Thus, as shown, the gates of the PMOS transistors 20 and 24 and
the gates of the NMOS transistors 22 and 26 are all connected
together. The drains of the PMOS transistor 24 and the NMOS
transistor 26 are connected to each other at a node N2, which is an
output of the clock buffer circuit 12.
[0017] The leakage current prevention circuit 14 is provided for
disconnecting the first and second CMOS transistors from the power
supply line. In one embodiment, the leakage current prevention
circuit includes a first switch or power supply blocking transistor
28 and a second switch or second power supply blocking transistor
30.
[0018] The first power supply blocking transistor 28 is formed from
a PMOS transistor and is connected between the first CMOS
transistor 16 (i.e., the source of the PMOS transistor 20) and the
power supply line VDD. The second power supply blocking transistor
30 also is formed from a PMOS transistor and is connected between
the second CMOS transistor 18 (i.e., the source of the PMOS
transistor 24) and the power supply line VDD. The first and second
power supply blocking transistors 28 and 30 have pre-defined widths
and lengths that are chosen such that the propagation delay of the
clock circuit 12 is not affected by the inclusion of the leakage
current prevention circuit 14.
[0019] The gates of the first and second power supply blocking
transistors 28 and 30 each receive an enable signal EN. The enable
signal EN can be provided from another circuit block of the
integrated circuit. In one embodiment, the enable signal is a low
active signal. Thus, when the gates of the first and second power
supply blocking transistors 28 and 30 receive a low enable signal
EN, the two transistors 28 and 30 are activated, which closes the
path between the power supply line VDD and the CMOS transistors 16
and 18, so the first and second CMOS transistors 16 and 18 receive
power supply voltage VDD.
[0020] On the other hand, when the gates of the first and second
power supply blocking transistors 28 and 30 receive a high enable
signal EN, the two transistors 28 and 30 are deactivated, which
blocks the supply of power supply voltage VDD to the first and
second CMOS transistors 16 and 18. When the supply voltage VDD is
not applied to the first and second CMOS transistors 16 and 18,
there is no leakage current produced from the first and second CMOS
transistors 16 and 18. That is, there is no potential difference
between the source and gate of the activated one of the MOS
transistors. Further, there is no potential difference between the
source and drain of the deactivated one of the MOS transistors.
Since there is no section that has a potential difference, leakage
current is not produced.
[0021] Leakage current is produced in the following manner. With
CMOS transistors, in the activated one of the MOS transistors, a
channel is formed and a source-gate potential difference is
produced. In the deactivated one of the MOS transistors, a channel
is inactive and a source-drain potential difference is produced. In
the sections in which the two potential differences are produced,
the potential difference results in the flow of leakage current. In
other words, gate leakage current flows in a section in which a
potential difference is produced between the source and gate, and
off current flows in a section in which a potential difference is
produced between the source and drain.
[0022] The clock buffer circuit 10 may be formed using high
threshold voltage (HVT) cells. As is known by those skilled in the
art, the use of HVT cells provides for less power loss, although
with slower switching speed than when a circuit is formed using low
threshold voltage (LVT) or standard threshold voltage (SVT) cells.
One trade-off being that LVT and SVT cells consume more power. When
forming the clock buffer circuit 10 from cells having a threshold
voltage set at a low potential (i.e., SVT or LVT cells), under a
high temperature (105.degree. C.), the leakage current would be
nearly twenty times greater than that when the clock buffer circuit
10 is formed from cells having a threshold voltage set at a high
potential such as a high voltage threshold (HVT) cell.
Additionally, if the clock buffer circuit 10 were formed from LVT
cells having a narrow line width and a threshold voltage set at a
low potential, the leakage current would further increase. Thus, in
the presently preferred embodiment, the clock buffer circuit is
formed using HVT cells. Even so, the clock buffer circuit 10 has a
high operation speed, one which is nearly the same as the operation
speed when forming a clock buffer from LVT cells. In addition, as
discussed above, the clock buffer circuit 10 reduces leakage
current considerably.
[0023] In one embodiment of the invention, the leakage current
prevention circuit 14 includes an output stabilization holding
transistor 32, which is formed from an NMOS transistor. The output
stabilization holding transistor 32 has a drain connected to the
output terminal of the clock driver circuit 12 at the node N2, a
source connected to the ground line GND, and a gate that receives
the enable signal EN in the same manner as the first and second
power supply blocking transistors 28 and 30.
[0024] Preferably, the output stabilization holding transistor 32
has a small size and a small driving capacity. When the gate of the
output stabilization holding transistor 32 receives a high enable
signal EN, the transistor 32 is activated so that the node N2 is
held at ground level GND. That is, when blocking the supply of
voltage VDD to the first and second CMOS transistors 16 and 18, the
output level of the clock buffer circuit 10 is held at the ground
GND level so as not to shift to a floating state. This prevents the
output level of the clock buffer circuit 10 from suddenly changing
when the supply voltage VDD is blocked and thereby prevents
erroneous operation of the circuits connected to the output of
clock buffer circuit 10 and receive their input signal from clock
circuit 10.
[0025] When the gate of the output stabilization holding transistor
32 receives a low enable signal EN, the transistor 32 is
deactivated. Thus, when the first and second CMOS transistors 16
and 18 are provided with the supply voltage VDD, the clock signal
CLK is output from the clock buffer circuit at the output terminal
(node N2).
[0026] Referring now to FIG. 2, a schematic block diagram of a
semiconductor integrated circuit (IC) 40 having a low leakage clock
network 42 and multiple circuit blocks 44 (only one is shown here)
is shown. A clock signal CLK is received by the clock network 42,
buffered, and then output to the circuit blocks 44. The circuit
blocks 44 can be predefined circuit blocks, often referred to by
designers as IPs, which means the blocks or IPs can be taken from
one IC and used in another IC. That is, the circuit blocks 44 can
be stand-alone circuits that can be re-used by other designers for
other designs, such as system-on-chip (SOC) ICs.
[0027] In one embodiment of the invention, the clock network
comprises a plurality of series connected clock circuit buffers 46.
Individual buffers 46a-46e are shown, however, as will be
understood by those skilled in the art, the clock network 42 could
contain more or fewer clock circuit buffers 46. The clock circuit
buffers 46 preferably have a circuit structure like the buffer
circuit 10 shown in FIG. 1 and described above. The circuit blocks
44 include a plurality of internal circuit elements, such as
latches 48 that receive the buffered clock signal CLK from the
clock network 42.
[0028] The clock network 42 also includes a low power clock gating
circuit (LPCG) 50. The LPCG 50 receives the clock signal CLK from
an external circuit such as a clock generator (not shown) and
receives an enable signal EN from a clock or power control module
(not shown). The LPCG 50 enables the clock signal CLK to be passed
to the clock buffer circuits 46. The LPCG 50 may be formed from
logic gates, such as an AND gate, as will be understood by those of
skill in the art. The use of an enable signal allows the clock
signal to be turned off and not provided to the circuit blocks 44.
In such a scenario, when the circuit block 44 does not receive an
active clock signal, the circuit block 44 is effectively turned off
and thus it does not consume active power.
[0029] In one embodiment of the invention, if the circuit block 44
is put in operation and clock signal CLK is therefore necessary to
operate the internal circuit elements 48 (operation state), a low
enable signal EN is input to the LPCG 50. On the other hand, if the
clock signal CLK is not required by the circuit block 44 (e.g.,
when the internal circuit elements 48 are not being used (standby
state)), a high enable signal EN is input to the LPCG 50.
[0030] In addition, since the buffer circuits 46a-46e have the
circuit structure of the buffer circuit 10 (FIG. 1) discussed
above, when the high enable signal EN is provided to the leakage
current prevention circuit 14, the first and second PMOS
transistors 28 and 30 are turned off so the CMOS transistors 16 and
18 do not receive the voltage signal VDD and thus the CMOS
transistors 16 and 18 do not leak, which saves on the leakage
component of power.
[0031] 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.
* * * * *