Electronic digital logic circuitry – Accelerating switching
Reexamination Certificate
2000-05-25
2003-02-18
Gossage, Glenn (Department: 2187)
Electronic digital logic circuitry
Accelerating switching
C327S374000, C327S404000
Reexamination Certificate
active
06522163
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the transfer of signals in logic circuits and, more specifically, to a balanced transfer gate circuit having a switching speed that is independent of the order in which individual series connected transfer gates are activated.
BACKGROUND OF THE INVENTION
In the design of logic circuits, particularly in the area of semiconductor memories, pass or transfer gates are utilized in a variety of applications to selectively transfer signals from one portion of a circuit to another. A typical transfer gate includes a p-channel metal oxide semiconductor (PMOS) transistor and an n-channel metal oxide semiconductor (NMOS) transistor having their drains coupled together to form an input, their sources coupled together to form an output, and the gates of the respective transistors receiving complementary control clock signals. The transfer gate couples its input to its output in response to the control clock signals, and thus operates as an electronic switch to transfer a signal placed on its input to its output. By utilizing both PMOS and NMOS transistors, the voltage level of signals that can be transferred by the transfer gate is not limited by the threshold voltages of the transistors.
The switching time of a transfer gate is the time it takes to transfer a signal from the input to the output. The switching time is a function of a number of factors including the gate-to-source and drain-to-source junction capacitances, and the channel resistance of the [MOS] metal oxide semiconductor (MOS) transistors. In addition, the switching time is a function of the load presented by circuitry coupled to the output of the transfer gate. When a number of transfer gates are connected in series, the output of an individual transfer gate may be loaded by other transfer gates in the series connected circuit. The load presented by such other transfer gates varies as the other transfer gates are selectively activated and deactivated. In addition, the load presented on the output of an individual transfer gate is a function of the order in which the other transfer gates are activated. For example, if four transfer gates are connected in series, the load presented on the output of the second transfer gate depends on whether transfer gates three and four are activated or deactivated, and the order in which they are activated.
FIG. 1
is a schematic of a conventional switching circuit
10
including two series-connected transfer gates
12
and
14
. The transfer gate
12
receives a pair of complementary clock signals CLK
0
and {overscore (CLK
0
)}, and couples its input IN to its output when the signals CLK
0
and {overscore (CLK
0
)} are high and low, respectively. The transfer gate
14
receives a pair of complementary clock signals CLK
1
and {overscore (CLK
1
)}, and couples its input to its output OUT when the signals CLK
1
and {overscore (CLK
1
)} are high and low, respectively. When both the clock signals CLK
0
and CLK
1
are high, the transfer gates
12
and
14
are activated, coupling the input IN to the output OUT. If either of the signals CLK
0
and CLK
1
is low, one of the transfer gates
12
or
14
is turned OFF isolating the input IN from the output OUT.
During operation of the switching circuit
10
, external circuitry (not shown in
FIG. 1
) develops the clock signals CLK
0
and CLK
1
to control activation of the transfer gates
12
and
14
. The external circuitry may at times activate the transfer gate
12
before the transfer gate
14
, and at other times the reverse will be true. The switching speed of the switching circuit
10
is the time it takes for a signal on the input IN to be coupled to the output OUT. Ideally, the switching speed is independent of the order in which the transfer gates
12
and
14
are activated. In the circuit
10
, however, the switching speed depends upon which transfer gate
12
or
14
is activated first. This is true because when the transfer gate
14
is activated first, the transfer gate
12
drives the load on the output OUT through the activated transfer gate
14
, and when the transfer gates
12
and
14
are activated in the reverse order, the transfer gate
14
directly drives the load on the output OUT. For example, assume both transfer gates
12
and
14
are initially deactivated so the input IN is isolated from the output OUT. Further assume the signal on the input IN is high. When the transfer gate
12
is activated, the high input signal is coupled to the output of the transfer gate
12
. When the transfer gate
14
is thereafter activated it must drive the load presented on the output OUT in order to drive the output OUT high. In contrast, when the transfer gate
14
is activated first, the transfer gate
12
can drive the load presented on the output OUT through the channel resistance impedance of the transfer gate
14
. Thus, when the load on the output OUT is capacitive, for example, the additional impedance of the transfer gate
14
and corresponding increased RC time constant result in the output OUT going high more slowly.
There is a need for a switching circuit including a number of series-connected transfer gates in which the switching speed of the switching circuit is independent of the sequence in which the transfer gates are activated.
SUMMARY OF THE INVENTION
A balanced switching circuit comprises a plurality of switch circuits, each switch circuit having an input terminal, an output terminal, and at least one control terminal adapted to receive a control signal. Each switch circuit is operable to couple the input terminal to the output terminal in response to the control signal. The plurality of switch circuits are arranged in N rows and N columns with the input and output terminals of the N switch circuits in each row connected in series between a first signal terminal and a second signal terminal. Each switch circuit in a given column has its control terminal connected to one of N clock terminals adapted to receive respective clock signals. Further, each clock terminal is coupled to the control terminal of only one switch circuit in each row and one switch circuit in each column. The balanced switching circuit is operable to couple the first signal terminal to the second signal terminal in response to the clock signals.
REFERENCES:
patent: 4622648 (1986-11-01), Whitaker
patent: 4912348 (1990-03-01), Maki et al.
patent: 4985647 (1991-01-01), Kawada
patent: 5159616 (1992-10-01), Douglas et al.
patent: 5239206 (1993-08-01), Yanai
patent: 5414745 (1995-05-01), Lowe
patent: 5534815 (1996-07-01), Badyal
patent: 5646898 (1997-07-01), Manning
patent: 5808483 (1998-09-01), Sako
patent: 5831453 (1998-11-01), Stamoulis et al.
patent: 5844424 (1998-12-01), Krishnamurthy et al.
Dorsey & Whitney LLP
Gossage Glenn
Micro)n Technology, Inc.
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