Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Output switching noise reduction
Reexamination Certificate
1999-08-16
2001-06-05
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Output switching noise reduction
C326S083000, C326S087000, C326S058000
Reexamination Certificate
active
06242942
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to integrated circuit devices, and more particularly to integrated circuit output buffers.
BACKGROUND OF THE INVENTION
Integrated circuits typically include buffer circuits therein for driving on-chip and off-chip loads. Dynamic output control (DOC) can also be provided by output buffers, such as those disclosed in application notes by Texas Instruments, Inc. (see, http://www.ti.com/sc/AVC). In particular, these output buffers having DOC circuitry may provide variable output impedance to reduce signal noise during output transitions. In these buffers, the DOC circuitry is stated as providing enough current to achieve high signaling speeds, while also having the ability to quickly switch the impedance level to reduce the undershoot and overshoot noise that is often found in high-speed logic. Such DOC circuitry may be used advantageously to eliminate the need for damping resistors which can limit noise but typically also increase propagation delay.
Additional buffer circuits are also disclosed in U.S. Pat. No. 5,894,238 to Chien, entitled “Output Buffer With Static and Transient Pull-Up and Pull-Down Drivers”. In particular, the '238 patent discloses an inverting buffer circuit that utilizes a signal fed back from an output node (DOUT) through logic gates to control the transfer of data input signals (OL or /OH) to inputs of a transient driver circuit containing an NMOS pull-down transistor
22
and a PMOS pull-up transistor
12
.
Notwithstanding such conventional output buffers with DOC circuitry, however, there still exists a need for output buffers that have excellent noise, propagation delay and impedance matching characteristics.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide improved integrated circuit output buffers.
It is another object of the present invention to provide integrated circuit output buffers having low propagation delay.
It is still another object of the present invention to provide integrated circuit output buffers having improved simultaneous-switching noise characteristics.
It is yet another object of the present invention to provide integrated circuit output buffers having reduced supply line-to-output coupling and improved impedance matching characteristics during DC operation.
These and other objects, advantages and features of the present invention may be provided by integrated circuit output buffers having pull-down and pull-up circuits and a control circuit that utilizes a preferred feedback technique that may facilitate a reduction in simultaneous-switching noise during pull-down and pull-up operations and may also improve the impedance matching characteristics of the output buffers during DC conditions. The preferred feedback technique may also limit the degree to which external noise can influence operation of the control circuit. Each of the pull-down and pull-up circuits may be provided by a respective pair of primary and secondary transistors.
In particular, the pull-down circuit is preferably configured so that the primary and secondary pull-down transistors (e.g., NMOS transistors) are electrically coupled to an output signal line (through an ESD protection resistor) and to a first reference signal line (e.g., Vss). The control circuit is designed to activate the pull-down circuit by turning on both the primary and secondary pull-down transistors during a leading portion of the pull-down time interval and by turning off the secondary pull-down transistor during a trailing portion of the pull-down time interval using a first feedback switch that is electrically coupled in series between the output signal line and a gate electrode of the secondary pull-down transistor so that a signal representing a potential of the output signal line can be passed through the first feedback switch to the gate electrode of the secondary pull-down transistor. In contrast, the pull-up circuit is preferably configured so that the primary and secondary pull-up transistors (e.g., PMOS transistors) are electrically coupled to an output signal line and to a second reference signal line (e.g., Vdd). Here, the control circuit is designed to activate the pull-up circuit by turning on both the primary and secondary pull-up transistors during a leading portion of the pull-up time interval and by turning off the secondary pull-up transistor during a trailing portion of the pull-up time interval using a second feedback switch that is electrically coupled in series between the output signal line and a gate electrode of the secondary pull-up transistor.
The control circuit also includes circuitry therein that is responsive to a data input signal (DI) and an output enable signal (OE). The control circuit controls tri-state operation of the output buffer, turns on the first feedback switch when the boolean expression OE
{overscore (DI)}=1 (where “
” represents a boolean AND operation), and turns on the second feedback switch when the boolean expression OE
DI=1. The control circuit may also comprise an NMOS pull-down transistor electrically connected in series (source-to-drain) between the gate electrode of the secondary pull-down transistor and the first reference signal line and a PMOS pull-up transistor electrically connected in series between the gate electrode of the secondary pull-up transistor and the second reference signal line. To operate these pull-down and pull-up transistors, the control circuit includes circuitry therein that turns on the NMOS pull-down transistor when the boolean expression OE
{overscore (DI)}=0 and turns on the PMOS pull-up transistor when the boolean expression OE
DI=0.
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Integrated Device Technology Inc.
Le Don Phu
Myers Bigel & Sibley & Sajovec
Tokar Michael
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