Transient Immune Input Buffer

The present invention relates to integrated circuits, and more particularly to input buffers for very large scale integrated (VLSI) circuits.

The trend in the design of integrated circuit chips is for ever smaller geometry sizes and increasingly faster switching times. To further improve system speed, bus widths have been expanded with additional parallel input and output lines. Each output pin on a chip has an output driver which provides a relatively large current in response to a weaker signal generated by the chip. As the load on the bus increases, higher current is provided by the output drivers.

Whenever a logic gate on a chip is switched, as for example a logic gate in an output driver, a transient change in the current is created as the load is charged or discharged. Such transient currents create voltage transients in the power or ground path of the chip. The magnitude of voltage transients increases with increased bus width as additional drivers switch at the same time. Similarly, voltage transients increase as a result of the higher current required to drive larger loads.

Logic gates on the same chip, which share the power or ground paths of the output drivers, are known to switch logic states in response to such power or ground path voltage transients. This creates problems for both output and input drivers. For example, a ground transient or "bounce" can cause a high input to be erroneously recognized as a low level by an input buffer of a chip. An input buffer includes an inverter with series connected pull-up and pull-down transistors. The pull-down transistor has its gate connected to the high input and its source connected to ground. When the ground plane is steady, the gate-to-source voltage is (V.sub.GS) is large enough to keep the pull-down transistor "on" which keeps the output low. However, the inverter shares a common ground plane with the output drivers on the same chip. If the ground plane bounces as a result of one or more of the outputs switching, V.sub.GS decreases, turning off the pull-down transistor, and erroneously switching the output of the inverter. A similar result occurs for the pull-up transistor as a result of a voltage transient on the power supply.

In the past, attempts to prevent the problems associated with power and ground bounce have concentrated on limiting the magnitude of the voltage transient. A typical method of reducing the voltage transient is to slow the rate at which nodes switch. A disadvantage of this approach is that it increases somewhat the propagation delays through an input/output buffer and pad driver. Furthermore, as chips become faster, input buffers become increasingly sensitive to smaller voltage transients. Further reduction of the magnitude of the voltage transient would considerably increase propagation delays through the input/output buffer and pad driver.

It is therefore an object of the present invention to provide a new and improved integrated circuit.

It is another object of the present invention to provide a new and improved input buffer for an integrated circuit.

It is a further object of the present invention to provide an input buffer for an integrated circuit which is immune to transients on the power and ground planes.

It is yet another object of the present invention to provide a transient immune input buffer with reduced propagations delays.

The present invention is a bistable integrated circuit such as an input buffer with reduced sensitivity to transients on a reference voltage terminal to the circuit. The circuit comprises a filter connected between an input and the reference voltage terminal for reducing the sensitivity of the circuit to a voltage transient on the terminal. To prevent transients on both the power and ground voltage terminal from affecting the circuit, filters may be provided on both inputs.

FIG. 1 is a prior art input buffer.

FIG. 2 is an input buffer with inverter according to one form of the present invention.

FIG. 3 is an integrated circuit layout of the inverter shown in FIG. 2.

FIG. 4 is a Schmitt trigger according to another form of the present invention.

FIG. 1 shows a prior art input buffer 10 with a filter 12 for reducing the sensitivity of buffer 10 to transients on the input line IN. Buffer 10 includes two CMOS inverters, each having complementary PMOS and NMOS field effect transistors P and N, respectively. Filter 12 includes a resistor R connected between the input and the control electrodes of transistors P and N (node 14) of the first inverter, and a capacitor C connected in series with resistor R and reference voltage terminal V.sub.SS. Filter 12 is a low pass filter for transients appearing on the input. Thus, high frequency voltage transients on the input do not appear on node 14. The typical layout of input filter 12 as an integrated circuit does not provide reduced sensitivity to transients on the ground plane V.sub.SS. Irrespective of the layout of filter 12 vis-a-vis the first inverter, buffer 10 is sensitive to transients on reference voltage terminals V.sub.DD.

FIG. 2 is an integrated circuit input buffer 16 having reduced sensitivity to voltage transients on reference voltage terminals V.sub.DD and V.sub.SS. Buffer 16 includes two CMOS inverters, a first inverter having complementary PMOS and NMOS field effect transistors P1 and N1, respectively, series connected between reference voltage terminals V.sub.DD and V.sub.SS, and a second inverter having complementary PMOS and NMOS field effect transistors P2 and N2, respectively, similarly series connected between reference voltage terminals V.sub.DD and V.sub.SS. The input of the second inverter is connected to the output terminal 20 of the first inverter.

A filter 26 which includes a capacitor C1 and resistor R1 is connected between input 18 and reference voltage terminal V.sub.DD. A filter 28 which includes a capacitor C2 and resistor R2 is connected between input 18 and reference voltage terminal V.sub.SS. As will be described more fully, filters 26 and 28 significantly reduce the sensitivity of circuit 16 to voltage transients on voltage terminals V.sub.DD and V.sub.SS, respectively, as well as reducing its sensitivity to transients on input terminal 18. Capacitor C1 is connected between reference voltage terminal V.sub.DD and node 22, the control electrode of transistor P1. Capacitor C2 is connected between reference voltage terminal V.sub.SS and node 24, the control electrode of transistor N1. Input terminal 18 is connected to nodes 22 and 24 through resistors R1 and R2, respectively. Output terminal 20 of the first inverter is connected to the common point between transistors P1 and N1 and is further connected to the input of the second inverter (transistors P2 and N2).

The sizing and layout of resistors R1 and R2, capacitors C1 and C2, and transistors P1 and P2 are important features of the present invention. Resistor R1 and capacitor C1 are sized to form a high pass filter between reference voltage terminal V.sub.DD and node 22, and to form a low pass filter between input terminal 18 and node 22. Similarly, resistor R2 and capacitor C2 are sized to form a high pass filter between reference voltage terminal V.sub.SS and node 24, and to form a low pass filter between input terminal 18 and node 24. In a preferred embodiment, for V.sub.DD =5 volts, and V.sub.SS =0 volts, transistor P1 has a length to width ratio of 5.5, transistor N1 has a length to width ratio of 20.0, resistor R1 is 1000 ohms, capacitor C1 is 1 pf, resistor R2 is 1000 ohms, and capacitor C2 is 3 pf.

The layout of resistors R1 and R2, capacitors C1 and C2, and transistors P1 and P2 is shown in FIG. 3. The gate or control electrode 22 of transistor P1 is tightly coupled to capacitor C1 and resistor R1 to minimize parasitic capacitance and resistance on electrode 22. Similarly, the gate or control electrode 24 of transistor N1 is tightly coupled to capacitor C2 and resistor R2 to minimize parasitic capacitance and resistance and resistance on electrode 24. In a preferred embodiment, the integrated circuit is laid out on a P-type substrate. PMOS transistor P1 is formed in a P+ region within an N- well, and NMOS transistor N1 is formed in an N+ region. Capacitor C1 is formed by the self-alignment of polysilicon layer 30 and the adjacent P+ regions, and capacitor C2 is formed by the self-alignment of polysilicon layer 32 and the adjacent N+ regions. It will be appreciated that capacitor C1 is formed by using the gate capacitance of a PMOS transistor with its source and drain connected to V.sub.DD, and capacitor C2 is formed by using the gate capacitance of an NMOS transistor with its source and drain connected to V.sub.SS. Parasitic capacitance on gate electrode 22 is significantly reduced by connecting electrode 22 to capacitor C1 by metalization runs 34. Parasitic capacitance on gate electrode 24 is significantly reduced by connecting electrode 24 to capacitor C2 by metalization runs 36. Resistor R1 is formed by a serpentine polysilicon run between input node 18 and electrode 22, and resistor R2 is formed by a serpentine polysilicon run between input node 18 and electrode 24. Parasitic resistance on gate electrode 22 is significantly reduced by the short metal runs 34, and parasitic resistance on gate electrode 24 is significantly reduced by the short metal runs 36.

FIG. 4 shows an alternative embodiment of the present invention in which the input buffer is a Schmitt trigger 38. The first stage of Schmitt trigger 38 includes two level sensitive elements: a PMOS transistor P3 and parallel connected transistors N1 and N2. Transistor P3 and parallel connected transistors N1 and N2 are series connected between reference voltage terminals V.sub.DD and V.sub.SS. As with input buffer 10 shown in FIG. 2, Schmitt trigger 38 includes a capacitor C1 and resistor R1 connected between reference voltage terminal V.sub.DD and input 40, and a capacitor C2 and resistor R2 connected between reference voltage terminal V.sub.SS and input 40. The common node 42 between capacitor C1 and resistor R1 is connected to the control electrode of transistor P3, and the common node 44 between capacitor C2 and resistor R2 is connected to one of the control electrodes of transistors N1 and N2. The layout of the input filters formed by resistor R1 and capacitor C1 and resistor R2 and capacitor C2 is similar to that shown in FIG. 3.

It will be clear to those skilled in the art that the present invention is not limited to the specific embodiment disclosed and illustrated herein. Numerous modifications, variations, and full and partial equivalents can be undertaken without departing from the invention as limited only by the spirit and scope of the appended claims.

What is desired to be secured by Letters Patent of the United States is as follows.