Voltage tolerant oscillator input cell

Details Voltage tolerant oscillator input cell Voltage tolerant oscillator input cell

1999-11-24

2001-01-30

Mis, David (Department: 2817)

Oscillators

With oscillator circuit protective means

C331S10800D, C331S1160FE, C331S1160FE, C331S158000, C361S001000, C361S111000

Reexamination Certificate

active

06181214

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to semiconductor integrated circuits and, more particularly, to an oscillator input cell for a crystal oscillator circuit.
Semiconductor integrated circuits typically include clock inputs for coupling to a crystal oscillator or other clock signal source, which controls the sequential operation of the various clocked elements within the integrated circuit. When using a crystal oscillator, one of the input-output (I/O) pads on the integrated circuit serves as an oscillator input pad, and another I/O pad serves as an oscillator feedback pad. An oscillator input cell is placed in the I/O region on the integrated circuit, across the two I/O pads. The oscillator input cell has an inverter, which provides negative feedback from the oscillator input pad to the oscillator feedback pad to insure instability of the crystal oscillator circuit. The inverter output is also coupled to the core of the integrated circuit for providing a clock signal to the integrated circuit.
A typical oscillator input cell further includes electrostatic discharge (ESD) protection circuits coupled to the oscillator input pad and the oscillator feedback pad for clamping any voltage spikes received on the pads to the core supply voltage levels. A typical ESD protection circuit includes N-channel pull-down transistors and P-channel pull-up transistors, which are coupled between the pad and the respective voltage supply rail. These transistors are biased in an off state during normal operation. However, large ESD events will cause either the N-channel or the P-channel transistors to turn on, thereby clamping the pad to the respective voltage supply rail.
Typical oscillator input cells such as those described above are not tolerant to input voltages that are greater than the core voltage levels of the integrated circuit. This limits the type of clock signal sources that can be coupled to the oscillator input pad. As core supply voltage levels of integrated circuits continue to decrease, it becomes more difficult to interface with other integrated circuits having older technology. Also, it may not be desirable to operate external clock signals with reduced voltage swings at the board level.
Improved oscillator input cells that are tolerant to higher voltage swings are desired.
SUMMARY OF THE INVENTION
One aspect of the present invention relates to an integrated circuit oscillator input cell, which has an oscillator input pad, an oscillator feedback pad, a core terminal, an inverter and an electrostatic discharge protection circuit. The inverter has an inverter input, which is coupled to the oscillator input pad, and an inverter output, which is coupled to the oscillator feedback pad and the core terminal. The electrostatic discharge protection circuit includes a plurality of N-channel protection transistors, which are coupled to the oscillator input pad. The N-channel protection transistors are the only protection transistors that are coupled to the oscillator input pad.
Another aspect of the present invention relates to an oscillator circuit, which includes a semiconductor integrated circuit and an external crystal oscillator circuit. The semiconductor integrated circuit includes an oscillator input pad, an oscillator feedback pad, a core terminal, an inverter and an electrostatic discharge protection circuit. The inverter has an inverter input, which is coupled to the oscillator input pad, and an inverter output, which is coupled to the oscillator feedback pad and the core terminal. The electrostatic discharge protection circuit includes a plurality of N-channel protection transistors, which are coupled to the oscillator input pad. The N-channel protection transistors are the only protection transistors that are coupled to the oscillator input pad. The crystal oscillator circuit is coupled across the oscillator input pad and the oscillator feedback pad.
Yet another aspect of the present invention relates to an integrated circuit oscillator input cell, which includes an oscillator input pad, an oscillator feedback pad, a core terminal, an inverter and an electrostatic discharge protection circuit. The oscillator input pad receives an oscillating signal having one of a first voltage swing and a second voltage swing, wherein the second voltage swing is greater than the first voltage swing. The oscillator feedback pad and the core terminal have the first voltage swing. The inverter has an inverter input, which is coupled to the oscillator input pad, and an inverter output, which is coupled to the oscillator feedback pad and the core terminal. The electrostatic discharge protection circuit is coupled to the oscillator input pad for clamping the oscillator input pad in response to electrostatic discharge voltage spikes received on the oscillator input pad and for drawing substantially no current when the oscillator input pad has a voltage level within the first and second voltage swings.


REFERENCES:
patent: 6078487 (2000-06-01), Partovi et al.

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