Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synchronizing
Reexamination Certificate
1999-08-02
2001-02-20
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synchronizing
C327S198000
Reexamination Certificate
active
06191626
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to input buffers. More specifically, this invention relates to a method and system for compensating for input threshold variations on input buffers.
BACKGROUND OF THE INVENTION
RC circuits are useful in a number of contexts. For example, RC circuits may be used for creating a fixed timeout from known resistance and capacitance values, and for measuring an unknown resistance or capacitance value from a known capacitance or resistance and a measured timeout. More specifically, an external RC circuit may be used in combination with a digital integrated circuit to generate a time dependent wave form to measure a threshold crossing for an input buffer.
When an RC circuit is used to generate such a timeout in combination with an input buffer, the timeout time is also a function of the threshold voltage. Nevertheless, threshold voltage may vary from input buffer to input buffer. These variations in threshold voltage can be minimized by using, for example, an analog fabrication technique as opposed to the typical digital CMOS fabrication process. Such fabrication techniques are, however, more complex and therefore more costly.
SUMMARY OF THE INVENTION
Therefore, a need has arisen for a method and apparatus to compensate for threshold voltage variations in input buffers. A method and apparatus according to the embodiments of the present invention compensate for variations in threshold voltage using a digital technique.
A system for compensating for variations in threshold voltage of a plurality of input buffers on a die is disclosed. The system includes a plurality of RC measurement circuits, each RC measurement circuit associated with one of the input buffers such that when a voltage across the capacitor of the RC measurement circuit exceeds the threshold voltage of the associated input buffer, the associated input buffer provides a logical one at its output. The system further includes a calibration circuit comprising a calibration capacitor having a known capacitance, a calibration resistor having a known resistance, a calibration input buffer connected to the calibration capacitor, the calibration input buffer operative to produce a logical one at its output when a voltage across the calibration capacitor exceeds a threshold voltage of the calibration input buffer, a voltage source operative to apply a voltage across the calibration capacitor, means for setting the voltage supplied by the voltage source and control circuitry for measuring an elapsed time between a time after which the voltage source applies an initial voltage across the calibration capacitor and a time at which the calibration input buffer provides a logical one at its output. The control circuitry is operative to compare the elapsed time with a reference time. The means for setting the voltage supplied by the voltage source is operative to adjust the initial voltage applied across the capacitor in response to the comparison. The system also includes means to apply the adjusted voltage to said plurality of RC measurement circuits.
The present invention also includes a method for compensating for variations in a threshold voltage of an input buffer that is operative to produce a logical one at its output when a voltage across a capacitor of an RC measurement circuit exceeds its threshold voltage. The method begins by setting the voltage across the capacitor to an initial voltage value. The method then measures an elapsed time between a time after which the voltage across the capacitor is set to the initial voltage value and a time at which the input buffer provides a logical one at its output. The elapsed time is then compared with a reference time, and the initial voltage value is adjusted based on the comparison.
The above-described embodiments of the present invention provide various technical advantages. For example, the embodiments of the present invention provide a simple and inexpensive method and apparatus for compensating for input threshold variations in an input buffer. In addition, the method and apparatus according to the embodiments of the present invention provide a digital compensation technique that is adjustable on the fly and thus capable of compensating for changes in input threshold that may occur due to varying operating conditions. Other technical advantages are apparent to one skilled in the art from the following figures, description and claims.
REFERENCES:
patent: 4447774 (1984-05-01), Stalberg
patent: 4607178 (1986-08-01), Sugie et al.
patent: 5479132 (1995-12-01), Verhaeghe et al.
Chaveriat Brett Walter
Prysby Daniel G.
Rotstein Ron
Sullivan Ronald Joseph
Brady W. James
Courtney Mark E.
Nguyen Hiep
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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