Method and apparatus for compensating input threshold variations on input buffers

An apparatus for compensating for variations in threshold voltage of an input buffer, said input buffer operative to produce a logical one at its output when a voltage across a capacitor of an RC measurement circuit exceeds the threshold voltage, said apparatus comprising:

• a voltage source operative to apply a voltage across the capacitor;
• means for setting the voltage supplied by the voltage source;
• control circuitry for measuring an elapsed time between a time after which the voltage source applies an initial voltage across the capacitor and a time at which the input buffer provides a logical one at its output;
• said control circuitry operative to compare the elapsed time with a reference time;
• and,
• said means for setting the voltage supplied by the voltage source operative to adjust the initial voltage applied across the capacitor in response to said comparison.

A system for compensating for variations in threshold voltage of a plurality of input buffers on a die comprising:

• a plurality of RC measurement circuits, each RC measurement circuit associated with one of said 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;
• a calibration circuit comprising:
• a calibration capacitor having a known capacitance;
• a calibration resistor having a known resistance;
• a calibration input buffer connected to said calibration capacitor, said calibration input buffer operative to produce a logical one at its output when a voltage across said calibration capacitor exceeds a threshold voltage of said 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;
• 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;
• said control circuitry operative to compare the elapsed time with a reference time;
• said means for setting the voltage supplied by the voltage source operative to adjust the initial voltage applied across the capacitor in response to said comparison;
• and,
• means to apply the adjusted voltage to said plurality of RC measurement circuits.