Nano-power capacitance-to-digital converter
An asynchronous capacitance-to-digital conversion is described that allows for very low-power operation when during inactive periods (when no conductive object is in contact or proximity to the sensing electrodes). Asynchronous operation of a capacitance-to-digital converter (CDC) provides for capacitance-to-digital conversion without the use of system resources and more power intensive circuit elements.
1. A method for detecting a change in capacitance on a sensing electrode, the method comprising:
applying a signal to the sensing electrode;
comparing the signal on the sensing electrode to a reference signal;
if the signal on the sensing electrode is less than the reference signal, increasing the signal applied to the sensing electrode;
repeating the steps of applying the signal, comparing the signal, and increasing the signal until the signal is greater than the reference signal;
when the signal is greater than the reference signal, decreasing the signal applied to the sensing electrode;
if a toggle condition is not detected, repeating the steps of applying the signal, comparing the signal, increasing the signal, and decreasing the signal until a toggle condition is detected, wherein a toggle condition is detected when the signal has been greater than the reference signal more than once and less than the reference signal more than once; and
storing a first digital value representative of the signal at which the toggle condition is detected.
2. The method of claim 1 , wherein a toggle condition is detected when the signal is alternately above and below the reference signal for a prescribed number of cycles.
3. The method of claim 2 , further comprising:
applying a signal corresponding to the stored first digital value to the sensing electrode; and
if a toggle condition is detected, determining that no capacitance change has occurred.
4. The method of claim 3 further comprising:
if a toggle condition is not detected, increasing or decreasing the signal until a toggle condition is detected; and
storing a second digital value representative of the signal at which the toggle condition is detected.
5. The method of claim 4 , further comprising:
comparing the second digital value to the first digital value; and
if a difference between the first digital value and the second digital value is great enough, determining that a conductive object is present on the sensing electrode.
6. The method of claim 1 , wherein:
the signal is a voltage; and
the reference signal is a reference voltage.
7. The method of claim 1 , wherein increasing the signal is achieved by incrementing a capacitive digital-to-analog converter (DAC).
8. The method of claim 7 , wherein the capacitive DAC setting when the toggle condition is detected corresponds to the first digital value.
9. The method of claim 1 , wherein the steps of increasing and decreasing the signal are achieved by a logic circuit for receiving an output of a comparator for comparing the signal to the reference signal.
10. A method for detecting a change in capacitance of a sensing electrode, the method comprising:
applying a signal on the sensing electrode;
comparing a resultant signal derived from the signal and a capacitance of the sensing electrode to a reference signal;
if the resultant signal is greater than the reference signal, decrementing the applied signal and repeating the steps of applying the signal and comparing the resultant signal;
if the resultant signal is less than the reference signal, incrementing the applied signal and repeating the steps of applying the signal and comparing the resultant signal;
detecting a toggle condition of the resultant signal about the reference signal, wherein the toggle condition is detected when the applied signal is alternately incremented and decremented more than once;
if a toggle condition is detected without first repeated increments or decrements of the applied signal, determining no capacitance change of the sensing electrode; and
if a toggle condition is detected after repeated increments or decrements before the toggle condition, quantifying the change in capacitance of the sensing electrode by measuring how much the signal is altered by the incrementing and decrementing.
11. The method of claim 10 , wherein the toggle condition is detected when the resultant signal is alternately above and below the reference signal for a prescribed number of cycles.
12. The method of claim 10 , wherein:
the resultant signal is a voltage; and
the reference signal is a reference voltage.