Multiple channel capacitive voltage divider scanning method and apparatus
Relative capacitance of a plurality of capacitive sensors may be monitored by using only one ADC conversion. A plurality of capacitive sensors individually charges a sample and hold capacitor. After all of the plurality of capacitive sensors have charged the sample and hold capacitor, a digital conversion of the resulting analog on the sample and hold capacitor is made and stored in a memory. This stored digital collective voltage is compared to a previously stored one and if different then a proximity/touch event may have occurred. Therefore, an entire panel of capacitive sensors may be quickly monitored for a change in the “group” capacitance thereof, or portions of the capacitive sensors may be monitored for a change in the “subgroup” capacitance thereof. By knowing which subgroup of capacitive sensors has changed its collective capacitive value, a more focused and selective capacitive sensor measurement can be made that uses less power.
1. A method for determining change in capacitance of at least one capacitive sensor of a plurality of capacitive sensors, said method comprising steps of:
coupling by first switches the plurality of capacitive sensors with a first voltage;
coupling by second switches a sample and hold capacitor with a second voltage;
coupling by the first and second switches the plurality of capacitive sensors with the sample and hold capacitor, wherein electron charge from the plurality of capacitive sensors is transferred to the sample and hold capacitor;
measuring by a processor a resulting voltage on the sample and hold capacitor; and
comparing by the processor the resulting voltage to a previously measured resulting voltage, wherein if the resulting voltage is different than the previously measured resulting voltage then the at least one capacitive sensor of the plurality of capacitive sensors has changed capacitance value.
2. The method according to claim 1 , further comprising the steps of:
coupling by the first switches a portion of the plurality of capacitive sensors with the first voltage;
coupling by the second switches the sample and hold capacitor with the second voltage;
coupling by the first and second the portion of the plurality of capacitive sensors with the sample and hold capacitor, wherein electron charge from the portion of the plurality of capacitive sensors is transferred to the sample and hold capacitor;
measuring by the processor a resulting voltage on the sample and hold capacitor; and
comparing by the processor the measured resulting voltage to a previously measured resulting voltage of the portion of the plurality of capacitive sensors, wherein if the measured resulting voltage is different than the previously measured resulting voltage then at least one capacitive sensor of the portion of the plurality of capacitive sensors has changed capacitance value.
3. The method according to claim 2 , further comprising if the measured resulting voltage is not substantially different than the previously measured resulting voltage then perform the steps of:
coupling by the first switches another portion of the plurality of capacitive sensors with the first voltage;
coupling by the second switches the sample and hold capacitor with the second voltage;
coupling by the first and second switches the another portion of the plurality of capacitive sensors with the sample and hold capacitor, wherein electron charge from the another portion of the plurality of capacitive sensors is transferred to the sample and hold capacitor;
measuring by the processor another resulting voltage on the sample and hold capacitor; and
comparing by the processor the another resulting voltage to a previously measured another resulting voltage of the another portion of the plurality of capacitive sensors, wherein if the resulting another voltage is different than the previously measured another resulting voltage then at least one capacitive sensor of the another portion of the plurality of capacitive sensors has changed capacitance value.
4. The method according to claim 2 , further comprising the steps of:
(a) coupling by the first switches a first one of the portion of the plurality of capacitive sensors with the first voltage;
(b) coupling by the second switches the sample and hold capacitor with the second voltage;
(c) coupling by the first and second switches the first one of the portion of the plurality of capacitive sensors with the sample and hold capacitor, wherein electron charge from the first one of the portion of the plurality of capacitive sensors is transferred to the sample and hold capacitor;
(d) measuring by the processor a resulting voltage on the sample and hold capacitor; and
(e) comparing by the processor the measured resulting voltage to a previously measured resulting voltage of the first one of the portion of the plurality of capacitive sensors, wherein if the measured resulting voltage is different than the previously measured resulting voltage then the first one of the portion of the plurality of capacitive sensors has changed capacitance value, if not repeat steps (a) through (e) with another one of the portion of the plurality of capacitive sensors.
5. The method according to claim 1 , wherein the first voltage is more positive than the second voltage.
6. The method according to claim 1 , wherein the second voltage is more positive than the first voltage.
7. The method according to claim 1 , wherein a step of measuring the resulting voltage comprises the step of converting the resulting voltage on the sample and hold capacitor to a digital value with an analog-to-digital converter (ADC).
8. The method according to claim 7 , wherein a step of comparing the measured resulting voltage to a previously measured resulting voltage comprises the steps of comparing the digital value to a previous digital value with a digital processor.
9. The method according to claim 8 , further comprising a step of waking up the digital processor from a low power sleep mode when the measured resulting voltage is different from the previously measured resulting voltage.
10. An apparatus, for determining change in capacitance of at least one capacitive sensor of a plurality of capacitive sensors, comprising:
a plurality of capacitive sensors;
a sample and hold capacitor;
a multiplexer having a plurality of inputs and an output;
a plurality of capacitive sensor switches coupled to the plurality of capacitive sensors, and adapted to couple the plurality of capacitive sensors to the first voltage, the second voltage or an input of the multiplexer;
a sample and hold switch coupled between the output of the multiplexer and the sample and hold capacitor;
an analog-to-digital converter (ADC) having an analog input coupled to the sample and hold capacitor; and
a digital processor coupled to an output of the ADC and adapted to control the plurality of capacitive sensor switches, multiplexer, and sample and hold switch such that:
the plurality of capacitive sensor switches couple the plurality of capacitive sensors with a first voltage;
the multiplexer couples the sample and hold capacitor with a second voltage;
the plurality of capacitive sensor switches and the multiplexer couple the plurality of capacitive sensors with the sample and hold capacitor, wherein electron charge from the plurality of capacitive sensors is transferred to the sample and hold capacitor;
a resulting voltage on the sample and hold capacitor is converted by the ADC into a digital value which is fed to the digital processor; and
the processor is further configured to compare the resulting voltage to a previously measured resulting voltage, wherein if the resulting voltage is different than the previously measured resulting voltage then the digital processor determines that the at least one capacitive sensor of the plurality of capacitive sensors has changed capacitance value.
11. The apparatus according to claim 10 , wherein the plurality of capacitive sensor switches, the multiplexer, the sample and hold capacitor, the ADC and the digital processor are provided in a microcontroller.
12. The apparatus according to claim 11 , further comprising a capacitive sensor scan controller adapted to control the plurality of capacitive sensor switches, multiplexer, and sample and hold switch while the digital processor is in a low power sleep mode.
13. The apparatus according to claim 12 , wherein the capacitive sensor scan controller wakes up the digital processor from the low power sleep mode when the at least one capacitive sensor of the plurality of capacitive sensors has changed capacitance value.