Capacitance sensing circuit and method of capacitance sensing
View Patent ↗A capacitance sensing circuit comprises a capacitive device having a capacitance, the device initially being at a first voltage level. The capacitance sensing circuit is capable of applying one or more pull-up currents to the device during one or more corresponding pull-up periods of time, for changing the first voltage level into one or more corresponding pull-up voltage levels; applying a measurement current to the device; and measuring a measurement period of time, during which one of the pull-up voltage levels changes into a second voltage level. A method of sensing a capacitance of a capacitive device comprises applying a first voltage to the device; applying one or more pull-up currents during corresponding pull-up times, for changing the first voltage into corresponding pull-up voltages; applying a measurement current; and measuring a time, during which one pull-up voltage changes into a second voltage.
1. A capacitance sensing circuit, comprising
a capacitive device having a capacitance;
said capacitance sensing circuit being capable of
applying in an initialization phase a pull-up current to said capacitive device, said capacitive device initially being at a first voltage level;
measuring an initialization period of time for changing said first voltage level into a second voltage level by means of said pull-up current;
determining a pull-up voltage level as a voltage level present at said capacitive device after a fraction of said initialization period of time;
applying in a measurement phase said pull-up current to said capacitive device, said capacitive device initially being at said first voltage level, during a pull-up period of time, for changing said first voltage level into said pull-up voltage level;
applying a measurement current to said capacitive device, said capacitive device being at said pull-up voltage level; and
measuring a measurement period of time, during which said pull-up voltage level changes into said second voltage level by means of said measurement current.
2. The capacitance sensing circuit as claimed in claim 1 , wherein a strength of said measurement current is smaller than a strength of at least one of said one or more pull-up currents.
3. The capacitance sensing circuit as claimed in claim 1 , comprising
a plurality of different current paths for applying said measurement current and each of said one or more pull-up currents to said capacitive device using a different one of said different current paths.
4. The capacitance sensing circuit as claimed in claim 3 , wherein at least one of said different current paths comprises a switch for switching off and/or on said at least one of said different current paths.
5. The capacitance sensing circuit as claimed in claim 3 , wherein each of said plurality of different current paths comprises a different resistance.
6. The capacitance sensing circuit as claimed in claim 5 , wherein said measurement current is provided through a measurement current path of said plurality of different current paths, at least one of said plurality of different current paths having a smaller resistance than said measurement current path.
7. The capacitance sensing circuit as claimed in claim 1 , wherein said capacitance is less than 10−9 Farad.
8. The capacitance sensing circuit as claimed in claim 1 , wherein said one or more pull-up currents and said measurement current are provided by different current sources.
9. The capacitance sensing circuit as claimed in claim 1 , wherein said first and said second voltage level are provided at a pin of a processing unit.
10. The capacitance sensing circuit as claimed in claim 9 , wherein at least one of said resistances is a resistance of an internal pull-up resistor of said pin of said processing unit.
11. The capacitance sensing circuit as claimed in claim 1 , wherein said capacitive device is a capacitive sensor.
12. A method of sensing a capacitance of a capacitive device, comprising
applying in an initialization phase a pull-up current to a capacitive device, said capacitive device initially being at a first voltage level;
measuring an initialization period of time for changing said first voltage level into a second voltage level by means of said pull-up current;
determining a pull-up voltage level as a voltage present at said capacitive device after applying said pull-up current for a fraction of said initialization period of time;
applying the first voltage level to said capacitive device;
applying in a measurement phase the pull-up current to said capacitive device, said capacitive device being at said first voltage level, during a pull-up period of time, for changing said first voltage level into said pull-up voltage level;
applying a measurement current to said capacitive device, said capacitive device being at said pull-up voltage level; and
measuring a measurement period of time, during which said pull-up voltage level changes into said second voltage level by means of said measurement current.
13. A non-transitory computer program product, comprising code portions for executing steps of a method as claimed in claim 12 .