Self capacitance sensing based on tangent of phase shift of drive signal
A touch screen controller includes driving circuitry coupled to a conductive line through a resistance and drives that conductive line with a driving signal passed through the resistance at a drive frequency. Sensing circuitry is coupled to that conductive line and senses a voltage at that conductive line, the voltage being a function of a capacitance seen by that conductive line. Analog to digital conversion circuitry is coupled to the sensing circuitry and samples the sensed voltage at a sampling frequency to produce samples. Processing circuitry is coupled to the analog to digital conversion circuitry and directly calculates a tangent of a phase shift of the voltage due to the resistance and the capacitance from the samples, and determines a self touch value for that conductive line as a function of the tangent of the phase shift of the voltage.
1. A touch screen controller, comprising:
driving circuitry coupled to a conductive line through a resistance and configured to drive that conductive line with a driving signal passed through the resistance at a drive frequency;
sensing circuitry coupled to that conductive line and configured to sense a voltage at that conductive line, the voltage being a function of a capacitance of that conductive line;
analog to digital conversion circuitry coupled to the sensing circuitry and configured to sample the sensed voltage at a sampling frequency to produce samples; and
processing circuitry coupled to the analog to digital conversion circuitry and configured to:
directly calculate, from the samples, a tangent of a phase shift of the voltage due to the resistance and the capacitance; and
determine a self touch value for that conductive line as a function of the tangent of the phase shift of the voltage.
2. The touch screen controller of claim 1 , wherein the sampling frequency is greater than the drive frequency.
3. The touch screen controller of claim 1 , wherein the sampling frequency is fixed.
4. The touch screen controller of claim 1 , wherein the processing circuitry is configured to directly calculate the tangent of the phase shift of the voltage by:
performing a Fast Fourier Transform (FFT) as a function of the drive frequency to determine real and imaginary components of a signal represented by the samples; and
determining the tangent of the phase shift of the voltage as a ratio of the real component of the signal represented by the samples to the imaginary component of the signal represented by the samples.
5. The touch screen controller of claim 4 , wherein the processing circuitry is configured to directly calculate the tangent of the phase shift of the voltage by:
multiplying the samples by a real multiplier and by an imaginary multiplier;
summing the samples after multiplication to produce the signal represented by the samples;
determining real and imaginary components of the signal represented by the samples; and
determining the tangent of the phase shift of the voltage as the ratio of the real and imaginary components of the signal represented by the samples.
6. The touch screen controller of claim 4 , wherein the processing circuitry performs the FFT by calculating the signal S represented by the samples at a frequency k of the drive signal as:
S
(
k
)
=
∑
n
=
0
N
s
(
n
)
*
e
-
j
2
π
k
n
N
.
7. The touch screen controller of claim 4 , wherein the processing circuitry performs the FFT by calculating the signal S represented by the samples at the drive frequency k as:
S
(
k
)
=
∑
n
=
0
N
s
(
n
)
*
e
-
j
2
π
k
n
N
=
∑
n
=
0
N
s
(
n
)
*
(
cos
(
2
π
k
n
N
)
-
j
*
sin
(
2
π
k
n
N
)
)
with the real component comprised of resulting expressions involving cosine and the imaginary component comprised of resulting expressions involving sine.
8. The touch screen controller of claim 1 , wherein the processing circuitry determines the self touch value as:
tan(Δphase)=2 πf ( C 1 −C 0 ),
wherein C 1 -C 0 represents a change in capacitance of the conductive line and f is the drive frequency.
9. A method of self capacitance sensing on a touch screen, the method comprising:
driving a plurality of conductive lines with a drive signal forced through a resistance at a drive frequency; and
for each of the plurality of conductive lines, during a single self sensing frame:
sampling a voltage at that conductive line at a sampling frequency to produce samples, the voltage being a function of a capacitance of that conductive line;
directly calculating, from the samples, a tangent of a phase shift of the voltage due to the resistance and capacitance; and
determining a self touch value for that conductive line during the single self sensing frame as a function of the tangent of the phase shift of the voltage.
10. The method of claim 9 , wherein the sampling frequency is greater than the drive frequency.
11. The method of claim 9 , wherein the sampling frequency is fixed.
12. The method of claim 9 , wherein the tangent of the phase shift of the voltage is directly calculated by:
performing a Fast Fourier Transform (FFT) as a function of the drive frequency to determine real and imaginary components of a signal represented by the samples; and
calculating the tangent of the phase shift of the voltage as a ratio of the real component of the signal represented by the samples to the imaginary component of the signal represented by the samples.
13. The method of claim 12 , wherein the tangent of the phase shift of the voltage is determined by:
multiplying the samples by a real multiplier and by an imaginary multiplier;
summing the samples after multiplication to produce the signal represented by the samples;
determining real and imaginary components of the signal represented by the samples; and
calculating the tangent of the phase shift of the voltage as the ratio of the real and imaginary components of the signal represented by the samples.
14. The method of claim 12 , wherein performing the FFT comprises calculating the signal S represented by the samples at the drive frequency k as:
S
(
k
)
=
∑
n
=
0
N
s
(
n
)
*
e
-
j
2
π
k
n
N
.
15. The method of claim 12 , wherein performing the FFT comprises calculating the signal S represented by the samples at the drive frequency k as:
S
(
k
)
=
∑
n
=
0
N
s
(
n
)
x
e
-
j
2
π
k
n
N
=
∑
n
=
0
N
s
(
n
)
*
(
cos
(
2
π
k
n
N
)
-
j
*
sin
(
2
π
k
n
N
)
)
with the real component comprised of resulting expressions involving cosine and the imaginary component comprised of resulting expressions involving sine.
16. The method of claim 9 , wherein the self touch value is determined as:
tan(Δphase)=2 πf ( C 1 −C 0 ),
wherein C 1 -C 0 represents a change in capacitance of the conductive line and f is the drive frequency.
17. A touch screen controller, comprising:
driving circuitry coupled to a conductive line and configured to drive that conductive line;
sensing circuitry coupled to that conductive line and configured to sense a voltage at that conductive line, the voltage being a function of a capacitance seen by that conductive line;
analog to digital conversion circuitry coupled to the sensing circuitry and configured to sample the sensed voltage to produce samples; and
processing circuitry configured to determine a self touch value for that conductive line as a function of a tangent of a phase shift of the sensed voltage based upon the samples.
18. The touch screen controller of claim 17 , wherein the processing circuitry is configured to directly calculate the tangent of the phase shift of the voltage by:
performing a Fast Fourier Transform (FFT) to determine real and imaginary components of a signal represented by the samples; and
determining the tangent of the phase shift of the voltage as a ratio of the real component of the signal represented by the samples to the imaginary component of the signal represented by the samples.
19. The touch screen controller of claim 18 , wherein the processing circuitry is configured to directly calculate the tangent of the phase shift of the voltage by:
multiplying the samples by a real multiplier and by an imaginary multiplier;
summing the samples after multiplication to produce the signal represented by the samples;
determining real and imaginary components of the signal represented by the samples; and
determining the tangent of the phase shift of the voltage as the ratio of the real and imaginary components of the signal represented by the samples.
20. A method of touch sensing on a touch screen, the method comprising:
driving conductive lines with a drive signal; and
for each of the conductive lines, during a single touch sensing frame:
sampling a voltage at that conductive line to produce samples, the voltage being a function of a capacitance seen by that conductive line;
directly calculating a tangent of a phase shift of the voltage due to the capacitance from the samples; and
determining a touch value for that conductive line during the single touch sensing frame as a function of the tangent of the phase shift of the voltage.
21. The method of claim 20 , wherein the tangent of the phase shift of the voltage is directly calculated by:
performing a Fast Fourier Transform (FFT) to determine real and imaginary components of a signal represented by the samples; and
calculating the tangent of the phase shift of the voltage as a ratio of the real component of the signal represented by the samples to the imaginary component of the signal represented by the samples.
22. The method of claim 21 , wherein the tangent of the phase shift of the voltage is determined by:
multiplying the samples by a real multiplier and by an imaginary multiplier;
summing the samples after multiplication to produce the signal represented by the samples; and
calculating the tangent of the phase shift of the voltage as the ratio of the real and imaginary components of the signal represented by the samples.