Phase error compensation in single correlator systems
Present disclosure provides techniques to compensate for the above-identified signal processing delays between an in-cell digitizer of the touch screen display system and a stylus (or pen). In one example, the delays may be compensated by estimating the stylus-digitizer phase error on the digitizer side and adjusting for the delay accordingly. Specifically, in this example, the digitizer may utilize a quadratic receiver to perform an in-phase and quadratic detection in time domain in order to estimate the phase error. Based on the in-phase and quadratic detection, the digitizer may adapt the subsequent stylus sampling windows to compensate for the detected phase error. In another example, the stylus may transmit a first signal and a second signal that is 90 degrees delayed during a predetermined time slot, and to adapt the stylus sampling windows on the digitizer side in order to compensate for the phase delay.
1. A method for time synchronizing a touch screen display with a stylus, comprising:
receiving, at a digitizer of the touch screen display, a downlink signal from the stylus associated with the touch screen display;
determining, at the digitizer, a timing misalignment between the touch screen display and the stylus by measuring self-capacitance of a sensel electrode on the touch screen display based on the downlink signal; and
adjusting, at the digitizer, a sampling window to compensate for the timing misalignment, based at least on a phase error between the digitizer and the stylus.
2. The method of claim 1 , wherein determining the timing misalignment between the touch screen display and the stylus comprises:
generating, at the digitizer, an uplink signal for transmission during a first time period; and
transmitting the uplink signal from the digitizer to the sensel electrode of the touch screen display.
3. The method of claim 2 , further comprising:
measuring an electric current (i) flow of electric charge into the sensel electrode during a second time period;
calculating a delay in time from the first time period to the second time period; and
adjusting a subsequent sampling window for transmissions of the uplink signal based on the delay.
4. The method of claim 3 , wherein calculating the delay in time from the first time period to the second time period comprises:
correlating a waveform of the uplink signal against a reference waveform to identify a phase at which correlation is maximized.
5. The method of claim 1 , wherein receiving the downlink signal from the stylus associated with the touch screen display, comprises:
receiving a first downlink signal from the stylus during a first time slot; and
receiving a second downlink signal from the stylus during a second time slot, wherein the second downlink signal is 90 degrees delayed from the first downlink signal.
6. The method of claim 5 , further comprising:
correlating an uplink signal for transmission against a quadrature signal of one of the first downlink signal or the second downlink signal; and
adjusting a subsequent sampling window for transmissions of the uplink signal at the digitizer based on the correlation.
7. The method of claim 1 , wherein the downlink signal is received from the stylus by measuring an electric current (i) flow of electric charge into the sensel electrode when the stylus applies voltage to electrodes located on a tip of the stylus.
8. The method of claim 1 , wherein the touch screen display is an in-cell touch screen.
9. A touch screen display, comprising:
a digitizer;
a memory coupled to the digitizer, wherein the memory includes instructions executable by the digitizer to:
receive, at the digitizer of the touch screen display, a downlink signal from a stylus associated with the touch screen display;
determine, at the digitizer, a timing misalignment between the touch screen display and the stylus by measuring self-capacitance of a sensel electrode on the touch screen display based on the downlink signal; and
adjust, at the digitizer, a sampling window to compensate for the timing misalignment, based at least on a phase error between the digitizer and the stylus.
10. The touch screen display of claim 9 , wherein the instructions to determine the timing misalignment between the touch screen display and the stylus further includes instructions executable by the digitizer to:
generate, at the digitizer, an uplink signal for transmission during a first time period; and
transmit the uplink signal from the digitizer to the sensel electrode of the touch screen display.
11. The touch screen display of claim 10 , wherein the instructions are further executable by the digitizer to:
measure an electric current (i) flow of electric charge into the sensel electrode during a second time period;
calculate a delay in time from the first time period to the second time period; and
adjust a subsequent sampling window for transmissions of the uplink signal based on the delay.
12. The touch screen display of claim 11 , wherein the instructions to calculate the delay in time from the first time period to the second time period are further-executable by the digitizer to:
correlate a waveform of the uplink signal against a reference waveform to identify a phase at which correlation is maximized.
13. The touch screen display of claim 9 , wherein the instructions to receive the downlink signal from the stylus associated with the touch screen display are further executable by the digitizer to:
receive a first downlink signal from the stylus during a first time slot; and
receive a second downlink signal from the stylus during a second time slot, wherein the second downlink, signal is 90 degrees delayed from the first downlink signal.
14. The touch screen display of claim 13 , wherein the instructions are further executable by the digitizer to:
correlate an uplink signal for transmission against a quadrature signal of one of the first downlink signal or the second downlink signal; and
adjust a subsequent sampling window for transmissions of the uplink signal at the digitizer based on the correlation.
15. The touch screen display of claim 9 , wherein the downlink signal is received from the stylus by measuring an electric current (i) flow of electric charge into the sensel electrode when the stylus applies voltage to electrodes located on a tip of the stylus.
16. The touch screen display of claim 9 , wherein the touch screen display is an in-cell touch screen.
17. A computer-readable medium for time synchronizing a touch screen display with a stylus comprising code for:
receiving, at a digitizer of the touch screen display, a downlink signal from the stylus associated with the touch screen display;
determining, at the digitizer, a timing misalignment between the touch screen display and the stylus by measuring self-capacitance of a sensel electrode on the touch screen display based on the downlink signal; and
adjusting, at the digitizer, a sampling window to compensate for the timing misalignment, based at least on a phase error between the digitizer and the stylus.
18. The computer-readable medium of claim 17 , wherein the code for determining the timing misalignment between the touch screen display and the stylus further includes code for:
generating, at the digitizer, an uplink signal for transmission during a first time period; and
transmitting the uplink signal from the digitizer to the sensel electrode of the touch screen display.
19. The computer-readable medium of claim 18 , further comprising code for:
measuring an electric current (i) flow of electric charge into the sensel electrode during a second time period;
calculating a delay in time from the first time period to the second time period; and
adjusting a subsequent sampling window for transmissions of the uplink signal based on the delay.
20. The computer-readable medium of claim 19 , wherein the code for calculating the delay in time from the first time period to the second time period comprises code for:
correlating a waveform of the uplink signal against a reference waveform to identify a phase at which correlation is maximized.