AC line synchronization for capacitive sensing
View Patent ↗A method includes powering multiple touch sensors using DC power derived from an AC power signal; monitoring the AC power signal and producing from the AC power signal a trigger signal that occurs repeatedly at a predetermined point in a cycle of the AC power signal; and initiating a scan of the touch sensors when the trigger signal occurs during each cycle of the AC power signal.
1. A touch sensing system, comprising:
a plurality of touch sensors operated using direct current (DC) power;
a digital controller;
logic to monitor an alternating current (AC) signal from which the DC power is derived and to produce a trigger signal to the digital controller; and
logic to cause the digital controller to wait for the trigger signal before scanning outputs of the touch sensors.
2. The touch sensing system of claim 1 , further comprising:
the logic to monitor the AC signal comprising an opto-electric isolation circuit.
3. The touch sensing system of claim 1 , further comprising:
the digital controller using half-wave demodulation to scan the touch sensors.
4. The touch sensing system of claim 3 , further comprising:
the touch sensors operating relative to a local ground reference and the AC signal being relative to an earth ground reference that is different from the local ground reference.
5. The touch sensing system of claim 1 , further comprising:
logic to cause the controller to scan multiple groups of touch sensors during one period of the AC signal, a scan of each group triggered at a different point of the AC signal period.
6. The touch sensing system of claim 1 , further comprising:
logic to convert the trigger signal to a digital value and to make the digital value available to the digital controller.
7. A device comprising:
an AC-DC power converter; and
a capacitive touch sensing system, comprising:
a plurality of touch sensors operated using DC power from the power converter;
a digital controller operated using DC power from the power converter;
logic to monitor an AC signal from which the DC power is derived and to produce a trigger signal to the digital controller; and
logic to cause the digital controller to respond to the trigger signal by initiating a scan of the touch sensors.
8. The touch sensing system of claim 7 , further comprising:
the logic to monitor the AC signal comprising an opto-electric isolation circuit.
9. The touch sensing system of claim 7 , further comprising:
the controller adapted to use half-wave demodulation to scan the touch sensors.
10. The touch sensing system of claim 9 , further comprising:
the touch sensors operating relative to a local ground reference and the AC signal relative to an earth ground reference that is different from the local ground reference.
11. The touch sensing system of claim 7 , further comprising:
logic to perform scans of multiple groups from among the touch sensors, a scan of each group triggered at a different point in a cycle of the AC signal.
12. The touch sensing system of claim 7 , further comprising:
logic to convert the trigger signal to a digital value and to make the digital value available to the digital controller.
13. A method comprising:
powering a plurality of touch sensors using DC power derived from an AC power signal;
monitoring the AC power signal and producing from the AC power signal a trigger signal that occurs repeatedly at a same point in a cycle of the AC power signal; and
delaying a scan of the touch sensors until the trigger signal occurs during each cycle of the AC power signal.
14. The method of claim 13 , further comprising:
converting a ramping output signal to a digital one or zero;
determining an edge type of the output signal; and
starting the scan of the touch sensors when the output signal has a particular edge type.
15. The method of claim 13 , further comprising:
producing an optical signal with an intensity that tracks a level of the AC power signal; and
applying the optical signal to produce the trigger signal in the electrical domain.
16. The method of claim 13 , further comprising:
producing the DC power from the AC power using an isolated power converter.
17. The method claim 13 , further comprising:
scanning the touch sensors using a half wave demodulation sensing technique.
18. A method comprising:
introducing AC noise into at least one of a plurality of DC powered touch sensors; and
mitigating effects of the AC noise by:
monitoring an AC power signal that produces the DC power;
producing from the AC power signal a trigger signal that occurs repeatedly at a same point in a cycle of the AC power signal; and
initiating a scan of the touch sensors by a digital controller when the trigger signal occurs during each cycle of the AC power signal.
19. The method claim 18 , further comprising:
scanning the touch sensors using a half wave demodulation sensing technique.
20. The method of claim 18 , further comprising:
converting a ramping value of an output signal to a digital one or zero;
determining an edge type of the output signal; and
initiating the scan of the touch sensors when the output signal has a particular edge type.