Pixel circuit and method of operating the same in an always-on mode

1. A method of operating an imaging device comprising a sensor array, the sensor array comprising a plurality of pixels, the method comprising:

capturing a first low-spatial resolution frame using a subset of the plurality of pixels of the sensor array;

generating, using a processor coupled to the sensor array, a first depth map using raw pixel values of the first low-spatial resolution frame;

capturing a second low-spatial resolution frame using the subset of the plurality of pixels of the sensor array;

generating, using the processor, a second depth map using raw pixel values of the second low-spatial resolution frame;

determining whether an object has moved in a field of view of the imaging device based on a comparison of the first depth map to the second depth map;

activating each of the plurality of pixels of the sensor array in response to a determination that the object has moved in the field of view of the imaging device;

capturing a high-spatial resolution frame using each of the plurality of pixels of the sensor array; and

generating a third depth map using raw pixel values of the high-spatial resolution frame.

2. The method of claim 1 , further comprising determining, based on the third depth map, at least one of a gesture performed by the object, a distance between the object and the imaging device, a feature of the object, or a motion characteristic of the object.

3. The method of claim 2 , wherein the object is a human being, and wherein the feature of the object comprises a facial feature of the object.

4. The method of claim 1 , wherein a power consumption of the imaging device during each of capturing the first low-spatial resolution frame and capturing the second low-spatial resolution frame is less than a power consumption of the imaging device during capturing the high-spatial resolution frame.

5. The method of claim 1 , wherein capturing the first low-spatial resolution frame using the subset of the plurality of pixels of the sensor array and capturing the second low-spatial resolution frame using the subset of the plurality of pixels of the sensor array comprises gating at least one of a clock driver circuit or a row control circuit coupled to the sensory array.

6. The method of claim 5 , wherein gating at least one of the clock driver circuit or the row control circuit coupled to the sensory array comprises power gating or signal gating the clock driver circuit or the row control circuit.

7. A method of operating an imaging device comprising a sensor array, the sensor array comprising a plurality of pixels, the method comprising:

capturing a first low-spatial resolution frame using a subset of the plurality of pixels of the sensor array, wherein raw pixel values of the first low-spatial resolution frame comprise a first time-of-flight measurement, a first brightness, and a first signal-to-noise ratio;

generating, using a processor coupled to the sensor array, a first depth map using raw pixel values of the first low-spatial resolution frame;

capturing a second low-spatial resolution frame using the subset of the plurality of pixels of the sensor array, wherein raw pixel values of the second low-spatial resolution frame comprise a second time-of-flight measurement, a second brightness, and a second signal-to-noise ratio;

generating, using the processor, a second depth map using raw pixel values of the second low-spatial resolution frame; and

determining whether an object has moved in a field of view of the imaging device based on a comparison of the first depth map to the second depth map.

8. The method of claim 7 , wherein generating the first depth map using raw pixel values of the first low-spatial resolution and generating the second depth map using raw pixel values of the second low-spatial resolution frame comprise:

constructing, using the processor, the first depth map based on the first time-of-flight measurement, the first brightness, and the first signal-to-noise ratio; and

constructing, using the processor, the second depth map based on the second time-of-flight measurement, the second brightness, and the second signal-to-noise ratio.

9. The method of claim 7 , wherein determining whether the object has moved in the field of view of the imaging device based on the comparison of the first depth map to the second depth map comprises:

determining that the object has not moved in the field of view of the imaging device in response to the first signal-to-noise ratio or the second signal-to-noise ratio being less than a pixel signal-to-noise ratio limit.

10. The method of claim 7 , wherein determining whether the object has moved in the field of view of the imaging device based on the comparison of the first depth map to the second depth map comprises:

determining that the object has moved in the field of view of the imaging device in response to a change between the first time-of-flight measurement and the second time-of-flight measurement or a change between the first brightness and the second brightness being greater than a threshold.

11. The method of claim 10 , wherein the threshold is a configurable predetermined limit, the configurable predetermined limit being dependent on an application of the imaging device.

12. A method of operating an imaging device comprising a sensor array, the sensor array comprising a plurality of pixels, the method comprising:

capturing a first low-spatial resolution frame using a subset of the plurality of pixels of the sensor array;

generating, using a processor coupled to the sensor array, a first depth map using raw pixel values of the first low-spatial resolution frame;

capturing a second low-spatial resolution frame using the subset of the plurality of pixels of the sensor array;

generating, using the processor, a second depth map using raw pixel values of the second low-spatial resolution frame;

determining whether an object has moved in a field of view of the imaging device based on a comparison of the first depth map to the second depth map;

designating the second depth map as the first depth map in response to a determination that the object has not moved in the field of view of the imaging device; and

repeating the steps of capturing the second low-spatial resolution frame, generating, the second depth map using raw pixel values of the second low-spatial resolution frame, and determining whether the object has moved in the field of view of the imaging device based on the comparison of the first depth map to the second depth map.

13. A method of operating an imaging device comprising a sensor array, the sensor array comprising a plurality of pixels, the method comprising:

providing a first timing signal to a row driver circuit and a column driver circuit to select a subset of the plurality of pixels;

based on the first timing signal, capturing a first low-spatial resolution frame using the subset of the plurality of pixels of the sensor array;

generating, using a processor, a first depth map using raw pixel values of the first low-spatial resolution frame;

providing a second timing signal to the row driver circuit and the column driver circuit to select the subset of the plurality of pixels;

based on the second timing signal, capturing a second low-spatial resolution frame using the subset of the plurality of pixels of the sensor array;

generating, using the processor, a second depth map using raw pixel values of the second low-spatial resolution frame; and

determining, using the processor, whether an object has moved in a field of view of the imaging device based on a comparison of the first depth map to the second depth map.

14. The method of claim 13 , further comprising:

activating each of the plurality of pixels of the sensor array in response to a determination that the object has moved in the field of view of the imaging device;

capturing a high-spatial resolution frame using each of the plurality of pixels of the sensor array; and

generating a third depth map using raw pixel values of the high-spatial resolution frame.

15. The method of claim 13 , wherein raw pixel values of the first low-spatial resolution frame comprise a first time-of-flight measurement, a first brightness, and a first signal-to-noise ratio, and wherein raw pixel values of the second low-spatial resolution frame comprise a second time-of-flight measurement, a second brightness, and a second signal-to-noise ratio.

16. The method of claim 15 , wherein generating the first depth map using raw pixel values of the first low-spatial resolution and generating the second depth map using raw pixel values of the second low-spatial resolution frame comprise:

constructing, using the processor, the first depth map based on the first time-of-flight measurement, the first brightness, and the first signal-to-noise ratio; and

constructing, using the processor, the second depth map based on the second time-of-flight measurement, the second brightness, and the second signal-to-noise ratio.

17. The method of claim 15 , wherein determining whether the object has moved in the field of view of the imaging device based on the comparison of the first depth map to the second depth map comprises:

determining that the object has not moved in the field of view of the imaging device in response to the first signal-to-noise ratio or the second signal-to-noise ratio being less than a pixel signal-to-noise ratio limit.

18. The method of claim 15 , wherein determining whether the object has moved in the field of view of the imaging device based on the comparison of the first depth map to the second depth map comprises:

determining that the object has moved in the field of view of the imaging device in response to a change between the first time-of-flight measurement and the second time-of-flight measurement or a change between the first brightness and the second brightness being greater than a threshold.

19. The method of claim 18 , wherein the threshold is a configurable predetermined limit, the configurable predetermined limit being dependent on an application of the imaging device.

20. The method of claim 13 , further comprising:

designating the second depth map as the first depth map in response to a determination that the object has not moved in the field of view of the imaging device; and

repeating the steps of capturing the second low-spatial resolution frame, generating, the second depth map using raw pixel values of the second low-spatial resolution frame; and

determining whether the object has moved in the field of view of the imaging device based on the comparison of the first depth map to the second depth map.

21. A method of operating an imaging device comprising a sensor array, the sensor array comprising a plurality of pixels, the method comprising:

capturing a first low-spatial resolution frame using a subset of the plurality of pixels of the sensor array;

generating, using a processor coupled to the sensor array, a first depth map using raw pixel values of the first low-spatial resolution frame;

capturing a second low-spatial resolution frame using the subset of the plurality of pixels of the sensor array;

generating, using the processor, a second depth map using raw pixel values of the second low-spatial resolution frame; and

determining whether an object has moved in a field of view of the imaging device based on a comparison of the first depth map to the second depth map;

when the object has moved in the field of view, activating each of the plurality of pixels of the sensor array, capturing a high-spatial resolution frame using each of the plurality of pixels of the sensor array, and generating a third depth map using raw pixel values of the high-spatial resolution third depth map using raw pixel values of the high-spatial resolution frame; and

when the object has not moved in the field of view, designating the second depth map as the first depth map, repeating the steps of capturing the second low-spatial resolution frame and generating the second depth map using raw pixel values of the second low-spatial resolution frame and then determining again whether the object has moved in the field of view of the imaging device based on the comparison of the first depth map to the second depth map.

22. The method of claim 21 , further comprising, when the object has moved in the field of view, determining, based on the third depth map, a gesture performed by the object, a distance between the object and the imaging device, a feature of the object, or a motion characteristic of the object.

23. The method of claim 21 , wherein raw pixel values of the first low-spatial resolution frame comprise a first time-of-flight measurement, a first brightness, and a first signal-to-noise ratio, and wherein raw pixel values of the second low-spatial resolution frame comprise a second time-of-flight measurement, a second brightness, and a second signal-to-noise ratio.