Conditional-reset, temporally oversampled image sensor

1. An image sensor comprising:

a plurality of pixel circuits disposed in rows and columns, each of the plurality of pixel circuits having a first light-detecting element operable to integrate photocharge, output circuitry operable to selectively output a signal indicative of photocharge integrated by the first light-detecting element, and individual pixel-circuit addressable reset circuitry operable to remove photocharge integrated by the first light-detecting element since the prior operation of the reset circuitry;

control circuitry operable to select subgroups of the pixel circuits to output the respective signals indicative of integrated photocharge, and operable such that each pixel circuit in the plurality is selected at least four times during a frame period;

a plurality of comparators, operable such that each pixel circuit in a given subgroup of the pixel circuits selected by the control circuitry is paired with one of the comparators to generate a binary pixel value that indicates whether the photocharge integrated by the corresponding light-detecting element exceeds a threshold; and

a plurality of reset signal generators each paired with a respective one of the comparators and each operable such that when the paired comparator generates a binary pixel value indicating threshold exceedance, the reset signal generator asserts a reset signal to cause the reset circuitry within the currently paired pixel circuit to reset the state of the light-detecting element.

2. The image sensor of claim 1 , integrated into an optical system wherein the plurality of pixel circuits is placed on a pixel pitch smaller than the diffraction limit of the optical system.

3. The image sensor of claim 1 wherein the output circuitry in each of the pixel circuits comprises:

an amplifying element coupled to a detection node of the light-detecting element to generate, at an output node of the amplifying element, the signal indicative of the integrated photocharge; and

an access element to switchably couple the output node of the amplifying element to the paired comparator based on a select signal supplied by the control circuitry.

4. The image sensor of claim 3 wherein the reset circuitry in each of the pixel circuits comprises circuitry coupled to the detection node of the light-detecting element to remove integrated photocharge based on both a signal supplied by the control circuitry and the reset signal supplied by the paired reset signal generator.

5. The image sensor of claim 1 , wherein each of the pixel circuits further comprises a second light-detecting element operable independently of the first light-detecting element.

6. The image sensor of claim 5 , wherein the first and second light-detecting elements have different sizes.

7. The image sensor of claim 1 further comprising accumulators paired respectively with pixel circuits to, when one of the reset signal generators generates a binary pixel value indicating threshold exceedance, add a corresponding pixel circuit value to a value stored in the paired accumulator.

8. The image sensor of claim 7 , wherein the pixel circuit value is binary.

9. The image sensor of claim 1 further comprising output logic to, when a reset signal generator generates a binary pixel value indicating threshold exceedance, transmit a corresponding pixel circuit value to another circuit.

10. The image sensor of claim 9 , wherein the pixel circuit value is binary.

11. The image sensor of claim 1 further comprising counter logic to generate a count value corresponding to a subgroup of the pixel circuits, the counter logic including circuitry to increment the count value, based on a first collection of at least two of the pixel circuits, when the state of the light-detecting element within any of the pixel circuits of the first collection of the pixel circuits is reset.

12. The image sensor of claim 1 , wherein the reset circuitry for at least two pixel circuits pairable with one of the comparators is connected to drive the light-detecting elements for those two pixel circuits to different reset voltages.

13. The image sensor of claim 1 , wherein for at least two pixel circuits pairable with one of the comparators, the comparator compares the photocharge integrated by the light-detecting elements of the two pixel circuits with respective, different thresholds.

14. A method of operation for capturing an image frame with a plurality of pixel circuits on an image sensor, the method comprising:

selecting a subgroup of the pixel circuits to output respective signals indicative of integrated photocharge, each pixel circuit being selected N times during the period of the image frame, where N is greater than one;

determining, for each pixel circuit in the selected subgroup, whether the signal indicative of integrated photocharge exceeds one or more thresholds; and

conditionally resetting each pixel circuit in the selected subgroup to a reset state depending on whether the corresponding signal indicative of integrated photocharge within that pixel circuit exceeds the one or more thresholds and independently of whether signals indicative of integrated photocharge in other pixel circuits in the selected subgroup exceed the one or more thresholds such that a time interval between successive resets of any one pixel circuit in the selected subgroup may be different from a time interval between successive resets of any other pixel circuit in the selected subgroup.

15. The method of claim 14 wherein the pixel circuits on the image sensor are disposed on the image sensor with a pitch smaller than a diffraction limit of an optical system into the image sensor is integrated.

16. The method of claim 14 further comprising incrementing a count value corresponding to the subgroup of the pixel circuits when any of the pixel circuits of the subgroup is reset.

17. The method of claim 14 wherein conditionally resetting each pixel circuit to a reset state comprises resetting a first one of the pixel circuits in the selected subgroup to a first reset voltage and resetting a second one of the pixel circuits in the selected subgroup to a second reset voltage, the second reset voltage being different from the first reset voltage.

18. The method of claim 14 wherein determining, for each pixel circuit in the selected subgroup, whether the signal indicative of integrated photocharge exceeds the one or more thresholds comprises comparing the signal indicative of integrated photocharge for a first one of the pixel circuits in the selected subgroup with a first threshold and comparing the signal indicative of integrated photocharge for a second one of the pixel circuits in the selected subgroup with a second threshold, the second threshold being different from the first threshold.

19. The method of claim 14 wherein N is greater than three.

20. An image sensor having a plurality of pixel circuits, the image sensor comprising:

means for concurrently selecting a subgroup of the pixel circuits to output respective signals indicative of integrated photocharge, each pixel circuit being selected at least four times during the period of the image frame;

means for determining, for each pixel circuit in the selected subgroup, whether the signal indicative of integrated photocharge exceeds one or more thresholds; and

means for conditionally resetting each pixel circuit in the selected subgroup to a reset state depending on whether the corresponding signal indicative of integrated photocharge within that pixel circuit exceeds the one or more thresholds and independently of whether signals indicative of integrated photocharge in other pixel circuits in the selected subgroup exceed the one or more thresholds such that a time interval between successive resets of any one pixel circuit in the selected subgroup may be different from a time interval between successive resets of any other pixel circuit in the selected subgroup.