SPAD array with gated histogram construction
A sensing device includes a first array of sensing elements, which output a signal indicative of a time of incidence of a single photon on the sensing element. A second array of processing circuits are coupled respectively to the sensing elements and comprise a gating generator, which variably sets a start time of the gating interval for each sensing element within each acquisition period, and a memory, which records the time of incidence of the single photon on each sensing element in each acquisition period. A controller sets, in each of at least some of the acquisition periods, different, respective gating intervals for different ones of the sensing elements.
1. A sensing device, comprising:
a first array of sensing elements, each sensing element configured to output a signal indicative of a time of incidence of a single photon on the sensing element during a certain gating interval in each of a succession acquisition periods; and
a second array of processing circuits, which are coupled respectively to the sensing elements and comprise:
a gating generator, which is configured to variably set a start time of the gating interval for each sensing element within each acquisition period;
a memory, which is coupled to record the time of incidence of the single photon on each sensing element in each acquisition period, responsively to the signal; and
a controller, which is configured to set, in each of at least some of the acquisition periods, different, respective gating intervals for different ones of the sensing elements and to modify the gating intervals such that the respective gating intervals for at least some of the sensing elements change over a series of image frames.
2. The device according to claim 1 , and comprising a radiation source, which is configured to direct a train of pulses of radiation, synchronized with the acquisition periods, toward a scene,
wherein the sensing elements are configured to detect photons of the radiation reflected from the scene, and wherein the processing circuits are configured to measure times of flight of the photons.
3. The device according to claim 2 , wherein the times of flight are indicative of depths of objects in the scene, and the controller is configured to set the different, respective gating intervals responsively to the depths.
4. The device according to claim 2 , wherein the controller is configured to control the gating generator during a first sequence of the acquisition periods so as to sweep the gating interval over the acquisition periods and to identify, responsively to the signal from each sensing element during the first sequence of the acquisition periods, a respective detection window for the sensing element, and during a second sequence of the acquisition periods, following the first sequence, to fix the gating interval for each sensing element to coincide with the respective detection window.
5. The device according to claim 1 , wherein the sensing elements comprise single-photon avalanche diodes (SPADs).
6. The device according to claim 1 , wherein the first array comprises a first two-dimensional matrix of the sensing elements formed on a first semiconductor chip, and wherein the second array comprises a second two-dimensional matrix of the processing circuits formed on a second semiconductor chip and coupled to the first matrix in a one-to-one correspondence between the sensing elements and the processing circuits.
7. The device according to claim 1 , wherein the processing circuits comprise a respective time-to-digital converter (TDC) coupled to receive the signal from each sensing element and to increment, responsively to the signal, a count in the memory of respective times of incidence of photons on the sensing element.
8. The device according to claim 7 , wherein the memory is configured to store respective counts of the photons that arrive in a plurality of different time bins within the gating interval during the succession of the acquisition periods.
9. The device according to claim 8 , wherein the controller is configured to process a histogram of the respective counts over the different time bins for each sensing element so as to derive and output a respective time-of-arrival value for the sensing element.
10. A method for sensing, comprising:
providing a first array of sensing elements, each sensing element configured to output a signal indicative of a time of incidence of a single photon on the sensing element during a certain gating interval in each of a succession acquisition periods;
coupling a second array of processing circuits respectively to the sensing elements, the processing circuits comprising a gating generator, which is configured to variably set a start time of the gating interval for each sensing element within each acquisition period, and a memory, which is coupled to record the time of incidence of the single photon on each sensing element in each acquisition period, responsively to the signal; and
controlling the gating generator so as to set, in each of at least some of the acquisition periods, different, respective gating intervals for different ones of the sensing elements and to modify the gating intervals such that the respective gating intervals for at least some of the sensing elements changes over a series of image frames.
11. The method according to claim 10 , and comprising directing a train of pulses of radiation, synchronized with the acquisition periods, toward a scene, wherein the sensing elements are configured to detect photons of the radiation reflected from the scene, and wherein the processing circuits are configured to measure times of flight of the photons responsively to the signals.
12. The method according to claim 11 , wherein the times of flight are indicative of depths of objects in the scene, and wherein controlling the gating generator comprises setting the different, respective gating intervals responsively to the depths.
13. The method according to claim 11 , wherein controlling the gating generator comprises:
during a first sequence of the acquisition periods, sweeping the gating interval over the acquisition periods; identifying, responsively to the signal from each sensing element during the first sequence of the acquisition periods, a respective detection window for each sensing element; and
during a second sequence of the acquisition periods, following the first sequence, fixing the gating interval for each sensing element to coincide with the respective detection window.
14. The method according to claim 10 , wherein the sensing elements comprise single-photon avalanche diodes (SPADs).
15. The method according to claim 10 , wherein the first array comprises a first two-dimensional matrix of the sensing elements formed on a first semiconductor chip, and wherein the second array comprises a second two-dimensional matrix of the processing circuits formed on a second semiconductor chip, and wherein the second matrix is coupled to the first matrix in a one-to-one correspondence between the sensing elements and the processing circuits.
16. The method according to claim 10 , and comprising incrementing, in response to the signal from each sensing element, a count in the memory of respective times of incidence of photons on the sensing element.
17. The method according to claim 16 , wherein incrementing the count comprises storing in the memory respective counts of the photons that arrive in a plurality of different time bins within the gating interval during the succession of the acquisition periods.
18. The method according to claim 17 , and comprising reading from the memory and processing a histogram of the respective counts over the different time bins for each sensing element so as to derive and output a respective time-of-arrival value for the sensing element.